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Postfix to Prefix conversion

With a given Postfix Expression, we will see how to convert Postfix Expression into Prefix Expression using stack.
 

Algorithm to convert Postfix expression to Prefix expression:

In this algorithm, we will use stack to store operands during the conversion. The step are as follows:
  • Read the postfix string
  • While the end of postfix string
    • symb = the current character
    • If symb is an operator
      • poped_sym1 = pop the stack
      • poped_sym2 = pop the stack
      • concat the string  STR = (operator)+(poped_sym2)+(poped_sym1)
      • push the string STR into stack
    • Else
      • push the operand symb into stack
    • End If
  • End While
  • prefix_str = pop the stack


 

Function to convert Postfix Expression to Prefix Expression:


void postfix_to_prefix(char postfix[], char prefix[]){
 char op[2]; //operator string
 char poped1[MAX];
 char poped2[MAX];
 char temp[MAX];
 int i = -1;
 op[1] = '\0';
 while(postfix[++i]){
  if(postfix[i] == ' '){
   continue;
  }
  if(isoperator(postfix[i])){
   pop(poped1);
   pop(poped2);
   op[0] = postfix[i]; //operator
   strcpy(temp, op);
   strcat(temp, poped2);
   strcat(temp, poped1);
   push(temp);
  }
  else{
   op[0] = postfix[i];//operand
   push(op);
  }
 }
 pop(prefix);
}
 

Program to convert Postfix Expression to Prefix Expression:


#include <stdio.h>
#include <string.h>
#define MAX 30

char stack[MAX][MAX];
int top = -1;

void push(char str[]){
 if(top != MAX-1){
  strcpy(stack[++top], str);
 }
 else{
  printf("Stack overflow : May be invalid postfix expression\n");
 }
}

void pop(char str[]){
 if(top != -1){
  strcpy(str,stack[top--]);
 }
 else{
  printf("Stack underflow : May be invalid postfix expression\n");
 }
}

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

void postfix_to_prefix(char postfix[], char prefix[]){
 char op[2]; //operator string
 char poped1[MAX];
 char poped2[MAX];
 char temp[MAX];
 int i = -1;
 op[1] = '\0';
 while(postfix[++i]){
  if(postfix[i] == ' '){
   continue;
  }
  if(isoperator(postfix[i])){
   pop(poped1);
   pop(poped2);
   op[0] = postfix[i]; //operator
   strcpy(temp, op);
   strcat(temp, poped2);
   strcat(temp, poped1);
   push(temp);
  }
  else{
   op[0] = postfix[i];//operand
   push(op);
  }
 }
 pop(prefix);
}

int main(){
 char postfix[] = "AB^C*D-EF/GH+/+"; //PREFIX : +-*^ABCD//EF+GH
 char prefix[MAX];
 postfix_to_prefix(postfix,prefix);
 printf("Input Postfix Expression : %s\n",postfix);
 printf("Output Prefix Expression : %s\n",prefix);
 
 return 0;
}
 
Input Postfix Expression : AB^C*D-EF/GH+/+
Output Prefix Expression : +-*^ABCD//EF+GH
 

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