Is my Switch statement incorrect when using strtol?

Question

#define MAX 100

int *p;
int *tos;
int *bos;

void push(int i);
int pop(void);   



int main ()
{    
int a,b;    
char s[80];

p = (int *) malloc(MAX*sizeof(int));    /* get stack memory */
if (!p)
{
    printf("Allocation Failure\n");
    exit(1);
}


tos = p;
bos = p + MAX-1;

printf("\nRPN Calculator\n");
printf("Enter 'i' for integer mode\n");
printf("Enter 'f' for floating point mode\n");
printf("Enter 'q' to quit\n");
char *endptr;


do {        
    printf("> ");
    scanf("%s", s);
    int val = strtol(s, &endptr, 10);

    if (*endptr == '\0')
    {

        //printf("Got only the integer: %d\n", val);


    }
    else{   printf("operator: %s\n", endptr); 
        printf("Got the integer: %d\n", val);
    }

        /* tests */


    switch(*endptr) {
        case 'i':
            printf("(Integer Mode)\n");
            break;
        case 'f':
            printf("(Floating Point Mode)\n");
            break;
        case '+':
            a = pop();
            b = pop();
            printf("%d\n",a);
            printf("%d\n",b);
            printf("%d\n", a+b);
            push(a+b);
            break;

                    case '-':
            a = pop();
            b = pop();
            printf("%d\n", b-a);
            push(b-a);
            break;  
        case '*':
            a = pop();
            b = pop();
            printf("%d\n", a*b);
            push(a*b);
            break;
        case '/':
            a = pop();
            b = pop();
            if(a == 0){
                printf("Cannot divide by zero\n");
                break;
            }

            printf("%d\n", b/a);
            push(b/a);
            break;
        case '.':
            a = pop();
            push(a);
            printf("Current value on top of stack: %d\n", a);

            break;  
        default:
            push(atoi(s));
    }

} while (*s != 'q'); 
        //end do while loop
    return 0;
}   


// Put an element on the stack

void push (int i)
{
if (p > bos){
    printf("Stack Full\n");
    return;
}
*p = i;
p++;
}

// Get the element from the top of the stack

int pop (void)
{
p--;
if(p < 0) {
    printf("Stack Underflow\n");
    return 0;
}
return *p;
}

I think my switch statement is incorrect. I use strtol to parse the integer and I can see that it's working:

2+
operator: +
got the integer: 2

but if I try this:

1
2+

I get:

operator: +
Got the integer: 2
1
0
1

I should have gotten a 2 instead of the 0 and a 3 instead of the sum of 1. Any ideas?

Answer 1

Hi Patrick,

I think you should include all of the code in your switch if you want us to check it for you.

You should probably show all of the code full stop. What does p point to, for e.g.?

Edit: following Patrick's extra code

Hi Patrick,

You never actually use your val variable. I suspect this is not what you want. You're getting the behaviour you see because (presumably) your stack is initialised to zeros and so your '+' is adding 1 to 0 and then pushing 1 back onto the stack. I'm not sure you're using strtol correctly.

Cheers, Dan

Answer 2

You need to check if the endptr is the same as the input pointer, to determine if no conversion took place:

if(endptr == s)
{
  /* no integer found */
}

Answer 3

It looks like the integer on the same line as the operator doesn't get pushed onto the stack, only the single value appears to be pushed. Also, no need to call atoi() when pushing the found integer, use val since it's the result of strtol().

The easy fix is to move the default behavior out of the switch, push(val) prior to the switch statement and removing the default should do the trick.

Answer 4

Incase of the +, *, / cases, just do one pop and use val directly instead of the second pop. I guess that will make your code work.