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

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Description

Convert an Infix Expression to a Prefix Expression

Demo Code

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

char prefixExp[60];
char infixExp[60];
char operatorStack[60];
int n=0, intTop=0;

void fillPre(char let)
{
  int m;/*  ww  w.ja va  2s . c o  m*/
  if(n == 0)
    prefixExp[0] = let;
  else {
    for(m = n + 1; m > 0; m--)
    prefixExp[m] = prefixExp[m - 1];
    prefixExp[0] = let;
  }
  n++;
}

int lowPriority(char opr, char oprStack)
{
  int k, p1, p2;
  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) {
        fillPre(operatorStack[--intTop]);
      }
    }
    operatorStack[intTop] = opr;
    intTop++;
  }
}

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

int main()
{
  char chrL;
  int length;
  printf("\n Enter Infix Expression : ");
  gets_s(infixExp);
  length = strlen(infixExp);
  while(length > 0) {              /* first while loop begins. */
    chrL = infixExp[--length];
    switch(chrL) {                 /* switch statement begins. */
      case ' ' :
                 break;
      case ')' :
      case '+' :
      case '-' :
      case '*' :
      case '/' :
      case '^' :
      case '%' :
                 pushOpr(chrL);
                 break;
      case '(' :
                 popOpr();
                 break;
      default :
                 fillPre(chrL);
    }                              /* switch statement ends. */
  }                                /* first while loop ends. */
  while( intTop > 0 ) {            /* second while loop begins. */
    fillPre( operatorStack[--intTop] );
    n++;
  }                                /* second while loop ends. */
  prefixExp[n] = '\0';
  printf("\n Infix Expression : %s ", infixExp);
  printf("\n Prefix Expression : %s ", prefixExp);
}

Result


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