How do I detect an operator vs. int in C using scanf?

Question

How do I read in the following input in my RPN calculator so that it will find the operator no matter what order?

2
2+
4

As of now my scanf only sees the first char in the string and I can only do this:

2
2
+
4

I'm also trying to add an option for integer vs floating point mode. (ex. when 'i' is entered, operate in floating point and vice versa.)

#include <stdio.h>
#include <stdlib.h>

#define MAX 100

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

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

int main (void)
{
int a, b;
//float c, d;
char s[80];
//char op;  //declare string of 80 chars

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");

 do {
 printf("> ");
 // gets(s);
 // scanf("%s", s);  //read integer
 scanf("%s", s);
 // switch (*s) {


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

 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;
 }

Answer 1

Your scanf reads the whole string. It's the following switch that judges by the first character and misses that + in 2+.

To improve it you can use the strtol function. It will parse an integer out of the string and return to you the location where the integer ended - if that's still not the end of the string, there may be an operator there.

A similar function for floating point numbers is strtod.

---

Here's some sample code of strtol applicable to your example:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>


int main()
{
    char* input = "25+";
    char* endptr;

    int val = strtol(input, &endptr, 10);

    if (*endptr == '\0')
    {
        printf("Got only the integer: %d\n", val);
    }
    else
    {
        printf("Got an integer %d\n", val);
        printf("Leftover: %s\n", endptr);
    }


    return 0;
}

Answer 2

I'm not sure if I fully understood your question, but you could iterate through the string like this:

for(i = 0; i < strlen(s); i++)
{
   // Here comes your switch section like this
   switch(s[i]) {
    .....
   }

}

Remember also to include string.h.

Answer 3

I really didn't understand your code.

If expect the user to enter one character each time, I mean one character + enter, you should use a simple char instead of char[]. And if you pretend to use a string you should receive it and parse it pzico said. You could do something like that. The problem would be in the treatment of numbers with multiple digits, but thinking a little bit you can fix this problem. I wrote an attempt, but I'm pretty sure it's not going to work.

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");
scanf("%c", s);

switch(*s){

case 'i':  
    printf("(Integer Mode)\n");  
break;  
case 'f':  
    printf("(Floating Point Mode)\n");  
break;  
case 'q':  
    printf("Bye Bye\n");  
    return;  
break;  

} printf("Enter the expression one character each time\n");

do {

scanf("%c", s);   
switch(s) {   
    case '+':   
        a = pop();      
        b = pop();     
        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:
        a = pop()*10+atoi(s);
        push(a);
}  

} while (s != 'q');

Another problem in your code is in your pop function. What do you want to do with this test:

if(p < 0) {
printf("Stack Underflow\n");
return 0;
}

You are expecting your pointer to reach the address 0?

Anyway I hope this is not your homework.