Hello,
I am a bit confused as to why this code compiles, but is not working, and how to fix it. Can you help? Thx
Code:#include <stdlib.h> typedef struct { int x; } Stype; void Fun(Stype a) { a.x = a.x + 1; } int main() { Stype sexample; sexample.x = 0; Stype arr[10]; arr[0] = sexample; Fun(arr[0]); printf("%d", arr[0].x); // why not 1? return 0; }
When you pass arr[0] to Fun, you pass it by copy. So, the change you make to the copy is not reflected back in main. If you want it the change to be reflected back in main, pass your value by reference (or maybe pointer).
If you want to see 1 as the output, you should do something like this:
Code:void Fun(Stype& a) { a.x++; } int main() { ..... Fun(arr[0]); ..... } // or maybe, void Fun(Stype* a) { *a.x++; } int main() { ..... Fun(&arr[0]); ..... }
Last edited by Zeus_; 03-27-2020 at 04:03 PM.
"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning." - Rick Cook, The Wizardry Compiled
But I thought I was passing an element of the array, not the whole array?
Are you sure your examples compile? This should work
Code:#include <stdlib.h> typedef struct { int x; } Stype; void Fun(Stype *a) { a->x = a->x + 1; } int main() { Stype sexample; sexample.x = 0; Stype arr[10]; arr[0] = sexample; Fun(&arr[0]); printf("%d", arr[0].x); // why not 1? return 0; }
No, it doesn't compile. Zeus is confused between C++ and C.Originally Posted by serge
Your code is correct, although using ++ is more usual.
Passing a pointer to the first element is essentially passing the whole array since in your function you could increment a to access the next element.But I thought I was passing an element of the array, not the whole array
A little inaccuracy saves tons of explanation. - H.H. Munro
My bad. I should have written (*a).x++
++ has higher precedence than dereferencing a pointer. I was busy with something else at the time you posted and I happened to write wrong code. SorryNonetheless, the reference version works right.
"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning." - Rick Cook, The Wizardry Compiled
Thx a lot. I am doing a pure c project atm, and it happens to me all the time.
