CS301 Data Structures Assignment No 01 idea solution spring 2011

[vukhan]

The program is used to convert a infix expression to a postfix expression using stack data structure

#include<iostream.h>
#include<stdio.h>
#include<conio.h>
#include<string.h>
#include<stdlib.h>



const int size =50;
char infix[size],postfix[size],stack[size];
int top=-1;

int precedence(char ch); // function to get the precedence of the operator
char pop(); //function to pop an element from the stack
char topelement(); // returns the top element of the stack
void push(char ch); // pushes an element into the stack



int main()
{
char ele,elem,st[2];
int prep,pre,popped,j=0,chk=0;
strcpy(postfix," ");

gets(infix);

for(int i=0;infix[i]!=0;i++)
{
if(infix[i]!='('&&infix[i]!=')'&&infix[i]!='^'&&infix[i]!='*'&&infix[i]!='/'&&infix[i]!='+'&&infix[i]!='-')
postfix[j++]=infix[i];
else if(infix[i]=='(')
{
elem=infix[i];
push(elem);
}
else if(infix[i]==')')
{
while(popped=pop() != '(')
postfix[j++]=popped;
}
else
{
elem=infix[i];
pre=precedence(elem);//stores the precedence of operator coming frm infix
ele=topelement();
prep=precedence(ele);//stores the precedence of operator at the top of the stack

if(pre > prep)
push(elem);

else
{
while(prep >= pre)
{
if(ele=='#')
break;
popped=pop();
ele=topelement();
postfix[j++]=popped;
prep=precedence(ele);
}
push(elem);
}
}
}

while((popped=pop())!='#')
postfix[j++]=popped;
postfix[j]='\0';

cout<<"\n post fix :"<<postfix<<endl;

system("pause");
return 0;
}

int precedence(char ch)
{
switch(ch)
{
case '^' : return 5;
case '/' : return 4;
case '*' : return 4;
case '+' : return 3;
case '-' : return 3;
default : return 0;
}
}

char pop() //function to pop the element from the stack
{
char ret;
if(top!=-1)
{ ret =stack[top];
top--;
return ret;
}
else
return '#';
}

char topelement() // function to return top element from the stack without popping
{
char ch;
if(top!=-1)
ch=stack[top];
else
ch='#';
return ch;
}

void push(char ch) // function to push an element in the stack
{
if(top!=size-1)
{
top++;
stack[top]= ch;
}
}