Thread: problem of reading Postfix Evaluation

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.

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);