lvalue required as left operand of assignment

Question

Why am I getting

lvalue required as left operand of assignment 

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

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

Thanks!

Answer 1

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.

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).

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:

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)

Answer 2

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.