Question

Here is my code:

void doubleValuesInArray(int *pointer) {
    for (int n = 0; n < 2; n++) {
        int a = (*pointer+n);
        a = a * 2;
        *pointer+n = a;
    }
}

int main(int argc, char** argv) {
    int myArray[] = {1,2};
    doubleValuesInArray(myArray);
    cout<<myArray[0]<<endl;
    return 0;
}

And the output is:

main.cpp: In function `void doubleValuesInArray(int*)’:
main.cpp:19: error: non-lvalue in assignment
make[2]: * [build/Debug/Cygwin-Windows/main.o] Error 1
make[1]: [.build-conf] Error 2
make: ** [.build-impl] Error 2

My question is:

int a = (*pointer+n);

works just fine. Variable ‘a’ gets the values 0 and 1 from the array by using *pointer + n.

However

 *pointer+n = a; 

does not seem to work.

If I use
pointer[n] = a;
it works as well.

Why doesn’t my first approach work?

Answer 1

You did not parenthesize your pointer expressions correctly: the * has higher priority, so you should add parentheses around the addition:

int a = *(pointer+n);
a = a * 2;
*(pointer+n) = a;