lvalue required as left operand of assignment

问题:

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!

回答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)

回答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.

回答3:

Change = to == i.e 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.

回答4:

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)

回答5:

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.

回答6:

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.