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[Page 358]

7.7. Searching Arrays with Linear Search

Often a programmer will be working with large amounts of data stored in arrays. It may be necessary to determine whether an array contains a value that matches a certain key value. The process of finding a particular element of an array is called searching. In this section we discuss the simple linear search. Exercise 7.33 at the end of this chapter asks you to implement a recursive version of the linear search. In Chapter 20, Searching and Sorting, we present the more complex, yet more efficient, binary search.

Linear Search

The linear search (Fig. 7.19, lines 3744) compares each element of an array with a search key (line 40). Because the array is not in any particular order, it is just as likely that the value will be found in the first element as the last. On average, therefore, the program must compare the search key with half the elements of the array. To determine that a value is not in the array, the program must compare the search key to every element in the array.

Figure 7.19. Linear search of an array.
(This item is displayed on pages 358 - 359 in the print version)

 1  // Fig. 7.19: fig07_19.cpp
 2  // Linear search of an array.
 3  #include <iostream>
 4  using std::cout;
 5  using std::cin;
 6  using std::endl;
 7
 8  int linearSearch( const int [], int, int ); // prototype
 9
10  int main()
11  {
12     const int arraySize = 100; // size of array a
13     int a[ arraySize ]; // create array a
14     int searchKey; // value to locate in array a
15
16     for ( int i = 0; i < arraySize; i++ )
17        a[ i ] = 2 * i; // create some data
18
19     cout << "Enter integer search key: ";
20     cin >> searchKey;
21
22     // attempt to locate searchKey in array a             
23     int element = linearSearch( a, searchKey, arraySize );
24
25     // display results
26     if ( element != -1 )
27        cout << "Found value in element " << element << endl;
28     else
29        cout << "Value not found" << endl;
30
31     return 0; // indicates successful termination
32  } // end main
33
34  // compare key to every element of array until location is     
35  // found or until end of array is reached; return subscript of 
36  // element if key or -1 if key not found                       
37  int linearSearch( const int array[], int key, int sizeOfArray )
38  {                                                              
39     for ( int j = 0; j < sizeOfArray; j++ )                     
40        if ( array[ j ] == key ) // if found,                    
41           return j; // return location of key                   
42                                                                 
43     return -1; // key not found                                 
44  } // end function linearSearch                                 

 Enter integer search key: 36
 Found value in element 18


 Enter integer search key: 37
 Value not found


The linear searching method works well for small arrays or for unsorted arrays (i.e., arrays whose elements are in no particular order). However, for large arrays, linear searching is inefficient. If the array is sorted (e.g., its elements are in ascending order), you can use the high-speed binary search technique that you will learn about in Chapter 20, Searching and Sorting.


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