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Saturday, 20 April 2013

C code to convert INFIX expression into POSTFIX expression

//C program to convert given INFIX expression into POSTFIX expression using stack//

#include<stdio.h>
#include<conio.h> 
#include<string.h>
#define MAX 20

char stack[MAX];
int top=-1;
char pop();
void push(char item);

int prcd(char symbol)
{
switch(symbol)
{
case '+':
case '-':return 2;
break;
case '*':
case '/':return 4;
break;
case '^':
case '$':return 6;
break;
case '(':
case ')':
case '#':return 1; 
break;
}
}
 
int isoperator(char symbol)
{
switch(symbol) 
{
case '+':
case '-':
case '*':
case '/':
case '^':
case '$':
case '(':
case ')':return 1;
break;
default:return 0;
}
}

 
void convertip(char infix[],char postfix[])
{
int i,symbol,j=0;
stack[++top]='#';
for(i=0;i<strlen(infix);i++)
{
symbol=infix[i];
if(isoperator(symbol)==0)
{
postfix[j]=symbol;
j++;
}
else{ 
if(symbol=='(')
push(symbol); 
else
if(symbol==')')
{
while(stack[top]!='(')

postfix[j]=pop();
j++; 
}
pop();//pop out 
}

else{
if(prcd(symbol)>prcd(stack[top]))
push(symbol);
else{
while(prcd(symbol)<=prcd(stack[top]))
{
postfix[j]=pop();
j++;
}
push(symbol);

}//end of else.

}//end of else.

}//end of else.

}//end of for.

while(stack[top]!='#')
{
postfix[j]=pop();
j++;
}
postfix[j]='\0';//null terminate string.
}
///**********************main()*********// 
void main()
{
char infix[20],postfix[20];
clrscr();
printf("Enter the valid infix string:\n");
gets(infix);
convertip(infix,postfix);
printf("The corresponding postfix string is:\n");
puts(postfix);
getch();
}

void push(char item)
{
top++;
stack[top]=item;
}

char pop()
{
char a;
a=stack[top];
top--;
return a;
}

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