Data-Structure-In-C/postfix.c at master · AugustineAykara/Data-Structure-In-C · GitHub

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


int top=-1, stack[15];


void push(char x)
{
    stack[++top] = x;
}


char pop()
{
	return stack[top--];
}


int precedence(char c)
{
  	if(c == '(')
   		return 0;
    else if(c == '+' || c == '-')
    	return 1;
    else if(c == '*' || c == '/')
    	return 2;
    else if(c == '^')
    	return 3;
}


void infixToPostfix(char infix[],char postfix[])
{
    char item, x;
    int i=0, j=0;

    while(infix[i] != '\0')
    {
       item = infix[i];

        if(isalnum(item))
        {
   		    postfix[j++] = item;
        }

        else if(item == '(')
        {
     	  	push(item);
        }

        else if(item==')')
        {
         	while((x = pop()) != '(')
         	{
          		postfix[j++] = x;
          }
        }

        else
        {
      	 	while(precedence(stack[top]) >= precedence(item))
          {
            postfix[j++] = pop();
          }
          push(item);
        }
        i++;
    }

    while(top != -1)
    {
   		postfix[j++] = pop();
    }
    postfix[j] = '\0';
}


void postfixEvaluation(char postfix[])
{
    char ch;
    int a,b,i;

    for(i=0; postfix[i] != '\0'; i++)
    {
        ch = postfix[i];

        if(isdigit(ch))
        {
            push(ch-'0');
        }

        else
        {
            b = pop();
            a = pop();

            switch(ch)
            {
              case '+': push(a+b);
                break;
              case '-': push(a-b);
                break;
              case '*': push(a*b);
                break;
              case '/': push(a/b);
                break;
            }
        }
    }

    printf("\n The postfix evaluation is   : %d", stack[top]);
}


void main()
{
    char infix[15],postfix[15];

    printf("\n Enter the infix expression  : ");
    scanf("%s", infix);

    infixToPostfix(infix, postfix);
    printf(" The postfix expression is   : %s",postfix);

    postfixEvaluation(postfix);

    printf("\n");
}