Finding with bisection, not stoping

Question

I have a problem with this method, when it gets "big" array in it. When I input array with like 10 numbers it's working fine. But if I insert like 10 milion numbers or even 20 the method never end and I can't find the problem.

int bisection(int el, int* a, int n) 
{
    int i = 0;

    if (n <= 0)
    {
       return -1;
    }
    else
    {

     do
     {
       int mid = (i + n) / 2;
       if(el == a[i])
       {
           return i;
       }
       else if(el < a[n])
       {
           n = mid - 1;
       }
       else if(el > a[i])
       {
           i = mid + 1;
       }

     }while(i <= n);
    }
}

I have to find first number for example if I have in array:

indexs:   0 1 2 3 4 5 6 7 8  
elements: 1,1,3,3,3,5,6,7,9

And i'm looking for number 3 i have to get this as

result: (index) 2

first occurrence.

What happens if the value is not contained in your array ? – Caduchon Mar 19 '14 at 09:44 — Caduchon, Mar 19 '14 at 09:44
The loop won't terminate if `el == a[n]` and `el > al[i]` – Skizz Mar 19 '14 at 10:03 — Skizz, Mar 19 '14 at 10:03

myusuf · Answer 1 · 2014-03-19T10:09:21.903

4

Are you using the wrong index and terminating condition ?

int mid;

do { mid = (i + n) / 2; if(el == a[mid] && (mid == 0 || a[mid-1] < el)) //for first occurence { returnmid; } else if(el <= a[mid]) { n = mid - 1; } else if(el > a[mid]) { i = mid + 1; } }while(i < n);

return -1;

edited Mar 19 '14 at 10:09

answered Mar 19 '14 at 09:41

myusuf

11,810
11
35
50

score 2 · Accepted Answer · edited Mar 14 '21 at 20:47

2

myusuf's answer is almost correct, but it doesn't always return the index of the first occurence. Here is my suggestion:

int bisection(int el, int* a, int n) 
{
    int i = 0;
    while(i < n)
    {
        // search in range i (inclusive) to n (exclusive)
        int mid = (i + n) / 2;
        if (el == a[mid] && (mid == 0 || a[mid-1] < el))
            return mid;
        if (el <= a[mid])
             n = mid;
        else
             i = mid + 1;
    }
    return -1;
}

edited Mar 14 '21 at 20:47

knarf

2,672
3
26
31

answered Mar 19 '14 at 10:01

Henrik

23,186
6
42
92

It's working fine if i'm looking numbers that are lower than mid. If i want to find numbers that are higher than mid I get -1 as result. – user3127680 Mar 19 '14 at 10:17
You are supposed to be looking for `el`. Mid is just a value calculated in the process `mid = (i+n)/2;` – myusuf Mar 19 '14 at 10:27
@user3127680 fixed a bug in my answer. – Henrik Mar 19 '14 at 13:05
Can someone explain to me what this code do -> if (el == a[mid] && (mid == 0 || a[mid-1] < el)) ? And how? – user3127680 Mar 22 '14 at 18:00
@user3127680 `el == a[mid]` matches any occurence of `el`. `el == a[mid] && a[mid-1] < el` matches only the first. But you need to take care of the special case `mid == 0` where `a[mid-1]` is undefined. – Henrik Mar 24 '14 at 08:22

score 0 · Answer 3 · answered Mar 19 '14 at 10:07

Maybe the problem comes from the maximum size of int. You have to check the sizeof(int) on your architecture.

For your algorithm, you can do something like that :

unsigned long int bisection(int value, int* vector, unsigned long int size)
{
  unsigned long int left = 0;
  unsigned long int right = size-1;
  unsigned long int previous_left;
  while(left < right)
  {
    unsigned long int i = (left + right) / 2;
    if(value < vector[i])
      right = i;
    else
    {
      previous_left = left;
      left = i;
    }
  }
  left = previous_left;
  while(left < right)
  {
    unsigned long int i = (left + right) / 2;
    if(value == vector[i])
      right = i;
    else
      left = i;
  }
  return left;
}

Finding with bisection, not stoping

3 Answers3