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)
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1 // Fig. 7.19: fig07_19.cpp 2 // Linear search of an array. 3 #include 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
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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.