CODEWITHSUNDEEP
carrays
user2754673
user2754673

Reputation: 459

Equality of arrays In c

I have to check equality of two arrays(1-D) with integer elements.

I understand that there is no direct comparison way. So i'm doing basic iteration and checking for equality of each element. 

 for ( int i = 0 ; i < len ; i++) {
    // Equality check

What is the most efficient way to test equality of arrays in C ? Can i get away with loops(for ..) somehow ?

Upvotes: 5

Views: 3579

Answers (3)

Punit Vara
Punit Vara

Reputation: 4214

You can refer for loop method

Or else memcmp is also an efficient way of comparing array. Refer memcmp

int memcmp(const void *s1, const void *s2, size_t n);

You can compare first n byte with this function

int array1[5],array2[5]; 
int x = memcmp(array1, array2, sizeof(array1));

If array size is different then:

  • sizeof(array1) is less than sizeof(array2) ,it returns negative value.
  • sizeof(array1) is greater than sizeof(array2) ,it returns positive value.

  • sizeof(array1) is equal to sizeof(array2), it returns zero.

    How to use memcmp ? Answer is also on SO.

REMEMBER memcmp is only useful when both memory element or arraysize are equal

Upvotes: 1

Peter
Peter

Reputation: 36617

As others have said, using memcmp() is effective.

A general answer, assuming actual arrays, is

int is_equal = sizeof(array1) == sizeof(array2) && !memcmp(array1, array2, sizeof(array1));

If the arrays are supplied as pointer arguments to a function, the size information is lost, and needs to be provided separately.

 int IsEqual(void *array1, void *array2, size_t size1, size_t size2)
 {
     return size1 == size2 && !memcmp(array1, array2, size1);
 }

 int main()
 {
      int arr1[] = { /* whatever */ };
      int arr2[] = { /* whatever */ };

      is_equal = IsEqual(arr1, arr2, sizeof(arr1), sizeof(arr2));
      return 0;
 }

or, preserve type information (i.e. knowledge of working with an array of int) as late as possible before converting to void pointers, and work with number of elements.

 int IsEqual2(int array1[], int array2[], size_t n1, size_t n2)
 {
      /*  n1 and n2 are number of ints in array1 and array2 respectively */
     return n1 == n2 && !memcmp(array1, array2, n1 * sizeof(int));
 }

 int main()
 {
      int arr1[] = { /* whatever */ };
      int arr2[] = { /* whatever */ };

      is_equal = IsEqual2(arr1, arr2, sizeof(arr1)/sizeof(*arr1), sizeof(arr2)/sizeof(*arr2));
      return 0;
 }

Upvotes: 2

haccks
haccks

Reputation: 106102

Use memcmp function to compare two arrays of equal length. 

int a = memcmp(arr1, arr2, sizeof(arr1));
if(!a)
    printf("Arrays are equal\n");
else
    printf("Arrays are not equal\n");

Upvotes: 6

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