one value required as left operand of assignment

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"lvalue required as left operand of assignment" When Writing to GPIO_REGs

Post by Fuzzyzilla » Sat May 13, 2017 6:04 pm

Code: Select all

Re: "lvalue required as left operand of assignment" When Writing to GPIO_REGs

Post by ESP_Sprite » Sun May 14, 2017 2:36 am

Post by Fuzzyzilla » Sun May 14, 2017 4:58 am

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How to setup defines in AVR Studio 6.0

Like basic C language I tried making a define as follows

I get an error which I havn't seen before:

lvalue required as left operand of assignment

What does this mean?

• programming

• 1 \$\begingroup\$ What AVR Compiler are you using? Have you included the specific AVR header files? \$\endgroup\$ –  Dean Jun 18, 2012 at 10:07
• \$\begingroup\$ yeah i'm using AVR studio 6.0, debugger is AVR dragon and the header files I have used are #include <avr/io.h> #include <util/delay.h> #include "iom16.h" \$\endgroup\$ –  David Norman Jun 18, 2012 at 10:12
• \$\begingroup\$ Whats the compiler? Winavr? Some of the headers are suggesting your using code for different compilers. The error is because it can't find the name of either the port or data directory register. This might be of some use \$\endgroup\$ –  Dean Jun 18, 2012 at 10:37

'L-value' and 'R-Value' describe attributes of the Left-hand side and Right-hand side, respectively, of an assignment. An R-value is a quantity of some kind (not necessarily numerical), usually the result of evaluating an expression, but also a constant, such as '5' or the string "Hello, World!\n". An L-value is a something that a program can assign an R-value to, such as a named variable, or a raw memory or port address. This WikiPedia article has a more complete description.

"lvalue required as left operand of assignment" simply means that the left side of your assignment isn't an assignable address or convertible to one. PA0 is #defined as a simple constant without l-value properties.

You're on the right track - here is an Arduino article about ports and the I/O registers associated with them, and direct manipulation of those registers. Despite its caveats, direct port manipulation is a useful technique for reducing both program size and timing-skew in some situations.

These statements:

will both define an l-value you can assign to (but note: it's a whole port, not just a pin) and change its value to turn on pin-0. The RED_ON expression reads: read PORTA, OR that with '1' (so you won't change any other bits in the port); the assignment writes the new value back to the port.

(Do note that your code as written - even assuming the assignment statement was correct - will repetitively turn on the LED, which is no different from turning it on once).

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Understanding lvalues and rvalues in C and C++

The terms lvalue and rvalue are not something one runs into often in C/C++ programming, but when one does, it's usually not immediately clear what they mean. The most common place to run into these terms are in compiler error & warning messages. For example, compiling the following with gcc :

True, this code is somewhat perverse and not something you'd write, but the error message mentions lvalue , which is not a term one usually finds in C/C++ tutorials. Another example is compiling this code with g++ :

Now the error is:

Here again, the error mentions some mysterious rvalue . So what do lvalue and rvalue mean in C and C++? This is what I intend to explore in this article.

A simple definition

This section presents an intentionally simplified definition of lvalues and rvalues . The rest of the article will elaborate on this definition.

An lvalue ( locator value ) represents an object that occupies some identifiable location in memory (i.e. has an address).

rvalues are defined by exclusion, by saying that every expression is either an lvalue or an rvalue . Therefore, from the above definition of lvalue , an rvalue is an expression that does not represent an object occupying some identifiable location in memory.

Basic examples

The terms as defined above may appear vague, which is why it's important to see some simple examples right away.

Let's assume we have an integer variable defined and assigned to:

An assignment expects an lvalue as its left operand, and var is an lvalue, because it is an object with an identifiable memory location. On the other hand, the following are invalid:

Neither the constant 4 , nor the expression var + 1 are lvalues (which makes them rvalues). They're not lvalues because both are temporary results of expressions, which don't have an identifiable memory location (i.e. they can just reside in some temporary register for the duration of the computation). Therefore, assigning to them makes no semantic sense - there's nowhere to assign to.

So it should now be clear what the error message in the first code snippet means. foo returns a temporary value which is an rvalue. Attempting to assign to it is an error, so when seeing foo() = 2; the compiler complains that it expected to see an lvalue on the left-hand-side of the assignment statement.

Not all assignments to results of function calls are invalid, however. For example, C++ references make this possible:

Here foo returns a reference, which is an lvalue , so it can be assigned to. Actually, the ability of C++ to return lvalues from functions is important for implementing some overloaded operators. One common example is overloading the brackets operator [] in classes that implement some kind of lookup access. std::map does this:

The assignment mymap[10] works because the non-const overload of std::map::operator[] returns a reference that can be assigned to.

Modifiable lvalues

Initially when lvalues were defined for C, it literally meant "values suitable for left-hand-side of assignment". Later, however, when ISO C added the const keyword, this definition had to be refined. After all:

So a further refinement had to be added. Not all lvalues can be assigned to. Those that can are called modifiable lvalues . Formally, the C99 standard defines modifiable lvalues as:

[...] an lvalue that does not have array type, does not have an incomplete type, does not have a const-qualified type, and if it is a structure or union, does not have any member (including, recursively, any member or element of all contained aggregates or unions) with a const-qualified type.

Conversions between lvalues and rvalues

Generally speaking, language constructs operating on object values require rvalues as arguments. For example, the binary addition operator '+' takes two rvalues as arguments and returns an rvalue:

As we've seen earlier, a and b are both lvalues. Therefore, in the third line, they undergo an implicit lvalue-to-rvalue conversion . All lvalues that aren't arrays, functions or of incomplete types can be converted thus to rvalues.

What about the other direction? Can rvalues be converted to lvalues? Of course not! This would violate the very nature of an lvalue according to its definition [1] .

This doesn't mean that lvalues can't be produced from rvalues by more explicit means. For example, the unary '*' (dereference) operator takes an rvalue argument but produces an lvalue as a result. Consider this valid code:

Conversely, the unary address-of operator '&' takes an lvalue argument and produces an rvalue:

The ampersand plays another role in C++ - it allows to define reference types. These are called "lvalue references". Non-const lvalue references cannot be assigned rvalues, since that would require an invalid rvalue-to-lvalue conversion:

Constant lvalue references can be assigned rvalues. Since they're constant, the value can't be modified through the reference and hence there's no problem of modifying an rvalue. This makes possible the very common C++ idiom of accepting values by constant references into functions, which avoids unnecessary copying and construction of temporary objects.

CV-qualified rvalues

If we read carefully the portion of the C++ standard discussing lvalue-to-rvalue conversions [2] , we notice it says:

An lvalue (3.10) of a non-function, non-array type T can be converted to an rvalue. [...] If T is a non-class type, the type of the rvalue is the cv-unqualified version of T. Otherwise, the type of the rvalue is T.

What is this "cv-unqualified" thing? CV-qualifier is a term used to describe const and volatile type qualifiers.

From section 3.9.3:

Each type which is a cv-unqualified complete or incomplete object type or is void (3.9) has three corresponding cv-qualified versions of its type: a const-qualified version, a volatile-qualified version, and a const-volatile-qualified version. [...] The cv-qualified or cv-unqualified versions of a type are distinct types; however, they shall have the same representation and alignment requirements (3.9)

But what has this got to do with rvalues? Well, in C, rvalues never have cv-qualified types. Only lvalues do. In C++, on the other hand, class rvalues can have cv-qualified types, but built-in types (like int ) can't. Consider this example:

The second call in main actually calls the foo () const method of A , because the type returned by cbar is const A , which is distinct from A . This is exactly what's meant by the last sentence in the quote mentioned earlier. Note also that the return value from cbar is an rvalue. So this is an example of a cv-qualified rvalue in action.

Rvalue references (C++11)

Rvalue references and the related concept of move semantics is one of the most powerful new features the C++11 standard introduces to the language. A full discussion of the feature is way beyond the scope of this humble article [3] , but I still want to provide a simple example, because I think it's a good place to demonstrate how an understanding of what lvalues and rvalues are aids our ability to reason about non-trivial language concepts.

I've just spent a good part of this article explaining that one of the main differences between lvalues and rvalues is that lvalues can be modified, and rvalues can't. Well, C++11 adds a crucial twist to this distinction, by allowing us to have references to rvalues and thus modify them, in some special circumstances.

As an example, consider a simplistic implementation of a dynamic "integer vector". I'm showing just the relevant methods here:

So, we have the usual constructor, destructor, copy constructor and copy assignment operator [4] defined, all using a logging function to let us know when they're actually called.

Let's run some simple code, which copies the contents of v1 into v2 :

What this prints is:

Makes sense - this faithfully represents what's going on inside operator= . But suppose that we want to assign some rvalue to v2 :

Although here I just assign a freshly constructed vector, it's just a demonstration of a more general case where some temporary rvalue is being built and then assigned to v2 (this can happen for some function returning a vector, for example). What gets printed now is this:

Ouch, this looks like a lot of work. In particular, it has one extra pair of constructor/destructor calls to create and then destroy the temporary object. And this is a shame, because inside the copy assignment operator, another temporary copy is being created and destroyed. That's extra work, for nothing.

Well, no more. C++11 gives us rvalue references with which we can implement "move semantics", and in particular a "move assignment operator" [5] . Let's add another operator= to Intvec :

The && syntax is the new rvalue reference . It does exactly what it sounds it does - gives us a reference to an rvalue, which is going to be destroyed after the call. We can use this fact to just "steal" the internals of the rvalue - it won't need them anyway! This prints:

What happens here is that our new move assignment operator is invoked since an rvalue gets assigned to v2 . The constructor and destructor calls are still needed for the temporary object that's created by Intvec(33) , but another temporary inside the assignment operator is no longer needed. The operator simply switches the rvalue's internal buffer with its own, arranging it so the rvalue's destructor will release our object's own buffer, which is no longer used. Neat.

I'll just mention once again that this example is only the tip of the iceberg on move semantics and rvalue references. As you can probably guess, it's a complex subject with a lot of special cases and gotchas to consider. My point here was to demonstrate a very interesting application of the difference between lvalues and rvalues in C++. The compiler obviously knows when some entity is an rvalue, and can arrange to invoke the correct constructor at compile time.

One can write a lot of C++ code without being concerned with the issue of rvalues vs. lvalues, dismissing them as weird compiler jargon in certain error messages. However, as this article aimed to show, getting a better grasp of this topic can aid in a deeper understanding of certain C++ code constructs, and make parts of the C++ spec and discussions between language experts more intelligible.

Also, in the new C++ spec this topic becomes even more important, because C++11's introduction of rvalue references and move semantics. To really grok this new feature of the language, a solid understanding of what rvalues and lvalues are becomes crucial.

For comments, please send me an email .

Demystifying the "Expression Must Be a Modifiable Lvalue" Error in C++

As C++ developers, we‘ve all likely encountered the dreaded "expression must be a modifiable lvalue" error at some point. This confusing compile-time error can be a roadblock, but if we take the time to truly understand it, it can help us write safer and more robust C++ code overall.

In this comprehensive guide, we‘ll demystify this error completely by looking at what it means, why it happens, how to diagnose it, and best practices to avoid it in the future. Let‘s get started!

What Exactly Does "Expression Must Be a Modifiable Lvalue" Mean in C++?

When the compiler throws this error, it‘s telling us that we tried to assign something to an expression that cannot be assigned to. Let‘s break it down word-by-word:

• Expression: This refers to any valid C++ expression like a variable, arithmetic operation, function call etc. Basically, any combination of literals, variables, operators and functions that evaluates to a value.
• Must be: The expression is expected to have a certain property.
• Modifiable: The property is that the expression can be modified/assigned to.
• Lvalue: An lvalue refers to an expression that represents a specific memory location. Lvalues have identifiable memory addresses that can be accessed and modified.

Put simply, the error occurs because we tried to assign a value to something that cannot be assigned to. The expression on the left hand side of the assignment operator = needs to be a modifiable lvalue.

This is because the assignment operator stores the rvalue (right hand side expression) at the location in memory represented by the lvalue (left hand side). For this modification of memory to work correctly, the lvalue must refer to a valid modifiable location.

Classifying Expressions as Lvalues and Rvalues

To really understand this error, we need to be clear on the difference between lvalues and rvalues in C++.

Lvalues are expressions that represent identifiable memory locations that can be accessed/modified in some way. Some key properties of lvalues:

• Have distinct memory addresses that can be accessed directly.
• Can appear on either side of the assignment operator ( = ).
• Can be used as operands with unary & address-of operator.
• Can be modified, if declared without const .

Some examples of lvalues:

• Named variables like int x;
• Dereferenced pointers like *ptr
• Array elements like myArray[5]
• Member fields like obj.member
• Function calls that return lvalue references like int& func()

Rvalues are expressions that do not represent identifiable memory locations. They cannot be assigned to. Some examples:

• Literals like 5 , 3.14 , ‘a‘ etc.
• Temporary values like those returned by functions
• Operators applied to operands producing temporary values like x + 5
• Functions returning non-reference types like int func()

So in summary, lvalues have identifiable memory locations and can appear on the left side of assignments whereas rvalues cannot.

Common Causes of the "Expression Must Be a Modifiable Lvalue" Error

Understanding lvalues vs rvalues, we can now look at some of the most common scenarios that cause this error:

1. Assigning to a Constant/Read-only Variable

When we declare a variable with const , it becomes read-only and its value cannot be changed. Trying to assign to it results in our error:

Similarly, assigning to a constant literal like a string also causes an error:

2. Assigning Inside a Condition Statement

Condition statements like if and while expect a boolean expression, not an assignment.

Accidentally using = instead of == leads to an assignment inside the condition, causing invalid code:

The condition x = 5 assigns 5 to x, when we meant to check if x == 5.

3. Confusing Precedence of Operators

The assignment operator = has lower precedence than other operators like + , * . So code like this:

Is treated as

This essentially tries to assign 10 to the temporary rvalue x + 5 , causing an error.

4. Assigning to the Wrong Operand in Declarations

When declaring multiple variables in one statement, we must take care to assign to the correct identifier on the left side:

This assigns 10 to y instead of x due to ordering.

5. Overloaded Assignment Operator Issues

Assignment operators can be overloaded in classes. If the overloaded assignment operator is not implemented correctly, it can lead to this error.

For example, overloaded assignment should return a modifiable lvalue but sometimes programmers mistakenly return a temporary rvalue instead.

6. Assigning to Temporary Rvalues

Temporary rvalues that get created in expressions cannot be assigned to:

Some other examples are trying to assign to literals directly or dereferencing an incorrect pointer location.

7. Assigning to Array Elements Incorrectly

Only modifiable lvalues pointing to valid memory can be assigned to.

Trying to assign to an out of bounds array index is invalid:

Diagnosing the Error in Your Code

Now that we‘ve seen common causes of the error, let‘s look at how to diagnose it when it shows up in our code:

• Examine the full error message – it will indicate the exact expression that is invalid.
• Ensure the expression can be assigned to based on its type – is it a modifiable lvalue?
• Double check precedences of operators on the line causing issues. Add parentheses if needed to force intended precedence.
• If assigning to a user-defined type, check if overloaded assignment operator is correct.
• For conditionals and loops, verify == and = are not mixed up.
• Make sure all variables being assigned to are valid, declared identifiers.
• If assigning to an array element, index boundaries must be correct.
• Look nearby for typos like swapping . and -> when accessing members.
• Enable all compiler warnings and pedantic errors to catch related issues.

With some careful analysis of the code and error messages, identifying the root cause becomes much easier.

Resolving the Error Through Correct Modifiable Lvalues

Once we diagnose the issue, it can be resolved by using a valid modifiable lvalue on the left hand side of the assignment:

• Declare normal variables without const that can be assigned to
• Use dereferenced pointers like *ptr = 5 instead of direct literals
• Call functions that return non-const lvalue references like int& func()
• Use array element indices that are in bounds
• Access member fields of objects correctly like obj.member not obj->member
• Store rvalues in temporary variables first before assigning
• Ensure operator precedence resolves properly by adding parentheses
• Return proper lvalue references from overloaded assignment operators

Let‘s see a couple of examples fixing code with this error:

1. Fixing Const Assignment

2. Fixing Conditional Assignment

3. Fixing Invalid Array Index

By properly understanding lvalues vs rvalues in C++, we can find and fix the root causes of this error.

Best Practices to Avoid "Expression Must Be a Modifiable Lvalue" Errors

Learning from the examples and causes above, we can establish coding best practices that help avoid these kinds of errors:

• Clearly understand difference between lvalues and rvalues before writing complex code.
• Enable all compiler warnings to catch errors early. Pay attention to warnings!
• Be very careful when overloading assignment operators in classes.
• Do not combine multiple assignments in one statement. Break them up over separate lines.
• Use const judiciously on values that do not need modification.
• Use static_assert to validate assumptions about types and constants.
• Use == for comparisons, = only for assignment. Avoid mixing them up.
• Be careful with operator precedence – add parentheses when unsure.
• Initialize variables properly when declared and avoid unintended fallthrough assignments.
• Use descriptive variable names and formatting for readability.
• Validate indices before accessing elements of arrays and vectors.
• Handle returned temporaries from functions carefully before assigning.
• Run regular static analysis on code to detect issues early.

Adopting these best practices proactively will help us write code that avoids lvalue errors.

Summary and Key Lessons

The cryptic "expression must be a modifiable lvalue" error in C++ is trying to tell us that we improperly tried assigning to something that cannot be assigned to in the language.

By learning lvalue/rvalue concepts deeply and following best practices around assignments, we can eliminate these kinds of bugs in our code.

Some key lessons:

• Lvalues represent identifiable memory locations that can be modified. Rvalues do not.
• Use non-const variables and valid references/pointers as left operands in assignments.
• Be extremely careful inside conditionals, loops and declarations.
• Understand and properly handle operator precedence and overloadable operators.
• Enable all compiler warnings to catch issues early.
• Adopt defensive programming practices with constants, arrays, temporaries etc.

Persistently applying these lessons will ensure we write assignment-bug-free C++ code!

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lvalue required as left operand of assignment PLEASE HELP ME!

Please help me AS SOON AS POSSIBLE! I have a projekt and i have the message: lvalue required as left operand of assignment

More informations: Arduino: 1.6.7 (Windows 8.1), Board: "Arduino/Genuino Mega or Mega 2560, ATmega2560 (Mega 2560)"

In function 'void setup()':

Vorw_rtsfahrwarner:17: error: lvalue required as left operand of assignment

pinMode(21, 20, 19, 18, 17, 15=OUTPUT);

Vorw_rtsfahrwarner:18: error: lvalue required as left operand of assignment

digitalWrite(15, 17, 18, 19, 20, 21=HIGH);

C:\Users\Stephan\Desktop\Vorw_rtsfahrwarner\Vorw_rtsfahrwarner.ino: In function 'void loop()':

Vorw_rtsfahrwarner:25: error: lvalue required as left operand of assignment

Vorw_rtsfahrwarner:57: error: lvalue required as left operand of assignment

digitalWrite(21=LOW);

Vorw_rtsfahrwarner:58: error: lvalue required as left operand of assignment

digitalWrite(20=HIGH);

Vorw_rtsfahrwarner:59: error: lvalue required as left operand of assignment

digitalWrite(19=LOW);

Vorw_rtsfahrwarner:60: error: lvalue required as left operand of assignment

digitalWrite(18=LOW);

Vorw_rtsfahrwarner:61: error: lvalue required as left operand of assignment

digitalWrite(17=LOW);

Vorw_rtsfahrwarner:62: error: lvalue required as left operand of assignment

digitalWrite(15=HIGH);

Vorw_rtsfahrwarner:66: error: lvalue required as left operand of assignment

Vorw_rtsfahrwarner:67: error: lvalue required as left operand of assignment

digitalWrite(20=LOW);

Vorw_rtsfahrwarner:68: error: lvalue required as left operand of assignment

Vorw_rtsfahrwarner:69: error: lvalue required as left operand of assignment

Vorw_rtsfahrwarner:70: error: lvalue required as left operand of assignment

digitalWrite(17=HIGH);

Vorw_rtsfahrwarner:71: error: lvalue required as left operand of assignment

digitalWrite(15=LOW);

Vorw_rtsfahrwarner:75: error: lvalue required as left operand of assignment

Vorw_rtsfahrwarner:76: error: lvalue required as left operand of assignment

Vorw_rtsfahrwarner:77: error: lvalue required as left operand of assignment

Vorw_rtsfahrwarner:78: error: lvalue required as left operand of assignment

digitalWrite(18=HIGH);

Vorw_rtsfahrwarner:79: error: lvalue required as left operand of assignment

Vorw_rtsfahrwarner:80: error: lvalue required as left operand of assignment

Vorw_rtsfahrwarner:84: error: lvalue required as left operand of assignment

Vorw_rtsfahrwarner:85: error: lvalue required as left operand of assignment

Vorw_rtsfahrwarner:86: error: lvalue required as left operand of assignment

digitalWrite(19=HIGH);

Vorw_rtsfahrwarner:87: error: lvalue required as left operand of assignment

Vorw_rtsfahrwarner:88: error: lvalue required as left operand of assignment

Vorw_rtsfahrwarner:89: error: lvalue required as left operand of assignment

Vorw_rtsfahrwarner:93: error: lvalue required as left operand of assignment

Vorw_rtsfahrwarner:94: error: lvalue required as left operand of assignment

Vorw_rtsfahrwarner:95: error: lvalue required as left operand of assignment

Vorw_rtsfahrwarner:96: error: lvalue required as left operand of assignment

Vorw_rtsfahrwarner:97: error: lvalue required as left operand of assignment

Vorw_rtsfahrwarner:98: error: lvalue required as left operand of assignment

Vorw_rtsfahrwarner:102: error: lvalue required as left operand of assignment

digitalWrite(21=HIGH);

Vorw_rtsfahrwarner:103: error: lvalue required as left operand of assignment

Vorw_rtsfahrwarner:104: error: lvalue required as left operand of assignment

Vorw_rtsfahrwarner:105: error: lvalue required as left operand of assignment

Vorw_rtsfahrwarner:106: error: lvalue required as left operand of assignment

Vorw_rtsfahrwarner:107: error: lvalue required as left operand of assignment

C:\Users\Stephan\Desktop\Vorw_rtsfahrwarner\Vorw_rtsfahrwarner.ino: At global scope:

Vorw_rtsfahrwarner:111: error: expected unqualified-id before '{' token

Vorw_rtsfahrwarner:117: error: expected declaration before '}' token

exit status 1 lvalue required as left operand of assignment

The code of my projekt:

#include <LiquidCrystal.h> int trigger=7; int echo=6; long dauer=0; LiquidCrystal lcd(12, 11, 5, 4, 3, 2);

long entfernung=0;

void setup() { Serial.begin (9600);

pinMode(trigger, OUTPUT); pinMode(echo, INPUT); pinMode(10, OUTPUT); lcd.begin(16, 2); pinMode(21, 20, 19, 18, 17, 15=OUTPUT); digitalWrite(15, 17, 18, 19, 20, 21=HIGH); }

void loop() { { delay(2000); digitalWrite(15, 17, 18, 19, 20, 21=HIGH); } digitalWrite(trigger, LOW);

delay(5); digitalWrite(trigger, HIGH); delay(10); digitalWrite(trigger, LOW); dauer = pulseIn(echo, HIGH);

entfernung = (dauer/2) / 29.1;

if (entfernung >= 500 || entfernung <= 0) { Serial.println("Kein Messwert"); } else { Serial.print(entfernung-1); Serial.println(" cm"); lcd.setCursor(0, 0);

lcd.print("Abstand[+/- 1cm]:");

lcd.setCursor(0, 1);

lcd.print(entfernung-1); lcd.print(" cm"); } { if (entfernung=2) { digitalWrite(21=LOW); digitalWrite(20=HIGH); digitalWrite(19=LOW); digitalWrite(18=LOW); digitalWrite(17=LOW); digitalWrite(15=HIGH); } if (entfernung=5) { digitalWrite(21=LOW); digitalWrite(20=LOW); digitalWrite(19=LOW); digitalWrite(18=LOW); digitalWrite(17=HIGH); digitalWrite(15=LOW); } if (entfernung=8) { digitalWrite(21=LOW); digitalWrite(20=LOW); digitalWrite(19=LOW); digitalWrite(18=HIGH); digitalWrite(17=LOW); digitalWrite(15=LOW); } if (entfernung=10) { digitalWrite(21=LOW); digitalWrite(20=LOW); digitalWrite(19=HIGH); digitalWrite(18=LOW); digitalWrite(17=LOW); digitalWrite(15=LOW); } if (entfernung = 12) { digitalWrite(21=LOW); digitalWrite(20=HIGH); digitalWrite(19=LOW); digitalWrite(18=LOW); digitalWrite(17=LOW); digitalWrite(15=LOW); } if (entfernung>12) { digitalWrite(21=HIGH); digitalWrite(20=HIGH); digitalWrite(19=LOW); digitalWrite(18=LOW); digitalWrite(17=LOW); digitalWrite(15=LOW); } } } { delay(1000); lcd.setCursor(0, 1); lcd.print(" "); digitalWrite(anzeige=LOW); } }

Please help me AS SOON AS POSSIBLE!

Vorw_rtsfahrwarner.ino (2.04 KB)

Have a read of the reference pages for the functions that are causing you the errors and see if you can spot what you're doing wrong:

. . . and when you've read that, please read the posting guidelines at the top of this section of the forum.

PS you may also want to review some of the comparisons in your if() statements.

I don't know what language that is, but it is not C/C++.

Look at the examples and copy that syntax.

Muss die Hausaufgabe morgen abgegeben werden?

Is the assignment due tomorrow?

Related Topics

【C言語】lvalue required as left operand of assignment

“lvalue required as left operand of assignment” というエラーメッセージは、C 言語の代入演算子（=）が左辺値でない式に対して使用された場合に発生します。

例えば、次のようなコードを見てみましょう。

このコードでは、最初の x = 456 の行では x という変数に = 演算子が適用されています。これは、= 演算子が適用できる左辺値であるため、問題ありません。

しかし、2番目の 123 = x の行では 123 というリテラルに = 演算子が適用されています。これは、= 演算子が適用できない左辺値であるため、このコードはエラーになります。

• Programming

• Rahul Awati

What is an operator in mathematics and programming?

In mathematics and computer programming , an operator is a character that represents a specific mathematical or logical action or process. For instance, "x" is an arithmetic operator that indicates multiplication, while "&&" is a logical operator representing the logical AND function in programming.

Depending on its type, an operator manipulates an arithmetic or logical value, or operand, in a specific way to generate a specific result. From handling simple arithmetic functions to facilitating the execution of complex algorithms, like security encryption , operators play an important role in the programming world.

Mathematical and logical operators should not be confused with a system operator , or sysop, which refers to a person operating a server or the hardware and software in a computing system or network.

Operators and logic gates

In computer programs, Boolean operators are among the most familiar and commonly used sets of operators. These operators work only with true or false values and include the following:

These operators and variations, such as XOR, are used in logic gates .

Boolean operators can also be used in online search engines , like Google. For example, a user can enter a phrase like "Galileo AND satellite" -- some search engines require the operator be capitalized in order to generate results that provide combined information about both Galileo and satellite.

Types of operators

There are many types of operators used in computing systems and in different programming languages. Based on their function, they can be categorized in six primary ways.

1. Arithmetic operators

Arithmetic operators are used for mathematical calculations. These operators take numerical values as operands and return a single unique numerical value, meaning there can only be one correct answer.

The standard arithmetic operators and their symbols are given below.

2. Relational operators

Relational operators are widely used for comparison operators. They enter the picture when certain conditions must be satisfied to return either a true or false value based on the comparison. That's why these operators are also known as conditional operators.

The standard relational operators and their symbols are given below.

3. Bitwise operators

Bitwise operators are used to manipulate bits and perform bit-level operations . These operators convert integers into binary before performing the required operation and then showing the decimal result.

The standard bitwise operators and their symbols are given below.

4. Logical operators

Logical operators play a key role in programming because they enable a system or program to take specific decisions depending on the specific underlying conditions. These operators take Boolean values as input and return the same as output.

The standard logical operators and their symbols are given below.

5. Assignment operators

Assignment operators are used to assign values to variables. The left operand is a variable, and the right is a value -- for example, x=3.

The data types of the variable and the value must match; otherwise, the program compiler raises an error, and the operation fails.

The standard assignment operators and their symbols are given below.

6. Increment/decrement operators

The increment/decrement operators are unary operators, meaning they require only one operand and perform an operation on that operand. They sometimes are called monadic operators .

The standard increment/decrement operators and their symbols are given below.

See also: proximity operator , search string , logical negation symbol , character and mathematical symbols .

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lvalue required as left operand of assignment 2

I'm new to c++ and don't know how to fix it, can someone help me?

• syntax-error

• 1 I know there is an answer to this question. But could you explain that you want x+1 && y=y-60; to do. Even x+1 && (y=y-60); as in the answer is an unusual expression. –  Richard Critten Dec 1, 2022 at 18:25
• Why would anyone want to write obscure code like this? Just break it up into multiple expressions: if ((m1+m2)>59) { x+=1; y-=60; } if ((s1+s2)>59) { y+=1; z-=60; } –  Remy Lebeau Dec 1, 2022 at 18:29
• i need to calculate degrees, minutes and seconds of an angle, and if there are more than 60 minutes this should add to degrees 1 and decrease value of minutes by 60 –  zxcmaxik Dec 1, 2022 at 18:29
• x + 1 and y + 1 are temporaries which are not assigned to anything in your scenario. did you mean x += 1 / y += 1 instead? –  The Dreams Wind Dec 1, 2022 at 18:31
• @zxcmaxik Then what you want is { ++x; y -= 60; } and similar for the other if test. –  Richard Critten Dec 1, 2022 at 18:32

The problem is that assignment operator = has the lowest precedence in your expressions:

if(m1+m2>59) x+1 && y=y-60; if(s1+s2>59) y+1 && z=z-60;

Thus, the compiler sees the expressions like this:

And the result of (x + 1 && y) and (y + 1 && z) cannot be assigned, because it's an rvalue.

Instead you probably want the assignment to take place prior to evaluating result of && :

• OP has clarified what the expression should be doing. You might want to update the answer. –  Richard Critten Dec 1, 2022 at 18:45
• @RichardCritten in order to meet the requirements OP provided, I'll have to rewrite this expression completely, as it's neither correct to increment x / y by one, nor decrement y / z by 60. The input can be any value, not limited to range of 60 seconds/minutes. Thus the answer then will be irrelevant to original question. As you suggested, OP may want to ask another question to sort this part properly –  The Dreams Wind Dec 1, 2022 at 18:56

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Expressions and operators

This chapter documents all the JavaScript language operators, expressions and keywords.

Expressions and operators by category

For an alphabetical listing see the sidebar on the left.

Primary expressions

Basic keywords and general expressions in JavaScript. These expressions have the highest precedence (higher than operators ).

The this keyword refers to a special property of an execution context.

Basic null , boolean, number, and string literals.

Array initializer/literal syntax.

Object initializer/literal syntax.

The function keyword defines a function expression.

The class keyword defines a class expression.

The function* keyword defines a generator function expression.

The async function defines an async function expression.

The async function* keywords define an async generator function expression.

Regular expression literal syntax.

Template literal syntax.

Grouping operator.

Left-hand-side expressions

Left values are the destination of an assignment.

Member operators provide access to a property or method of an object ( object.property and object["property"] ).

The optional chaining operator returns undefined instead of causing an error if a reference is nullish ( null or undefined ).

The new operator creates an instance of a constructor.

In constructors, new.target refers to the constructor that was invoked by new .

An object exposing context-specific metadata to a JavaScript module.

The super keyword calls the parent constructor or allows accessing properties of the parent object.

The import() syntax allows loading a module asynchronously and dynamically into a potentially non-module environment.

Increment and decrement

Postfix/prefix increment and postfix/prefix decrement operators.

Postfix increment operator.

Postfix decrement operator.

Prefix increment operator.

Prefix decrement operator.

Unary operators

A unary operation is an operation with only one operand.

The delete operator deletes a property from an object.

The void operator evaluates an expression and discards its return value.

The typeof operator determines the type of a given object.

The unary plus operator converts its operand to Number type.

The unary negation operator converts its operand to Number type and then negates it.

Bitwise NOT operator.

Logical NOT operator.

Pause and resume an async function and wait for the promise's fulfillment/rejection.

Arithmetic operators

Arithmetic operators take numerical values (either literals or variables) as their operands and return a single numerical value.

Exponentiation operator.

Multiplication operator.

Division operator.

Remainder operator.

Addition operator.

Subtraction operator.

Relational operators

A comparison operator compares its operands and returns a boolean value based on whether the comparison is true.

Less than operator.

Greater than operator.

Less than or equal operator.

Greater than or equal operator.

The instanceof operator determines whether an object is an instance of another object.

The in operator determines whether an object has a given property.

Note: => is not an operator, but the notation for Arrow functions .

Equality operators

The result of evaluating an equality operator is always of type boolean based on whether the comparison is true.

Equality operator.

Inequality operator.

Strict equality operator.

Strict inequality operator.

Bitwise shift operators

Operations to shift all bits of the operand.

Bitwise left shift operator.

Bitwise right shift operator.

Bitwise unsigned right shift operator.

Binary bitwise operators

Bitwise operators treat their operands as a set of 32 bits (zeros and ones) and return standard JavaScript numerical values.

Bitwise AND.

Bitwise OR.

Bitwise XOR.

Binary logical operators

Logical operators implement boolean (logical) values and have short-circuiting behavior.

Logical AND.

Logical OR.

Nullish Coalescing Operator.

Conditional (ternary) operator

The conditional operator returns one of two values based on the logical value of the condition.

Assignment operators

An assignment operator assigns a value to its left operand based on the value of its right operand.

Assignment operator.

Multiplication assignment.

Division assignment.

Remainder assignment.

Addition assignment.

Subtraction assignment

Left shift assignment.

Right shift assignment.

Unsigned right shift assignment.

Bitwise AND assignment.

Bitwise XOR assignment.

Bitwise OR assignment.

Exponentiation assignment.

Logical AND assignment.

Logical OR assignment.

Nullish coalescing assignment.

Destructuring assignment allows you to assign the properties of an array or object to variables using syntax that looks similar to array or object literals.

Yield operators

Pause and resume a generator function.

Delegate to another generator function or iterable object.

Spread syntax

Spread syntax allows an iterable, such as an array or string, to be expanded in places where zero or more arguments (for function calls) or elements (for array literals) are expected. In an object literal, the spread syntax enumerates the properties of an object and adds the key-value pairs to the object being created.

Comma operator

The comma operator allows multiple expressions to be evaluated in a single statement and returns the result of the last expression.

Specifications

Browser compatibility

BCD tables only load in the browser with JavaScript enabled. Enable JavaScript to view data.

• Operator precedence