How to work with a Boolean function in C in this code

Question

What is wrong in this code?

#include <stdio.h>

bool func(char *, int);

void main()
{
   char *a = "Interview";
   if(func(a, 9))
   {
      printf("True");
   }
   else
   {
      printf("False");
   }
}

bool func(char *s, int len)
{
   if(len < 2)
      return true;
   else
      return s[0] == s[len-1] && func(&s[1], len-2);
}

I believe this function always returns TRUE. This is an interview question. But, when I try to compile this, it shows six errors...

You're missing #include <stdbool.h>. The return type of main shall be int. — Daniel Fischer
– Daniel Fischer, Commented Nov 7, 2012 at 16:45
you may use int instead of bool and 1/0 instead of true/false. or #define bool int, #define true 1, #define false 0 — J X
– J X, Commented Nov 7, 2012 at 16:48
@JX yes i may, but it was an interview question.. so i can't make any correction on that — Ravi
– Ravi, Commented Nov 7, 2012 at 16:51

Peter Mortensen · Accepted Answer · 2023-08-17 20:59:27Z

17

I'm going to guess it doesn't know what bool and true are. bool is not a primitive data type in C. You need an extra include:

#include <stdbool.h>

The second part of your question? Does it always return TRUE?

No:

When you come into the function, you'll skip the first return because your length is 9. So instead you'll return if true only if:

return s[0] == s[len-1] && func(&s[1], len-2)

Is true. You can skip the recursive logic here because it's not modifying your string. Just look at the first part:

s[0]     // This has to be 'I'
==       // We need so what we compare against has to be 'I' as well
s[len-1] // This will be 'w'

So... that's not going to return true... Who cares about ANDing (&&) the recursive part? I suspect the compiler will even optimize that out because everything here is hard coded.

edited Aug 17, 2023 at 20:59

Peter Mortensen

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answered Nov 7, 2012 at 16:45

Mike

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8 Comments

Fingolfin Over a year ago

Correction, bool is not a primitive data type, but it is standard in C99 and following(hence stdbool )

Ravi Over a year ago

please correct me... the particular function will always return TRUE or not??

Ravi Over a year ago

because... s[len-1] && func(&s[1], len-2); at last func(&s[1], len-2); return TRUE, which means it become s[len-1] && true and anything except 0 is also known as TRUE. so thats why. pls correct me. if my logic is wrong.

Mike Over a year ago

@coders - right... so s[len-1] && true will be true, but it doesn't end there... you need to have s[0] == s[len-1] for it to be true as well. That can't happen. Check out the edit to my post regarding the logic

Ravi Over a year ago

@Mike thanks !! but program showing error.fatal error C1083: Cannot open include file: 'stdbool.h': No such file or directory why so.. ??

|

md5 · Accepted Answer · 2012-11-07 16:46:02Z

6

You just have to include the right header.

#include <stdbool.h>

Or, you can use _Bool type, which don't need any inclusion. bool is just an alias from this type. By the way, don't forget to compile in C99.

answered Nov 7, 2012 at 16:46

md5

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Comments

Tyler2P · Accepted Answer · 2023-07-15 10:09:34Z

-2

true and false are defined as 1 and 0, so you should do this:

#define true 1
#define false 0
int code() {
  //code
}

edited Jul 15, 2023 at 10:09

Tyler2P

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answered Jul 10, 2023 at 13:46

windows.sys

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1 Comment

Peter Mortensen Over a year ago

Re "true and false are defined as 1 and 0": Do you have a source?

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