Incorrect Difference

I'm trying to make a simple calculator program....but when I start to subtract numbers..it gives my incorrect answer....

Code:

---

#include <stdio.h>
int main()
{
        int a[2],i, diff=0;
       
        printf("\n\tSubtract Numbers\n\n");
        for(i=0; i<2; i++)
        {
                printf("Enter a number: ");
                scanf("%d", &a[i]);
                diff-=a[i];
        }
        printf("\n\tThe difference is %d\n", diff);
}

---

Output:

Quote:

---

Enter a number: 13
Enter a number: 15
The difference is -28

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It should be -2 right??...because 13-15=-2

No, the code you have now is equivalent to doing this:
diff = diff - a[0] - a[1];

Since you want to subtract one number from the other, I recommend deleting line 11, and writing this after line 12 but before printf():
diff = a[0] - a[1];

Quote:

---

Originally Posted by whiteflags

No, the code you have now is equivalent to doing this:
diff = diff - a[0] - a[1];

Since you want to subtract one number from the other, I recommend deleting line 11, and writing this after line 12 but before printf():
diff = a[0] - a[1];

---

what if the number of elements is also determined by the user?..

Like this:

Code:

---

    int a[2],i, diff=0, n;
   
    printf("Enter number of elements: ");
    scanf("%d", &n);
    for(i=0; i<n; i++)
    {
        printf("Enter a number: ");
        scanf("%d", &a[i]);
    }

---

I get what you're going for. The actual solution might be odd, because you need to start with at least two operands.

Maybe something like this. (It's untested, turn in/copy at your own risk)

Code:

---

char op = '\0';
double a[2] = {0.0, 0.0};
scanf("%lf%c%lf", &a[0], &op, &a[1]);
a[0] =
  (op == '+')? a[0] + a[1] :
  (op == '-')? a[0] - a[1] :
  (op == '*')? a[0] * a[1] :
  (op == '/' && a[1] != 0.0) ? a[0] / a[1] :
  0.0;
while (op != '=')
{
  printf("\t%f\n", a[0]);
  scanf("%c", &op);
  if (op == '=')
  {
      printf("\t%f\n", a[0]);
      break;
  }
  else
  {
      scanf("%lf", &a[1]);
      if (op == '+')
          a[0] += a[1];
      else if (op == '-')
          a[0] -= a[1];
      else if (op == '*')
          a[0] *= a[1];
      else if (op == '/')
      {
          if (a[1] == 0)
          {
              printf("\t%s\n", "Error.");
              break;
          }
          else
              a[0] /= a[1];
      }
  }
}

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This way the user can do things like

2+2
4
*3
12
:
:

a[0] always stores the running answer.