C Programming - Integer value within struct becomes "random" after assignment [duplicate] - c

Question

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How do I get a function to modify any of the parameters?
(1 answer)
Closed 4 years ago.
I've searched for a fix to this issue but haven't been able to find an explanation. I have a two dimensional struct which has an integer variable within it.
typedef struct
{
int example;
} Example;
typedef struct
{
Example two_dimensional_array[5][5];
} Example_Outer;
I then set this variable to 0 for all fields using the following function and print the current values.
void initialise(Example_Outer example)
{
for(int i = 0; i < 5; i++){
for(int j = 0; j < 5; j++){
example.two_dimensional_array[i][j].example = 0;
}
}
print_example(example);
}
During this print, the values all appear as 0's like they should.
Output:
0, 0, 0, 0, 0,
0, 0, 0, 0, 0,
0, 0, 0, 0, 0,
0, 0, 0, 0, 0,
0, 0, 0, 0, 0,
I then run a new function that utilises the exact same print code, and receive the following output:
0, 0, 0, 0, 9,
0, -394918304, 32551, -2138948520, 32764,
1, 0, 1, 0, 1775692253,
21904, -394860128, 32551, 0, 0,
1775692176, 21904, 1775691312, 21904, -2138948320,
print method:
void print_example(Example_Outer example)
{
for(int i = 0; i < 5; i++){
for(int j = 0; j < 5; j++){
printf("%d, ", example.two_dimensional_array[i][j].example);
}
printf("\n");
}
}
main method:
int main( int argc, const char* argv[] )
{
Example_Outer example;
initialise(example);
printf("---------------\n");
print_example(example);
}
Why do the variables not remain set to 0? What is causing this and how can I fix it? Thank you!

Answer 1

First, you can initialize your struct like bellow in a simple way :
Example_Outer example = { 0 };
Or in second way :
typedef struct
{
Example two_dimensional_array[5][5] = { 0 };
} Example_Outer;
Now in your code , you forgot about * in your void initialise(Example_Outer example) function, and in this case you just passing a copy of struct in function.
So you should use of struct's address as parameter to the function with pointer (*) :
void initialise(Example_Outer *example)
{
for (int i = 0; i < 5; i++) {
for (int j = 0; j < 5; j++) {
example->two_dimensional_array[i][j].example = 0;
}
}
print_example(*example);
}
Finally you can pass your struct's address as following : ( Test it online ) :
int main(int argc, const char* argv[])
{
Example_Outer example;
initialise(&example);
printf("---------------\n");
print_example(example);
}