problem of reading Postfix Evaluation

For example, 1 2 3 * + 4 -. A space between two data(operand or operator).
How to read it step by step and save as a proper parameter? for example, I declare two variable `int operand; char operator`. If it is scanned a data(operand or operator), how to save this data to a proper variable(operand or operator variable)? Example below,

Code:

---

1 2 3 * + 4 -
(Now It is read "1", save as operand)
....
(Now It is read "*", save as operator)

---

By this method, I can pop or push the data properly. How to implement it?

Read everything as a string
- if it looks like an int, push it onto the stack
- it it looks like an operator, pop the required number of operands and push the result.

can I use strtok to read it easily? or other method?

Use fgets() to read the line.

Use strtok() / sscanf() / strchr() / strspn() or any other method you like to break the string into meaningful tokens.

thank Salem. I have other problem about passing by value of variable "operator" at function "void operation(char operator)". The output is very strange. It cannot display "+", "-" or "*" properly. What's wrong?

Code:

---

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define DELIM " "
#define MAXWORD 100
#define MAXLEN 10
#define MAX_PS 1000

int postStack[MAX_PS];
int top;

void push(int x)
{
        top++;
        postStack[top] = x;
}

int pop()
{
        if (top >= 0) {
                top--;
                return postStack[top+1];
        }
}

void initStack()
{ top = -1; }

void operation(char operator)
{
        int a, b, result;
        a = pop();
        b = pop();
        if (operator == '+')
                result = a+b;
        else if (operator == '-')
                result = a-b;
        else if (operator == '*')
                result = a*b;
        push(result);
        printf("%d %c %d = %d\n", a, b, operator, result);
}

int main()
{
        char words[MAXWORD][MAXLEN];
        char buff[BUFSIZ];
        int n, i;
        initStack();
        n = 0;
        if (fgets(buff, sizeof buff, stdin) != NULL) {
                char *sep = strtok(buff, DELIM);
                while (sep != NULL) {
                        strcpy(words[n++], sep);
                        sep = strtok(NULL, DELIM);
                }
        }

        for (i = 0; i < n; i++) {
                if ((words[i][0] != '+') && (words[i][0] != '-') && (words[i][0] != '*'))
                        push(atoi(words[i]));
                else operation(words[i][0]);
        }

        return 0;
}

---

data input/output

Code:

---

input:
6 4 * 2 +
output:
4  42 = 0
2 ▒ 43 = 0

---

printf("%d %c %d = %d\n", a, b, operator, result);
should be

printf("%d %c %d = %d\n", a, operator, b , result);

change your operator variable name to something else something like v_operator. operator is key word in c++. Even through u wont get any error message when it is saved as .c ext. Would,'t be recommended variable name

ssharish2005

thanks ssharish2005, I haven't throught that it will be crashed with C++ variaible name of operator.