Question

This code in C converts infix to postfix and evaluates it. The problem is that it only evaluates one digit expressions. I need to fix it so that it can evaluate 2 digits expressions as well.

#include <stdio.h>
#include <ctype.h>
#include <string.h>
#include <math.h>
#define SIZE 100

char s[SIZE];
int top=-1;

void infixToPostfix(char *infix, char *postfix);
void postfixEvaluation(char *postfix);

void push(char elem){
s[++top]=elem;
}

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

int pr(char elem){ // Order of precedence
switch (elem)
{
case '(': return 1;
case '+': return 2;
case '-': return 2;
case '*': return 3;
case '/': return 3;
case '%': return 3; //modulo
case '^': return 4; //exponent
}
return -1;
}

int main ()
{
char infix[50];
  
printf("\nEnter an infix expression:\n");
scanf("%s", infix);
push('=');
  
char postfix[50];
infixToPostfix(infix, postfix);
  
postfixEvaluation(postfix);
  
return 0;
}

void infixToPostfix(char *infix, char *postfix){
  
char ch, elem;
int i=0, j=0;
  
for (i=0; infix[i] !='\0'; i++)
{
if (infix[i] == '(')
push(infix[i]);
  
else if (isalnum(infix[i]))
{
postfix[j++]=infix[i];
  
// if (infix[i+1] != '\0' && isalnum(infix[i+1]) !=0){ //This line check if there's another number aside
// postfix[j++]=infix[++i];
// }
// postfix[j++]=' '; //this line add an extra space
}
else if (infix[i] == ')')
{
while( s[top] != '(')
{
postfix[j++] = pop();
}
elem=pop();
}
else
{
while( pr(s[top]) >= pr(infix[i]) ){
postfix[j++] = pop();
}
push(infix[i]);
  
// postfix[j++]=' '; //This line separate the operators from operand
}
}
while ( s[top] != '=')
postfix[j++]=pop();
postfix[j]='\0';
  
printf("\n%s = %s", infix, postfix);
}

void postfixEvaluation(char *postfix){
  
int i=0, j=0, postfix_int[50];
  
while (top != -1)
pop();
  
int value=0, op1, op2;
  
for (i=0; postfix[i] != '\0'; i++)
{
  
if (isdigit(postfix[i]))
push(postfix[i]);
  
else {
op1 = pop() - '0';
op2 = pop() - '0';
switch (postfix[i]) {
case '+':
value = op2 + op1;
break;
case '-':
value = op2 - op1;
break;
case '*':
value = op2 * op1;
break;
case '/':
value = op2 / op1;
break;
case '%':
value = op2 % op1;
break;
case '^':
value = pow(op2, op1); // exponet math library
break;
}
push(value + '0');
}
}
printf(" = %d", pop() - '0');
}

Answer 1

Dear student,

The way you have implemented this program it wont work for 2 or more digit numbers.

You can use any of the below two code both are written by me.

i would recommend you to use Code 1

Code 1

#include<stdio.h>
#include<string.h>
#include<math.h>
#include<stdlib.h>
#include<ctype.h>
#define SPACE ' '
#define TAB '\t'
#define MAX 100
char infix[MAX], postfix[MAX];
int stack[MAX];
int top;

void push(int symbol)
{
   if(top>MAX)
   {
       printf("Stack overflow\n");
       exit(1);
   }
   stack[++top]=symbol;
}
//This function returns the priority of the operator
int priority(char symbol)
{
   switch(symbol)
   {
   case '(':
       return 0;
   case '+':
   case '-':
       return 1;
   case '*':
   case '/':
   case '%':
       return 2;
   case '^':
       return 3;
   default :
       return 0;
   }
}/*End of priority()*/
//This function is used to check to see is the char is a space
int white_space(char symbol)
{
   if( symbol == SPACE || symbol == TAB )
       return 1;
   else
       return 0;
}
int isEmpty()
{
   if(top==-1)
       return 1;
   else
       return 0;
}
long int pop()
{
   return (stack[top--]);
}
void intopost()
{
   int i,p=0,flag=0;
   char next;
   char symbol;
   for(i=0;i<strlen(infix);i++)
   {
       symbol=infix[i];
       if(!white_space(symbol))
       {
           switch(symbol)
           {
           case '(':
               push(symbol);
               break;
           case ')':
               while((next=pop())!='(')
                   postfix[p++] = next;
               break;
           case '+':
           case '-':
           case '*':
           case '/':
           case '%':
           case '^':
               while( !isEmpty( ) && priority(stack[top])>= priority(symbol) )
               {
                   postfix[p++]=' ';
                   postfix[p++]=pop();
               }
               push(symbol);
               break;
           case '\n':
               break;
           default: /*if an operand comes*/
               postfix[p++]=symbol;
               flag=1;
           }
           if(flag != 1)
               postfix[p++]=' ';
           flag=0;
       }
   }
   while(!isEmpty( ))
   {
       postfix[p++]=pop();
       postfix[p++]=' ';
   }
   postfix[p]='\0'; /*End postfix with'\0' to make it a string*/
}/*End of intopost()*/

long int eval()
{
   int temp1,temp2;
   int a,b,temp,result;
   unsigned int i;
   for(i=0;i<strlen(postfix);i++)
   {
       if(isalnum(postfix[i]))
       {
           temp1=postfix[i]-'0';
           while(isalnum(postfix[i+1]))
           {
               i++;
               temp2=postfix[i]-'0';
               temp1=temp1*10+temp2;
           }
           push(temp1);
       }
       else if(postfix[i] == 32)
       {
       }
       else
       {
           a=pop();
           b=pop();
           switch(postfix[i])
           {
           case '+':
               temp=b+a; break;
           case '-':
               temp=b-a;break;
           case '*':
               temp=b*a;break;
           case '/':
               temp=b/a;break;
           case '%':
               temp=b%a;break;
           case '^':
               temp=pow(b,a);
           }
           push(temp);
       }
   }
   result=pop();
   return result;
}/*End of eval */
int main()
{
   //Right here I declare the int value to be used to display the evaluated postix number
   int value;
   //set top to -1 for the stack
   top=-1;
   //here is where the program prints and receives an input from the user
   printf("Please input your infix expression: ");
   fgets(infix,MAX,stdin);
   //Right here is where the program takes the infix and converts it over to a postfix then prints out what the postfix expression is
   intopost();
   printf("Your infix to postfix expression is: %s\n",postfix);
   //These last two lines is where the the int value is set to equal what the funct eval() evaluated
   value=eval();
   printf("The evaluated postfix expression is: %d\n",value);
}

Screenshot of output:

Code 2

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

char stack[100];
int top = -1;
int eval_top = -1;
int eval_stack[100];

void push(char x) // Push char into stack
{
   stack[++top] = x;
}
char pop() // Pop char to top of stack
{
   if (top == -1)
       return -1;
   else
       return stack[top--];
}

/* functions for evaluation of postfix expression */
// push function
void eval_push(int x) { // Find push result
   eval_stack[++eval_top] = x;
}
// pop function
int eval_pop() { // Find pop result
   if (eval_top == -1) {
       return -1;
   } else {
       return eval_stack[eval_top--];
   }
}

int priority(char x) // check priority order
{
   if (x == '(')
       return 0;
   if (x == '+' || x == '-')
       return 1;
   if (x == '*' || x == '/')
       return 2;
}
// function to evaluate the postfix expression
void EvalPostfix(char postfix[]) {
   int temp1,temp2;
   int A, B;
   int val;
   char ch;
   int i;

   //find postfix
   for (i = 0; postfix[i] != ')'; i++) {
       ch = postfix[i];
       if (isdigit(ch))
       {
           temp1=ch - '0';
           i++;
           while(postfix[i] != ' ')
           {
               ch = postfix[i];
               temp2=ch - '0';
               temp1 = temp1 * 10 + temp2;
               i++;
           }
           eval_push(temp1);

       }
       else if (ch == '+' || ch == '-' || ch == '*' || ch == '/') {

           A = eval_pop();
           B = eval_pop();

           switch (ch)
           {
           case '*':
               val = B * A;
               break;

           case '/':
               val = B / A;
               break;

           case '+':
               val = B + A;
               break;

           case '-':
               val = B - A;
               break;
           }

           eval_push(val);//send value on top of stack
       }
   }
   printf("\n Result: %d \n", eval_pop());
}

int main()
{
   int i = 0;
   int j = 0;
   char * e, x;
   char postfix[100]; // store postfix for later evaluation
   char exp[100];
   printf("Infix expression : ");
   scanf("%s", exp); // asking the user to enter the infix expression
   printf("Postfix expression: ");
   while ( exp[j] != '\0')
   {
       if (isalnum( exp[j]))
       { // if character is alphabet or number , it is printed
           while(isalnum(exp[j]))
           {
               printf("%c",exp[j]);
               postfix[i++] = exp[j];
               j++;
           }
           j--;
       }
       else if ( exp[j] == '(') // if it is open parenthesis, it is pushed into the stack without any priority
           push( exp[j]);
       else if ( exp[j] == ')') // if it is closed parenthesis , pop the elements in the stack and print them until the we see ( symbol
       {
           while ((x = pop()) != '(')
           {
               printf("%c", x);
               postfix[i++] = x;
           }
       }
       else // if character is symbol like +, -, *, / then based on their priority character is pushed if it high priority otherwise high priority symbols are popped and it is pushed
       {
           while (priority(stack[top]) >= priority(exp[j])) {
               x = pop();
               printf("%c", x);
               postfix[i++] = x;
           }
           push(exp[j]);
       }
       printf(" ");
       postfix[i++] = ' ';
       j++;
   }
   while (top != -1) // printing remaining elements in the stack
   {
       x = pop();
       printf("%c", x);
       postfix[i++] = x;
   }
   postfix[i] = ')'; // this is to add at the end for detecting end by the evaluation function
   EvalPostfix(postfix);
}

Screenshot of output

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