Student Projects > Software Development > Java > Java Programs > Advanced programs > Java program that converts infix expression into postfix form

Java program that converts infix expression into postfix form

Editorial Team Advanced programs 
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import java.io.*;
class stack
{
char stack1[]=new char[20];
int top;
void push(char ch)
{
top++;
stack1[top]=ch;
}
char pop()
{
char ch;
ch=stack1[top];
top--;
return ch;
}
int pre(char ch)
{
switch(ch)
{
case '-':return 1;
case '+':return 1;
case '*':return 2;
case '/':return 2;
}
return 0;
}
boolean operator(char ch)
{
if(ch=='/'||ch=='*'||ch=='+'||ch=='-')
return true;
else
return false;
}
boolean isAlpha(char ch)
{
if(ch>='a'&&ch<='z'||ch>='0'&&ch=='9')
return true;
else
return false;
}
void postfix(String str)
{
char output[]=new char[str.length()];
char ch;
int p=0,i;
for(i=0;i<str.length();i++)
{
ch=str.charAt(i);	
if(ch=='(')
{
push(ch);
}
else if(isAlpha(ch))
{
output[p++]=ch;
}
else if(operator(ch))
{
if(stack1[top]==0||(pre(ch)>pre(stack1[top]))||stack1[top]=='(')
{
push(ch);
}
}
else if(pre(ch)<=pre(stack1[top]))
{
output[p++]=pop();
push(ch);
}
else if(ch=='(')
{
while((ch=pop())!='(')
{
output[p++]=ch;
}
}
}
while(top!=0)
{
output[p++]=pop();
}
for(int j=0;j<str.length();j++)
{
System.out.print(output[j]);	
}
}
}
class intopost
{
public static void main(String[] args)throws Exception
{
String s;
BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
stack b=new stack();
System.out.println("Enter input string");
s=br.readLine();
System.out.println("Input String:"+s);
System.out.println("Output String:");
b.postfix(s);
}
}

Output:

Enter input string
a+b*c
Input String:a+b*c
Output String:
abc*+

Enter input string
a+(b*c)/d
Input String:a+(b*c)/d
Output String:
abc*d/)(+

Editorial Team
Editorial Team

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11 thoughts on “Java program that converts infix expression into postfix form”

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  4. thanks a lot dude… i’ll be very happy if you guide how could i code for postfix to infix in java

  5. “top” is undefined.

    “stack1” declaration is constantly being recognized as an illegal start of the expression.

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