How to treat negative indices in C? - c

Question

I have been trying to use all possible combinations but I always get segmentation fault
my first guess was to use int *ary = &storage[0] but it doesn't work :( can someone explain me what it is that I'm doing wrong?(Im beginner in C)
thank you!
This is the problem
#include <stdio.h>
#define MINN -50
#define MAXN 50
int main() {
int storage[MAXN - MINN + 1] = {0};
int *ary = &storage[0];
for (int i = MINN; i <= MAXN; i++)
ary[i] = i;
for (int i = MINN; i <= MAXN; i++)
printf("%d ", ary[i]);
The output should be "-50, -49,-48 ... 49, 50"
I have to be doing something wrong here
int *ary = &storage[0]

Answer 1

Arrays always start with index 0.
You have to use an offset to ensure accessing the array accordingly.
Your MINN is useable for that.
Though I recommend to define the macro constants in a more paranoid way, because macro expansion can do weird things with operators.
#include <stdio.h>
#define MINN (-50)
#define MAXN (50)
int main(void) {
int storage[MAXN - MINN + 1] = {0};
int *ary = &storage[0];
for (int i = MINN; i <= MAXN; i++)
ary[i-MINN] = i;
for (int i = MINN; i <= MAXN; i++)
printf("%d ", ary[i-MINN]);
}
Output (from https://www.tutorialspoint.com/compile_c_online.php):
-50 -49 -48 -47 -46 -45 -44 -43 -42 -41 -40 -39 -38 -37 -36 -35 -34 -33 -32 -31 -30 -29 -28 -27 -26 -25 -24 -23 -22 -21 -20 -19 -18 -17 -16 -15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

Answer 2

Don't mix up array sizes and the values you store in them. In this case you wish to store 50 + 50 + 1 numbers in an array. So make a separate constant for its size, for example:
#define ARR_SIZE (-MINN + MAXN + 1)
int storage [ARR_SIZE] = ...;
Now we don't have to worry about what values those other constants have nor do any arithmetic involving them. Simply use ARR_SIZE. Same thing in the loop, don't mix up array indices with array values:
int val = MINN;
for(size_t i=0; i<ARR_SIZE; i++)
{
arr[i] = val;
val++;
}
Overall keep it simple and there will be no bugs.

Answer 3

With no defines (but malloc), and no parallel counter (but a formula) it could be:
#include <stdio.h>
#include <stdlib.h>
const int AMPL = 50;
/* Store and print numbers from -AMPL to AMPL */
void main(void) {
extern const int AMPL; // not needed
const int SZ = 2*AMPL + 1;
int *ary = malloc(SZ * sizeof *ary);
for (int i = 0; i < SZ; i++) {
ary[i] = i - AMPL;
printf("%d ", ary[i]);
}
}
This is only for symmetric series of course. What exactly are you trying?