How to do binary search in C?

Question

C++ / C#

It turns out an infinite loop, what to do? And how to find out if the number is found?

#include <stdio.h>
#define SIZE 10

int main (void){
  int array_1[SIZE], count_1, count_2, reserve, search, left, right, mid;
 
  for (count_1 = 0; count_1 < SIZE; count_1++){
    printf ("Enter array (%d): \t", SIZE - count_1);
    scanf ("%d", &array_1[count_1]);
  }  
  
  printf ("%s%10s \n", "Element", "Value");

  for (count_1 = 0; count_1 < SIZE; count_1++){
    printf ("%7d%10d \n", count_1, array_1[count_1]);
  }

  for (count_1 = 0; count_1 < SIZE - 1; count_1++){
    for (count_2 = 0; count_2 < SIZE - count_1 - 1; count_2++){
      if (array_1[count_2] > array_1[count_2 + 1]){
        reserve = array_1[count_2];
        array_1[count_2] = array_1[count_2 + 1];
        array_1[count_2 + 1] = reserve;
      }
    }
  }  

  printf ("%s%10s \n", "Element", "Value");
  
  for (count_1 = 0; count_1 < SIZE; count_1++){
    printf ("%7d%10d \n", count_1, array_1[count_1]);
  }

  printf ("Enter value: \t");
  scanf ("%d", &search);

  left = array_1[0];
  right = array_1[SIZE - 1];
  mid = (left + right) / 2;

  while (left <= right){
    if (search < array_1[mid]){
      left = mid - 1; 
    } else{
      right = mid + 1;  
    }
  }

  return 0;
}

Answer 1

It is quite logical that the cycle is infinite.
Firstly, initially left and right are for some reason not set by indexes, but by values ​​from the array. Secondly, mid must be calculated at each step of the loop. Thirdly, there is no check to find the desired value. Fourth, the left and right borders are shifted incorrectly inside the loop.
Use a debugger and step through the section of the program with control of all variables, or even better, do it in your mind, writing down the result of each step on paper.

Answer 2

I don't know how correct this is, but it works =) Thank you.

#include <stdio.h>
#define SIZE 10

int main (void) {
  int arrayOne [SIZE], countOne, countTwo, reserve, search, left, right, mid;
  
  for (countOne = 0; countOne < SIZE; countOne++) {
    printf ("Enter array (%d): \t", SIZE - countOne);
    scanf ("%d", &arrayOne [countOne]);
  }

  printf ("%s%10s \n", "Element", "Value");

  for (countOne = 0; countOne < SIZE - 1; countOne++) {
    for (countTwo = 0; countTwo < SIZE - countOne - 1; countTwo++) {
      if (arrayOne [countTwo] > arrayOne [countTwo + 1]) {
        reserve = arrayOne [countTwo];
        arrayOne [countTwo] = arrayOne [countTwo + 1];
        arrayOne [countTwo + 1] = reserve;
      } 
    }
  }

  for (countOne = 0; countOne < SIZE; countOne++) {
    printf ("%7d%10d \n", countOne, arrayOne [countOne]);
  }

  printf ("Enter value: \t");
  scanf ("%d", &search);

  left = 0;
  right = SIZE - 1;

  while (left < right) {
    mid = (left + right) / 2;
    if (search <= arrayOne [mid]) {
      right = mid;
    } else {
      left = mid + 1;
    }
  }

  if (search == arrayOne [right]) {
    printf ("Ok! \n");
  } else {
    printf ("No! \n");
  }

  return 0;
}

Answer 3

How to do binary search in C?

It's already done - the bsearch() function.