C++ Notes: Algorithms: Linear Search

Look at every element

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 is returned.


For small arrays, linear search is a good solution because it's so straightforward. In an array of a million elements linear search on average will take 500,000 comparisons to find the key. For a much faster search, take a look at binary search.


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

Related Pages

Binary Search