This is a very straightforward loop comparing every element in the array with the key. As soon as an equal value is found, it returns. If the loop finishes without finding a match, the search failed and -1 isreturned.
For small arrays, a linear search is a good solution because it’s so straightforward. In an array of a million elements, a linear search will take, on average, 500,000 comparisons to find the key. For a much faster search, take a look at binary search. Example:
int linearSearch(int a[], int first, int last, int key) { // function: // Searches a[first]..a[last] for key. // returns: index of the matching element if it finds key, // otherwise -1. // parameters: // a in array of (possibly unsorted) values. // first, last in lower and upper subscript bounds // key in value to search for. // returns: // index of key, or -1 if key is not in the array. for (int i=first; i<=last; i++) { if (key == a[i]) { return i; } } return -1; // failed to find key}