We normally write arithmetical expressions using infix notation, meaning that th

Question

We normally write arithmetical expressions using infix notation, meaning that the operator appears between its two operands, as in "4 + 5". In postfix notation, the operator appears after its operands, as in "4 5 +". Here is a slightly more complex postfix expression: "25 12 7 - 2 * /". The equivalent infix expression is: "25 / ((12 - 7) * 2)". The result of that expression should be 2.5 (beware integer division). Postfix expressions don't require parentheses.

Write a function named postfixEval that uses a stack<double> (from the Standard Template Library) to evaluate postfix expressions. It should take a C-style string parameter that represents a postfix expression. The only symbols in the string will be +, -, *, /, digits and spaces. '+' and '-' will only appear in the expression string as binary operators - not as unary operators that indicate the sign of a number. The return type should be double. You may find the isdigit() function useful in parsing the expression. You may also use strtok() and atof().

Explanation / Answer

#include <stdio.h>
#include <ctype.h>
#define MAX 100

float stack[MAX];
int TOP=-1;
int f=0;
float pop()
{   
return(stack[TOP--]);
}

float push(float elem)
{
if(f==1){
float num;
num=pop();
stack[++TOP]=elem+10*num;
}
else if(f==0){
stack[++TOP]=elem;
f=1;
}
}


int main()
{   
char postfixExp[50],ch;
int i=0;float a,b;
printf("Enter the Postfix Expression :");
fgets(postfixExp,100,stdin);
while( (ch=postfixExp[i++]) != ' ')
{
if(isdigit(ch)) push(ch-'0');
else if(ch==' ')
f=0;
else
{   
f=0;
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;
default:
printf("Error ");
return 0;
}
}
}
printf("Result after evaluation: %f ",stack[TOP]);
}

===============================

Output:

akshay@akshay-Inspiron-3537:~/Chegg$ gcc postfix.c
akshay@akshay-Inspiron-3537:~/Chegg$ ./a.out
Enter the Postfix Expression :25 12 7 - 2 * /
Result after evaluation: 2.500000

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