Prefix to Post fix using C - c

Question

The question is:
WAP to convert a given Prefix expression into its equivalent Postfix
expression and evaluate it using stack.
I have written this code, it takes the input but doesn't display the output.
//header files
#include<stdio.h>
#include<string.h>
//global variables
char stack[50];
int top = -1;
void push(char s)
{
stack[++top]=s;
}
//function to pop an element
char pop()
{
return stack[top--];
}
// funtion to check if the character is operator or not
int is_operator(char x)
{
switch (x)
{
case '+':
case '-':
case '/':
case '*':
return 1;
}
return 0;
}
//function to Convert prefix to Postfix
void convert()
{
int i,l;
char op1,op2,tmp;
char exp[50];
printf("Enter the prefix expression: ");
gets(exp);
//length of expression
l = strlen(exp);
//scanning from right to left
for(i = l - 1; i >= 0; i--)
{
//checking if the symbol is an operator
if (is_operator(exp[i]))
{
//popping two operands from stack
op1 = stack[top];
pop();
op2 = stack[top];
pop();
//concating the operands and operator
tmp = op1 + op2 + exp[i];
//Pushing the temporary string to stack
push(tmp);
}
//if it is an operand
else
{
//push the operand to the stack
push((exp[i]));
}
}
//printf("The postfix expression is: %s",stack[top].c_str());
printf("%s ",stack[top]);
}
//main function
int main()
{
convert();
return 0;
}
when i run the code it takes the prefix expression but immediately after that it crashes and returns to the terminal?
Can anyone please help me with the code i am having difficulty in understanding what mistake i have made.
Also if possible give a little explanation what mistake i did in the code.

Answer 1

In this algorithm, a stack of strings is required. But you are using an array of char.
The exp[i] returns a char. So just can not expect tmp = op1 + op2 + exp[i] to concat them into a string in c.
In c strcpy() can be used to copy a string and strcat() can be used to concat strings.
There are several ways to convert a char to a string of one char tailing NULL. In this code (char[2]){(char)exp[i], '\0'} is used to get a string containing exp[i] and NULL char.
In the solution code there is a memory limitation. The input expression should be in 50 chars (including '\0') .
Solution code :
#include<stdio.h>
#include<string.h>
char stack[50][50];
int top = -1;
void clear_stack() {
top = -1 ;
}
void push(char *s)
{
strcpy(stack[++top], s) ;
}
char* pop()
{
return stack[top--];
}
int is_operator(char x)
{
if(x == '+' ||x == '-'||x == '*'||x == '/'){
return 1;
}
else{
return 0;
}
}
//function to Convert prefix to Postfix
void convert(char *exp)
{
clear_stack() ;
int i,l;
char op1[50],op2[50];
l = strlen(exp);
//scanning from right to left
for(i = l - 1; i >= 0; i--)
{
//checking if the symbol is an operator
if (is_operator(exp[i]))
{
//popping two operands from stack
strcpy(op1, pop()) ;
strcpy(op2, pop()) ;;
//concating the operands and operator
strcat(op1 , strcat(op2 , (char[2]) {(char)exp[i], '\0'})) ;
//Pushing the temporary string to stack
push(op1);
}
//if it is an operand
else
{
//push the operand to the stack
push((char[2]){(char)exp[i], '\0'});
}
}
//printf("The postfix expression is: %s",stack[top].c_str());
printf("%s\n",stack[top]);
}
//main function
int main()
{
convert("*-A/BC-/AKL");
convert("+ab");
convert("*+abc");
convert("*a+bc");
convert("+/ab/cd");
convert("*+ab+cd");
convert("-*+abcd");
return 0;
}
Output :
ABC/-AK/L-*
ab+
ab+c*
abc+*
ab/cd/+
ab+cd+*
ab+c*d-