infix.c

//Write a program to evaluate infix expression using Stacks.
// header files
#include<stdio.h>
#include<stdlib.h>
#include<ctype.h>
#include<string.h>
//Global variables
int numbers[50],tn=-1,to=-1;
char op[50];
//function to push digits
void push_num(int n)
{
numbers[++tn]=n;
}
//function to push operators
void push_op(char ch)
{
op[++to]=ch;
}
// function to pop digits
int pop_num()
{
return numbers[tn--];
}
//function to pop operators
char pop_op()
{
return op[to--];
}
//evaluating the expression
int infix_eval(int numbers[50], char op[50])
{
int x, y;
char ope;
//taking first two operands
x=pop_num();
y=pop_num();
//taking the operators between them
ope=pop_op();
//executing the operation
switch(ope)
{
case '+':
    return x+y;
case '-':
    return y-x;
case '*':
    return x*y;
case '/':
    if(x==0)
    {
        printf("\n cannot divide by 0");
        exit(0);
    }
    else
        return y/x;
}
return 0;
}
//Function to check whether the character is an operator or not
int is_operator(char ch)
{
    return (ch=='+'||ch=='-'||ch=='*'||ch=='/'||ch=='^');
}
//the precedence of the operators
int precedence (char c)
{
    switch (c)
    {
    case '+':
        return 1;
    case '-':
        return 1;
    case '*':
        return 2;
    case '/':
        return 2;
    case '^':
        return 3;
    }
    return -1;
}
//function to evaluate an infix expression
int eval(char exp[20])
{
    int i, num, output,r;
    char c;
    for (i=0;exp[i]!='\0';i++)
    {
        //taking each character from the expression
        c = exp[i];
       //checking if it is number
       if(isdigit(c)!=0)
       {
           num = 0;
           while (isdigit(c))
           {
               num = num*10 + (c-'0');
               i++;
               if (i<strlen(exp))
                c = exp[i];
               else
                break;
           }
           i--;
           //pushing the number into stack of numbers
           push_num(num);
       }
       else if(c=='(')
       {
           //pushing the operators into the stack
           push_op(c);
       }
       //If we get close bracket, evaluate the entire brackets
       else if(c==')')
       {
           while (op[to]!='(')
           {
               r = infix_eval (numbers, op);
               //pushing the result back to stack
               push_num(r);
           }
           pop_op();
       }
       //if the current character is operator
       else if (is_operator(c))
       {
           //evaluating the expression
           while (to!=-1 && precedence (c)<= precedence(op[to]))
           {
               output = infix_eval(numbers,op);
               //pushing the result back to stack
               push_num(output);
           }
           //pushing the current operator to stack
           push_op(c);
       }
    }
    //if there is any remaining expression, evaluate them
    while(to!=-1)
    {
        output = infix_eval(numbers,op);
        //pushing it back to stack
        push_num(output);
    }
    return pop_num();
}
//Main function
int main()
{
    char exp[50];
    //taking the expression to evaluate
    printf("Enter the infix expression to evaluate:");
    gets(exp);
    //calling the function and printing the result
    printf("%d",eval(exp));
    return 0;
}