Int variable subtraction affecting for loop index - c

Question

I am using a for loop to walk through the elements of char array teste. Within this loop, I am using an int variable called stack_index, for some controls. The program below runs fine, see that, its result is the value of stack_index repeated 18 times (size of char array teste).
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
size_t char_size (char* c) {
return strlen(c);
}
int main() {
char teste[] = "Este *teste* string";
size_t n = char_size(teste);
char stack[20];
int stack_index = 0;
for(int i; i < n; i++){
printf("%d", stack_index);
}
}
Result of program above:
0000000000000000000
But, when I add just the line stack_index-- inside the loop, the loop index breaks in some crazy way. See that the result of this program is the values -1, -2 and -3. This result is indicating that the for loop is running only 3 times (instead of 18). But why this is happening? Why subtracting (or adding) 1 from the stack_index variable at each iteration, is affecting the loop index variable i? This makes no sense to me, since the i variable is the index variable of the loop, not stack_index.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
size_t char_size (char* c) {
return strlen(c);
}
int main() {
char teste[] = "Este *teste* string";
size_t n = char_size(teste);
char stack[20];
int stack_index = 0;
for(int i; i < n; i++){
stack_index--;
printf("%d", stack_index);
}
}
Result of program above:
-1-2-3

Answer 1

#Some programmer dude comment answer the problem. I did not initialize the i variable with some initial value (like 0). This mistake was breaking the loop, not the stack_index-- line.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
size_t char_size (char* c) {
return strlen(c);
}
int main() {
char teste[] = "Este *teste* string";
size_t n = char_size(teste);
char stack[20];
int stack_index = 0;
for(int i = 0; i < n; i++){
stack_index--;
printf("%d", stack_index);
}
}
Result of program above:
-1-2-3-4-5-6-7-8-9-10-11-12-13-14-15-16-17-18-19