Unwanted symbol result when evaluating postfix expression - c

Question

I'm trying to make a C program to evaluate postfix expressions and when doing so an unwanted symbol is being printed on the screen for the input 45+.
P.S. Please tell me the mistake (except of that gets() I am studying right now how to use fgets())
// to Evaluate a postfix expression
#include<stdio.h>
#include<conio.h>
int is_operator(char);
void answer();
char stack[100];
int top =-1;
void push(char);
char pop();
void main()
{
char postfix[100],item;
int i=0;
clrscr();
printf("Enter Postfix Expression");
gets(postfix);
while(postfix[i]!='\0')
{
item=postfix[i];
if(is_operator(item)==2)
{
push(item);
}
if(is_operator(item)==1)
{
char op;
int n1,n2,n3;
op=item;
n1=pop();
n2=pop();
switch(op)
{
case '+':
n3=n1+n2;
case '-':
n3=n1-n2;
case '*':
n3=n1*n2;
case '/':
n3=n1/n2;
}
push(n3);
}
i++;
}//end while
answer();
getch();
}
void push(char c)
{
top++;
stack[top]=c;
}
char pop()
{
char c;
c=stack[top];
top--;
return(c);
}
int is_operator(char i)
{
char ch=i;
if(ch=='+'||ch=='-'||ch=='*'||ch=='/')
{
return(1);
}
else
{
return(2);
}
}
void answer()
{
char ans;
ans=stack[top];
printf("Answere is %c",ans);
}

Answer 1

There are a lot of mistakes in your code.Try to properly type cast.
Go through comments to understand the mistakes.
Go through this for understanding character pointer and arrays.
// to Evaluate a postfix expression
#include<stdio.h>
int is_operator(char);
void answer();
int stack[100];//Use integer array since operands are integer
int top =-1;
void push(int);//Arguments changed to integer type since the stack is integer
int pop(); //Return type to integer
void main()
{
char* postfix;//Use character pointer for iterating through loop smoothly
int item;
int i=0;
printf("Enter Postfix Expression");
gets(postfix);
char c;
while(*postfix!='\0')
{
c=*postfix;
if(is_operator(c)==2)
{
push((c-'0')); //Converting char to int before pushing it into the stack
}
if(is_operator(c)==1)
{
char op;
int n1,n2,n3;
op=*postfix;
n1=pop();
n2=pop();
switch(op)
{
case '+':
n3=n1+n2;
break;
case '-':
n3=n1-n2;
break;
case '*':
n3=n1*n2;
break;
case '/':
n3=n1/n2;
break;
}
push(n3);
}
postfix++;
}//end while
answer();
}
void push(int c)
{
top++;
stack[top]=c;
}
int pop()
{
int c;
c=stack[top];
top--;
return(c);
}
int is_operator(char i)
{
char ch=i;
if(ch=='+'||ch=='-'||ch=='*'||ch=='/')
{
return(1);
}
else
{
return(2);
}
}
void answer()
{
char ans;
ans=stack[top];
printf("Answere is %d",ans);
}
I hope it is helpful....

Answer 2

The problems I see with your code are: your switch() is missing break statements on the individual case clauses (and a default case might be nice too); when you push your non-operators (aka single digit numbers) on the stack, you push them as character codes rather then converting them to numbers so the math doesn't make sense; you're not properly handling the order of non-communitive operations like subtraction and division (use Unix dc command as a comparison tool); finally, don't reinvent booleans. Below is a rework of your code with the above changes and some style adjustments:
// Evaluate a postfix expression
#include <ctype.h>
#include <stdio.h>
#include <stdbool.h>
char stack[100];
int top = -1;
void push(char);
char pop(void);
bool is_operator(char);
void answer(void);
void push(char c)
{
stack[++top] = c;
}
char pop()
{
return stack[top--];
}
bool is_operator(char op)
{
return (op == '+' || op == '-' || op == '*' || op == '/');
}
void answer()
{
printf("Answer is %d\n", stack[top]);
}
int main()
{
char item, postfix[100];
int i = 0;
printf("Enter Postfix Expression: ");
gets(postfix);
while ((item = postfix[i++]) != '\0')
{
if (is_operator(item))
{
char n1 = pop();
char n2 = pop();
char n3 = 0;
switch (item)
{
case '+':
n3 = n1 + n2;
break;
case '-':
n3 = n2 - n1;
break;
case '*':
n3 = n1 * n2;
break;
case '/':
n3 = n2 / n1;
break;
}
push(n3);
} else if (isdigit(item)) {
push(item - '0');
}
} // end while
answer();
return 0;
}
Example (note this evaluator only operates on single digit numbers):
> ./a.out
Enter Postfix Expression: 6 4 - 7 * 1 +
Answer is 15
> dc
6 4 - 7 * 1 + p
15