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lvalue required as left operand of assignment

Why am I getting

lvalue required as left operand of assignment


with a single string comparison? How can I fix this in C?

C

if (strcmp("hello", "hello") = 0)


Thanks!

6 Answers
6

You need to compare, not assign:

if (strcmp("hello", "hello") == 0)
^


Because you want to check if the result of strcmp("hello", "hello") equals to 0.

strcmp("hello", "hello")

0

About the error:

lvalue required as left operand of
assignment

lvalue means an assignable value (variable), and in assignment the left value to the = has to be lvalue (pretty clear).

lvalue

=

lvalue

Both function results and constants are not assignable (rvalues), so they are rvalues. so the order doesn't matter and if you forget to use == you will get this error. (edit:)I consider it a good practice in comparison to put the constant in the left side, so if you write = instead of ==, you will get a compilation error. for example:

rvalue

rvalue

==

=

==

int a = 5;
if (a = 0) // Always evaluated as false, no error.

//...



vs.

int a = 5;
if (0 = a) // Generates compilation error, you cannot assign a to 0 (rvalue)

//...



(see first answer to this question: https://stackoverflow.com/questions/2349378/new-programming-jargon-you-coined)

=

==

=

You cannot assign an rvalue to an rvalue.

if (strcmp("hello", "hello") = 0)


is wrong. Suggestions:

if (strcmp("hello", "hello") == 0)
^


= is the assign operator.
== is the equal to operator.

I know many new programmers are confused with this fact.

=

==

Change = to ==
i.e
if (strcmp("hello", "hello") == 0)

=

==

if (strcmp("hello", "hello") == 0)

You want to compare the result of strcmp() to 0. So you need ==. Assigning it to 0 won't work because rvalues cannot be assigned to.

strcmp()

==

You are trying to assign a value to a function, which is not possible in C. Try the comparison operator instead:

if (strcmp("hello", "hello") == 0)


I found that an answer to this issue when dealing with math is that the operator on the left hand side must be the variable you are trying to change. The logic cannot come first.

coin1 + coin2 + coin3 = coinTotal; // Wrong

coinTotal = coin1 + coin2 + coin3; // Right


This isn't a direct answer to your question but it might be helpful to future people who google the same thing I googled.

if (strcmp("hello", "hello") = 0)


Is trying to assign 0 to function return value which isn't lvalue.

Function return values are not lvalue (no storage for it), so any attempt to assign value to something that is not lvalue result in error.

Best practice to avoid such mistakes in if conditions is to use constant value on left side of comparison, so even if you use "=" instead "==", constant being not lvalue will immediately give error and avoid accidental value assignment and causing false positive if condition.

Thank you for your interest in this question.
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