Convert an Infix Expression to a Postfix Expression - C Data Structure

C examples for Data Structure:Algorithm

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Description

Convert an Infix Expression to a Postfix Expression

Demo Code

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

char postfixExp[60];
char infixExp[60];
char operatorStack[60];
int i=0, j=0, intTop=0;

int lowPriority(char opr, char oprStack)
{
  int k, p1, p2;//from   w ww  .j  a v a  2  s  . c om
  char oprList[] = {'+', '-', '*', '/', '%', '^', '('};
  int prioList[] = {0,0,1,1,2,3,4};
  if( oprStack == '(' )
    return 0;
  for(k = 0; k < 6; k ++) {
    if(opr == oprList[k])
    p1 = prioList[k];
  }
  for(k = 0; k < 6; k ++) {
    if(oprStack == oprList[k])
    p2 = prioList[k];
  }
  if(p1 < p2)
    return 1;
  else
    return 0;
}

void pushOpr(char opr)
{
  if(intTop == 0) {
    operatorStack[intTop] = opr;
    intTop++;
  }
  else {
    if(opr != '(' ) {
       while(lowPriority(opr, operatorStack[intTop-1]) ==
       1 && intTop > 0) {
        postfixExp[j] = operatorStack[--intTop];
        j++;
      }
    }
    operatorStack[intTop] = opr;
    intTop++;
  }
}

void popOpr()
{
  while(operatorStack[--intTop] != '(' ) {
    postfixExp[j] = operatorStack[intTop];
    j++;
  }
}

int main()
{
  char k;
  printf("\n Enter Infix Expression : ");
  gets_s(infixExp);
  while( (k=infixExp[i++]) != '\0') { /* while statement begins. */
    switch(k) {                      /* switch statement begins. */
      case ' ' :
                 break;
      case '(' :
      case '+' :
      case '-' :
      case '*' :
      case '/' :
      case '^' :
      case '%' :
                 pushOpr(k);
                 break;
      case ')' :
                 popOpr();
                 break;
       default :
                 postfixExp[j] = k;
                 j++;
    }                                /* switch statement ends. */
  }                                  /* while state ment ends. */
  while(intTop >= 0) {               /* while statement begins. */
    postfixExp[j] = operatorStack[--intTop];
    j++;
  }                                  /* while statement ends. */
  postfixExp[j] = '\0';
  printf("\n Infix Expression : %s ", infixExp);
  printf("\n Postfix Expression : %s ", postfixExp);
}

Result


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