Guess a person's secret number between 0 - 100 using binary search

Question

I converted a code that I know how to construct in python in C Language, but everytime I run the program in CodeBlocks, the program crashes! And I have NO idea why this is happening, can someone help me?

The program is suppose to guess a person's number (between 0 - 100), using binary search. For example, if my number is 66, the program asks if my number is 50, since 66 is higher than 50, the number 50 becomes the lower boundary while 100 remains to be the higher boundary, and so on...

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

int main()
{
    int x;
    printf("Please think of a number between 0 and 100\n\n");
    x = binarysearch();
    printf("%d", x);


}
int binarysearch()
{
int hi,lo,guess;
hi = 100;
lo = 0;
char user_inp;
while (1){
    guess = round(((hi + lo)/2));
    printf("Is your secret number %d?\n\n", guess);
    printf("Enter 'h' to indicate the guess is too high. \nEnter 'l' to indicate the guess is too low.\nEnter 'c' to indicate I guessed correctly. \n");
    scanf("%c", &user_inp);
    if (strcmp(user_inp, "c") == 0){
        break;
    }
    else if  (strcmp(user_inp, "h")==0){
        hi = guess;
    }
    else if (strcmp(user_inp, "l")==0){
        lo = guess;
    }
    else{
        printf("Sorry, I did not understand your input.");
        continue;
    }
}
printf("Game over. Your secret number was");
return guess;

}

Answer 1

1. Your binary search is incorrect, you need to swap the check of 'h' and 'l'.
2. Because you compare chars and not strings, use == and not strcmp().
3. You don't need to include <math.h> because guess is an int, so it'll automatically round floats.
4. You can use getchar() to clear the buffer after the scanf()
5. You need to declare your function before main (possibly by defining the function before main).

#include <stdio.h>
#include <stdlib.h>
// WITHOUT <MATH.H>

int binarysearch(void);

int main(void)
{
    int x;
    printf("Please think of a number between 0 and 100\n\n");
    x = binarysearch();
    printf("%d", x);
    return 0;    // RETRUN 0    
}

int binarysearch(void)
{
    int hi,lo,guess;
    hi = 100;
    lo = 0;
    char user_inp;
    int flag = 1;    // USE FLAG, NOT BREAK AND CONTINUE
    while (flag){
        guess = ((hi + lo)/2);   // WITHOUT ROUND
        printf("Is your secret number %d?\n\n", guess);
        printf("Enter 'h' to indicate the guess is too high. \nEnter 'l' to indicate the guess is too low.\nEnter 'c' to indicate I guessed correctly. \n");
        scanf("%c", &user_inp);
        getchar(); // CLEAR THE BUFFER
        if (user_inp == 'c'){   // MAKE FLAG 0
            flag = 0;
        }
        //  USE '==', NOT STRCMP
        else if  (user_inp == 'l'){  // YOU NEED TO SWAP 'L' & 'H'
            hi = guess;
        }
        else if (user_inp == 'h'){
            lo = guess;
        }
        else{
            printf("Sorry, I did not understand your input.");
        }
    }
    printf("Game over. Your secret number was ");
    return guess;
}

Answer 2

As per the comments, the problem was very likely the incorrect use of strcmp:

char *string = "fish";
char not_a_string = 'f';
if (0 == strcmp( not_a_string, string  ))
  ...

The character 'f' has ASCII value 0x66. strcmp would blindly use this as a pointer (expecting it to point to a valid string) which would cause a crash as you access memory that's not yours (a segmentation fault).

You would have got away with strcmp( &not_a_string, string ) in this case, but that's good fortune, not correct code.

To compare the user's character input with another character, you can just use a straightforward equality (since they're both really integers):

if ( user_inp == 'c' ) ...

---

So that's your code fixed, but how did you ever get to run it in the first place? For me GCC immediately complained:

In function 'int binarysearch()': so.cpp:17:29: error: invalid conversion from 'char' to 'const char*' [-fpermissive] 
if (strcmp(user_inp, "c") == 0){

and didn't produce an output. It's telling you the same thing I just did (albeit fractionally more cryptically).

Lessons to learn: listen to your compiler's complaints (and make your compiler as complainy as possible)

---

@pmg also noted:

add a space before the conversion specifier: scanf(" %c", &user_inp)

Without it, every time you hit Enter:

Sorry, I did not understand your input.Is your secret number 25?

ie you get a spurious complaint. But with the space it works as desired.

(I hate scanf, so have no idea why this works ;) )