Infix Expression: Any expression in the standard form like "AB-C/D" is an Infix(Inorder) expression.
Postfix Expression: The Postfix form of the above expression is "ABCD/-".

here is the program to convert infix to postfix expression

#include <iostream.h>
#include <string.h>
#include <ctype.h>

#define MAX 10
#define EMPTY -1

struct stack
{
char data[MAX];
int top;
};

int isempty(struct stack s)
{
return (s->top == EMPTY) ? 1 : 0;
}

void emptystack(struct stack s)
{
s->top=EMPTY;
}

void push(struct stack* s,int item)
{
if(s->top == (MAX-1))
{
cout<<"\nSTACK FULL";
}
else
{
++s->top;
s->data[s->top]=item;
}
}

char pop(struct stack* s)
{
char ret;
if(!isempty(s))
{
ret= s->data[s->top];
--s->top;
}
return ret;
}

void display(struct stack s)
{
while(s.top != EMPTY)
{
cout<<"\n"<<s.data[s.top];
s.top--;
}
}

int isoperator(char e)
{
if(e == '+' || e == '-' || e == '' || e == '/' || e == '%')
return 1;
else
return 0;
}

int priority(char e)
{
int pri = 0;

if(e == '' || e == '/' || e =='%')
pri = 2;
else
{
if(e == '+' || e == '-')
pri = 1;
}
return pri;
}

void infix2postfix(char* infix, char * postfix)
{
char i,p;
struct stack X;
char n1;
emptystack(&X);
i = &infix[0];
p = &postfix[0];

while(i)
{
while(i == ' ' || i == '\t')
{
i++;
}

if( isdigit(i) || isalpha(i) )
{
while( isdigit(i) || isalpha(i))
{
p = i;
p++;
i++;
}
/SPACE CODE/
p = ' ';
p++;
/END SPACE CODE/
}

if( i == '(' )
{
push(&X,i);
i++;
}

if( i == ')')
{
n1 = pop(&X);
while( n1 != '(' )
{
p = n1;
p++;
/SPACE CODE/
p = ' ';
p++;
/END SPACE CODE/
n1 = pop(&X);
}
i++;
}

if( isoperator(i) )
{
if(isempty(&X))
push(&X,i);
else
{
n1 = pop(&X);
while(priority(n1) >= priority(i))
{
p = n1;
p++;
/SPACE CODE/
p = ' ';
p++;

/END SPACE CODE/
n1 = pop(&X);
}
push(&X,n1);
push(&X,i);
}
i++;
}
}
while(!isempty(&X))
{
n1 = pop(&X);
p = n1;
p++;
/SPACE CODE/
p = ' ';
p++;

/END SPACE CODE/
}
p = '\0';
}

int main()
{
char in[50];
char post[50];

cout<<"Enter Infix Expression : ";
cin.getline(in, 50);
infix2postfix(&in[0],&post[0]);
cout<<"Postfix Expression is : \n"<<post;

return 0;
}

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