How can i show all the 3 values like one low index, one high and last one is the highest which wont be found?

Question

Implementing Binary search in the iterative procedure. Searching 3 values one by one, the first one which will be placed in low and mid-index, 2nd one which will be placed in the mid and high index and the last one value will be the highest value of your array and will not be found.

#include <stdio.h>
int main()
{
  int c, first, last, middle, n, search, array[100];

  printf("Enter number of elements\n");
  scanf("%d", &n);

  printf("Enter %d integers\n", n);

  for (c = 0; c < n; c++)
    scanf("%d", &array[c]);

  printf("Enter first value to find\n");
  scanf("%d", &search);

  printf("Enter second value to find\n");
  scanf("%d", &search);

  printf("Enter third value to find\n");
  scanf("%d", &search);


  first = 0;
  last = n - 1;
  middle = (first+last)/2;

  while (first <= last) {
    if (array[middle] < search)
      first = middle + 1;
    else if (array[middle] == search) {
      printf("%d found at location %d.\n", search, middle+1);
      break;
    }
    else
      last = middle - 1;

    middle = (first + last)/2;
  }
  if (first > last)
    printf("Not found! %d isn't present in the list.\n", search);

  return 0;
}

Answer 1

The correct way is to put code that should be repeated for different values either in a loop or in a function. Here you could easily use a loop:

#include <stdio.h>
int main()
{
    int c, first, last, middle, n, search, array[100];
    const char* label[] = { "first", "second", "third" };

    printf("Enter number of elements\n");
    scanf("%d", &n);

    printf("Enter %d integers\n", n);

    for (c = 0; c < n; c++)
        scanf("%d", &array[c]);

    // idiomatic way for an array length
    unsigned repeat = sizeof(label) / sizeof(*label);
    for (unsigned count = 0; count < repeat; count++) {

        printf("Enter %s value to find\n", label[count]);
        scanf("%d", &search);

        first = 0;
        last = n - 1;
        middle = (first + last) / 2;

        while (first <= last) {
            if (array[middle] < search)
                first = middle + 1;
            else if (array[middle] == search) {
                printf("%d found at location %d.\n", search, middle + 1);
                break;
            }
            else
                last = middle - 1;

            middle = (first + last) / 2;
        }
        if (first > last)
            printf("Not found! %d isn't present in the list.\n", search);
    }

    return 0;
}

But you should always control the return value of an input function (scanf): if you type a non numeric character (e.g. an a) all the following scanf will return 0 instead of 1 and leave the variable untouched. Your code will then use uninitialized value which invokes Undefined Behaviour (the hell for C programmers...).

All your scanf calls should look like:

if (1 != scanf("%d", &n)) {
    // process error
    fprintf(stderr, "Incorrect input\n");
    exit(1);
}

You will stop the program in an ordered way and warn the user for the reason instead of risking a crash or unexpected results with no error message.