Infix to postfix implementation using linked lists in C

I've been trying to debug this program for a long time. It works fine when i input expressions
like a+b-c or a/b+c where the first operator has a greater or equal precedence than the second. But for expressions like a-b/c where the first operator has a lesser precedence than the second, the compiler throws a breakpoint.

struct stack
{
char ele;
struct stack *next;
};

void push(int);
int pop();
int precedence(char);

struct stack *top = NULL;

int main()
{
char infix[20], postfix[20];
int i=0,j=0;

printf("ENTER INFIX EXPRESSION: ");
gets(infix);

while(infix[i]!='')
{
if(isalnum(infix[i]))
postfix[j++]=infix[i];
else
{
if(top==NULL)
push(infix[i]);

else
{
while(top!=NULL && (precedence(top->ele)>=precedence(infix[i])))
postfix[j++]=pop();
push(infix[i]);
}
}

++i;
}

while(top!=NULL)
postfix[j++]=pop();

postfix[j]='';
puts(postfix);
getchar();
return 0;
}

int precedence(char x)
{
switch(x)
{
case '^': return 4;
case '*':
case '/': return 3;
case '+':
case '-': return 2;

default: return 0;
}
}

void push(int x)
{
int item;
struct stack *tmp;

if(top==NULL)
{
top=(struct stack *)malloc(sizeof(struct stack));
top->ele=x;
top->next=NULL;
}

else
{
tmp=top;
top->ele=x;
top->next=tmp;
}
}

int pop()
{
struct stack *tmp;
int item;

if(top==NULL)
puts("EMPTY STACK");

else if(top->next==NULL)
{
tmp=top;
item=top->ele;
top=NULL;
free(tmp);
}

else
{
tmp=top;
item=top->ele;
top=top->next;
free(tmp);
}

return item;
}


Any advice on how to improve my coding would be helpful.
Thanks for the help, i really appreciate it :)

1 + 2

+

1

2

NULL

1 Answer
1

it works fine for me.


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

struct node
{

char data;
struct node *next;


}*top=NULL,*pstart=NULL;
/*-------------------- insertion in postfix expression linked list -------*/


void insert(char ch)
{
struct node *t,*baby;
baby=(struct node *)malloc(sizeof(struct node));
baby->next=NULL;
baby->data=ch;
t=pstart;

if(pstart==NULL)
{
pstart=baby;
}
else
{
while(t->next!=NULL)
t=t->next;
t->next=baby;

}
//printf(" inserted in list- %c",baby->data);

}

/* --------- push operation ------- */



void push (char symbol)
{

struct node *p;
p=(struct node *)malloc(sizeof(struct node));
p->data=symbol;
if(top==NULL)
{
top=p;
p->next=NULL;

}
else
{

p->next=top;
top=p;

}
}

char pop()
{
struct node *x,*y;
char k;
if(top==NULL)
{
printf("stack underflown");
return 0;

}
else
{
x=top;
top=top->next;
k=x->data;
//printf("node %d is deletedn",top->data);
free(x);
x=NULL;
return k;


}



}


void displaypost()
{
struct node *to;
if(pstart==NULL)
printf("");
else`enter code here`
{
to=pstart;
while(to!=NULL)
{
printf("%c",to->data);
to=to->next;

}

}


}


/*============== precedence selector ================= */

int precedence(char ch)
{

if(ch=='^')
return (5);
else if(ch=='*' || ch== '/')
return (4);
else if (ch== '+' || ch== '-')
return (3);
else
return (2);
}


/*=================== infix to postfix conversion ================ */

void intopost(char infix)
{

int len;
int index=0;
char symbol,temp;
len= strlen(infix);
//printf("%d",len);
while(len>index)
{
symbol=infix[index];

switch(symbol)
{

case '(':
push(symbol);
break;

case ')':
temp=pop();
while(temp!='(')
{
insert(temp);
temp=pop();
}
break;

case '^':
case '+':
case '-':
case '*':
case '/':
if(top==NULL)
{
push(symbol);
// break;

}
else
{
while(top!=NULL && (precedence(top->data)>=precedence(symbol)))
{
temp=pop();
insert(temp);

}
push(symbol);

}
break;
default:
insert(symbol);

}
index=index+1;


}
while(top!=NULL)
{
temp=pop();
insert(temp);

}
displaypost();
return;




}


int main()
{
char infix[50];
system("clear");
printf("enter infix expression: ");
gets(infix);

printf("nn equivalent postfix expression is---> ");
intopost(infix);
getchar();
return 0;
}
`


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