How to write a binary algorithm for C/C++

Question

I am having trouble to write the binary algorithm in C/C++.
My question is like that:

Apply binary algorithm to search for a number from 1 to 100 in a number guessing game.
The user will respond with 'y' for a correct guess, 'h' if the guess is too high or 'l' if the guess is too low.

I don't have any idea to apply it. Can someone just give me an example of the code.

beginners do mistakes and they dont deserve one down.i am making it +1 — Vijay
– Vijay, Commented Dec 9, 2009 at 9:26
@benjamin Nothing to do with mistakes. People should at least try to solve the problem before asking for help. So -1 — qrdl
– qrdl, Commented Dec 9, 2009 at 9:29
There's no rule on SO that states that people have to try and solve their problems before asking for help. — Andreas Brinck
– Andreas Brinck, Commented Dec 9, 2009 at 10:28

Andreas Brinck · Accepted Answer · 2009-12-09 09:25:02Z

3

Detailed instructions here plus various implementations.

int low = 1;
int high = 100;
while (low <= high) {
    int mid = (low + high) / 2;
    char answer = evaluateGuess(mid); //return l, h or y;
    if ('y'==answer) {
       return mid;
    }
    if ('l' == answer) {
        low = mid + 1;
    } else {
        high = mid - 1;
    }
}
// If you get here the human player lied and the answer wasn't in [1..100]

edited Dec 9, 2009 at 9:25

answered Dec 9, 2009 at 9:19

Andreas Brinck

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Comments

Yacoby · Accepted Answer · 2009-12-09 09:22:22Z

I assume you mean binary search. Wikipedia has loads of information. You also haven't specified if you can use the stl.

The basic pseudo code is

  min := 1;
  max := N; {array size: var A : array [1..N] of integer}
  repeat
    mid := (min + max) div 2;
    if x > A[mid] then
      min := mid + 1
    else 
      max := mid - 1;
  until (A[mid] = x) or (min > max);

So in you case, min is 0, max is 100, where could alter the above algorithm to that it supports user input. All that needs to happen is rather than the comparison checks on an array, you just need to check user input.

  min := 1;
  max := 100;
  repeat
    mid := (min + max) div 2;
    print mid;
    c := getChar();
    if c == 'h' then
      min := mid + 1
    else if c == 'l'
      max := mid - 1;
    else if c == 'y'
      return mid
  until (min > max);

However if you want more help, you will need to post your code so far.

David Sykes · Accepted Answer · 2009-12-09 13:38:34Z

0

getRandomNumber(lower, upper){
  return random number between lower and upper;
}

main(){
 lower = 0;
 upper = 101;
 num = getRandomNumber(lower, upper);
 response = askUser(num);
 while(response != Y){
  if (response==H)
    //if secret is higher than num
    lower = num;
  else
    //if secret is lower than num
    upper = num;

  num = getRandomNumber (lower, upper);
  response = askUser(num);
 }
}

edited Dec 9, 2009 at 13:38

David Sykes

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answered Dec 9, 2009 at 9:35

Blessed Geek

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1 Comment

David Sykes Over a year ago

This is not really a 'binary' search, though it will get there in the end

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