Objective-C: Exclamation Point strcmp in "if" statement

Question

I am looking over some OpenGL ES code to multiplay matrices, but I'm not sure about how this if statement works:

for (int i = 0; i <_uniformArraySize; i++) {
    **if (!strcmp(_uniformArray[i].Name, "ModelViewProjectionMatrix")) {**

        GLKMatrix4 modelViewProjectionMatrix = GLKMatrix4Multiply(_projectionMatrix, _modelViewMatrix);
glUniformMatrix4fv(_uniformArray[i].Location, 1, GL_FALSE, modelViewProjectionMatrix.m);
    }
}

Does !strcmp mean that the strings are equal or not equal? I looked at the strcmp documentation and it returns numbers. So how does this exclamation point in an if statement affect a number (being the return value of strcmp)?

Thanks

score 3 · Accepted Answer · answered Jul 15 '13 at 16:53

Since Objective C, like C, allows integers in conditionals, using !expr is a common shorthand for expr== 0.

Your statement is equivalent to

if (strcmp(_uniformArray[i].Name, "ModelViewProjectionMatrix") == 0) {
    ...
}

Since strcmp returns zero when strings are equal to each other, the condition checks if the content of two C strings is the same.

score 0 · Answer 2 · answered Jul 15 '13 at 16:53

strcmp() function returns the total number of differences found between each string, so a zero means there are no differences. It is kind of un-intuitive when you think of it in terms of strcmp true/false.

The exclamation mark is the 'NOT' operator. This means that it will test whether the value in front of it is NOT true, so the result is essentially a reversal of the original boolean value.

In this case, if !strcmp() means if the result of strcmp is NOT > 0 then the result is true.

score 0 · Answer 3 · answered Jul 15 '13 at 16:57

0

strcmp will return zero when strings are equal. The exclamation point is the negation operator so the program will enter the if statement when strings are equal.

answered Jul 15 '13 at 16:57

dbv

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Objective-C: Exclamation Point strcmp in "if" statement

3 Answers3