Binary Search in C - Optimization

I know that there is a function bsearch present in stdlib.h but still I want to implement this.

This is my code for binary search. Any and all reviews are welcome.

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

int compare (const void * a, const void * b)
{
return (*(int*)a - *(int*)b);
}

int bin_search(int arr[], int min_index, int max_index, int element)
{
/*
Searches for an element in the array arr
Returns fist index of element if present else returns -1
*/
if (min_index > max_index){
return -1;
}
else
{
//Don't change this assignment of mid_point. It avoids overflow
int mid_point = min_index + (max_index - min_index)/2;

if (arr[mid_point] > element){
return bin_search(arr, min_index, mid_point - 1, element);
}
else if (arr[mid_point] < element){
return bin_search(arr, mid_point + 1, max_index, element);
}
else{
return mid_point;
}
}
}

int main()
{
int length;
while (1)
{
printf("Enter a positive length: ");
scanf("%d", &length);
if (length > 1){
break;
}
else{
printf("You entered length = %d\n\n", length);
}
}

int *arr = malloc(sizeof(int) * length);
if (arr == NULL)
{
perror("The following error occurred");
exit(-1);
}

for (int i = 0; i < length; i++){
scanf("%d", &arr[i]);
}

int element;
printf("\nEnter the element to be searched: ");
scanf("%d", &element);

qsort(arr, length, sizeof(int), compare);
int index = bin_search(arr, 0, length - 1, element);

if (index == -1){
printf("\nElement not in the array");
}
else{
printf("\nIndex of element is %d", index);
}

free(arr);
return 0;
}

• Any discussion regarding this question can be done in this chat room. @codesparkle Just leave this comment if you want to delete all others – Aseem Bansal Jul 9 '13 at 18:34
• Why make element a const and not min and max? Such const markup on parameters passed by value does no harm but is not part of the function's interface - in other words the caller does not care. From an interface point of view, const only has meaning for pointers, such as arr. It could be considered to have some utility in telling the reader that the call parameters remain unchanged (but personally I rarely, if ever, use it for parameters passed by value). – William Morris Jul 10 '13 at 18:51
• @WilliamMorris I am still confused about what is the best way to use const. I commented on ruds' answer about this confusion as he added 1 more const here. Perhaps you can answer my comments there. – Aseem Bansal Jul 11 '13 at 4:06

The functions says it "Returns fist index of element if present". If I give it the numbers 2, 2, 2 and ask it to find 2 it says the 1, but the first index with value 2 is clearly 0.

Some minor comments on the code:

• the arr parameter to bin_search should be const. This tells the compiler and the reader that the array is not changed by the function. The compiler will then enforce this if you, by mistake, try to modify the array data. The reader/user knows that her data is unchanged after the call.

• the parameter names min_index and max_index could be shortened to min and max. Giving names an appropriate size is a service to the reader (auto-completion by the IDE is a service to you). In general, the shorter the names, the less dense the code and the easier it is to read. This can be taken too far of course, once names become meaningless.

• Note that it would be more normal to pass the start of the array and its size instead of the array plus two offsets.

• functions could be static. This is of no significance in a one-file program but becomes important with bigger programs. Making functions and global variables static restricts their scope to the file which allows extra optimisation and reduces namespace pollution.

• the output message needs a trailing \n

• there is no prompt for the input values - ok that is trivial. Personally I find this sort of test better with values entered on the command line.

• exit status is normally 1 (EXIT_FAILURE) on failure, not -1. On success it is 0 (EXIT_SUCCESS). These are UNIX conventions.

EDIT

You questioned the use of const. Its utility when used on parameters passed by reference - pointers/array passed into functions - is, I think clear.

However, its use with parameters passed by value is not so clear. It plays no part in the interface seen by callers of a function, as it can have no influence on the caller. You can declare a function prototype in a public header like this

int bin_search(const int arr[], int min, int max, int element);


and then define the function implementation like this:

int bin_search(const int arr[], const int min, const int max, const int element) {...}


And the compiler will be quite happy with the difference.

So it is purely an implementation issue. Hence you should definitely not use const on pass-by-value parameters in public prototypes, only in the implementation (if at all). Used in the implementation, const tells the reader and the compiler that a parameter is not (and cannot be) changed. This gives the reader some extra information and of course the compiler will enforce this read-only behaviour.

So shouldn't you use it on all parameters that are not changed (and some might say that parameters should never be changed)? Good question! I never use const on call parameters but would have no hesitation in adding const to a local variable

int cirle_area(double radius)
{
const double pi = 3.14;
}


A lot of good programming style concerns consistency, so my inconsistency here is troubling. And my previous comment - that if you make the element parameter const, then you should be consistent and do the same for min and max uses consistency as an argument!

I'm afraid I can do no better than that. I've programmed for 20 years in C and have rarely seen const applied to parameters. But maybe it should be.

• I'll take care of the incorrect comment. About arr being const why? I used min and max but max is defined in C++ and the IDE that I use(Code:blocks) starts to highlight it as a keyword so I thought to change them. It has auto-completion so length of variables isn't a problem. Any particular reason why I should use static functions? Can you tell me what it is this called so I can read up a bit(like static for variables is storage class)? About exit status, is that a convention or what? – Aseem Bansal Jul 9 '13 at 17:19
• So const should be used whenever I am passing pointers but not changing the contents. Correct? – Aseem Bansal Jul 9 '13 at 17:55
• Yes, that is correct – William Morris Jul 9 '13 at 19:57
• Asked a question on programmers about use of the const keyword. Please give your comments and suggest improvements. – Aseem Bansal Jul 11 '13 at 14:07
• Signaling the array as const is a good idea, for the other parameters const it's meaningless and just bloats the code. As for the the identifiers, I would argue that max_index is better than just max. A good rule of thumb (that do not work for every case) is to have a noun in the mix. – Marcelo De Zen Jul 23 '16 at 10:47

If your average case is that the looked-for element is distributed uniformly randomly (and that your array is pulled from the same distribution), you actually gain in the average case by not returning early if arr[mid] == element.

William Morris's comment about static was most likely meant to apply to your compare helper function; presumably bin_search will be declared in a header file to be included in other translation units, in which case it must not be declared static.

int bin_search(const int arr[], int min, int max, const int element)
{
/*
Searches for an element in the array arr
Returns index of element if present else returns -1
*/
if (min >= max) {
return arr[max] == element ? max : -1;
}

//Don't change this assignment of mid. It avoids overflow
const int mid = min + (max - min) / 2;

if (arr[mid] < element) {
return bin_search(arr, mid + 1, max, element);
} else {
return bin_search(arr, min, mid, element);
}


This has exactly 2 * lg(N) + 1 comparisons (2 for each halving of the size, plus 1 on the final return). Your version has worst-case 3 * lg(N) comparisons and is worse in the average case (2.5 average comparisons per layer, saving on average (N-1)*2^-N + (N-2)*2^(N-1) + ... + 1*2^-1 layers, means that you get about 2.5 * (lg(N)-2) comparisons on average).

• My solution has the downside that it accesses uninitialized memory when max < 0 initially, which may happen e.g. when code doesn't check for an empty array before calling bin_search – ruds Jul 9 '13 at 19:54
• Any function that needs to called in other files by including the header files shouldn't be static. But these functions can access the static functions from the files from which they are called. Am I correct? As an example of what I think you have said, in this question the merge_parts function should be static but the merge_sort shouldn't be. Declaring merge_parts as static won't effect the use of merge_sort in another file. Correct me if I am wrong. – Aseem Bansal Jul 10 '13 at 8:53
• About declaring mid and element as const, should any variable, that isn't changed in the called function, be decalred const? I understand that declaring arr as const is relevant because it is a pointer that can be changed but the calling function's original value passed to the variable element cannot be effected here. Is this declaration of const for the purposes of clarity and perhaps a security measure in the scope of the called function? – Aseem Bansal Jul 10 '13 at 9:06
• Your code is missing a brace after the last else and shouldn't it be 2 * lg(N) + 2. In the final case there are 2 comparisons, 1 for the if and 1 inside it. – Aseem Bansal Jul 10 '13 at 9:13
• After some thinking I think I might have found the best of both worlds. I'll update with a code that uses the same number of comparisons as you have suggested while still taking care of accessing uninitialized memory. – Aseem Bansal Jul 12 '13 at 14:55

Update bin_search with a best-case scenario O(1), if the array has only 1 element.

int bin_search (int arr[], int min_index, int max_index, int element)
{
// this is the best case scenario
if(min_index == max_index)
{
if(arr[min_index] == element)
return min_index;
else
return -1;
}
if (min_index > max_index)
{
return -1;
}
else
{
int mid_index = (min_index + max_index) / 2;
if (arr[mid_index] > element)
{
return bin_search(arr, min_index, mid_index - 1, element);
}
else if (arr[mid_index] < element)
{
return bin_search(arr, mid_index + 1, max_index, element);
}
else
{
return mid_index;
}
}
}


There is also an iterative process.

int bin_search (const int arr[], int min, int max, int element)
{
int high = max+1;
int low = min;
while(low < (high-1))
{
int mid = (low + high)/2;
if(element < arr[mid])
high = mid;
else
low = mid;
}
if(arr[low] == element)
return low;
else

• @AseemBansal: Looks like if you compile with -O2, -O3, or -Os, or add -foptimize-sibling-calls to the command line, it should at least try. At that point, the result depends on how good the detection is. – cHao Jul 9 '13 at 18:05