Thread: 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:

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

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];

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];
       }
   }
}

This way the user can do things like

2+2
4
*3
12
:
:

a[0] always stores the running answer.