7.4.6. Using the Elements of an array as Counters

Image

Sometimes, programs use counter variables to summarize data, such as the results of a survey. In Fig. 6.8, we used separate counters in our die-rolling program to track the number of occurrences of each side of a die as the program rolled the die 6,000,000 times. An array version of this program is shown in Fig. 7.10. This version also uses the new C++11 random-number generation capabilities that were introduced in Section 6.8.

Click here to view code image

 1   // Fig. 7.10: fig07_10.cpp
 2   // Die-rolling program using an array instead of switch.
 3   #include <iostream>
 4   #include <iomanip>
 5   #include <array>
 6   #include <random>
 7   #include <ctime>
 8   using namespace std;
 9
10   int main()
11   {
12      // use the default random-number generation engine to
13      // produce uniformly distributed pseudorandom int values from 1 to 6
14      default_random_engine engine( static_cast< unsigned int >( time(0) ) );
15      uniform_int_distribution< unsigned int > randomInt( 1, 6 );
16
17      const size_t arraySize = 7; // ignore element zero
18      array< unsigned int, arraySize > frequency = {}; // initialize to 0s
19
20      // roll die 6,000,000 times; use die value as frequency index
21      for ( unsigned int roll = 1; roll <= 6000000; ++roll )
22         ++frequency[ randomInt( engine ) ];
23
24      cout << "Face" << setw( 13 ) << "Frequency" << endl;
25
26      // output each array element's value
27      for ( size_t face = 1; face < frequency.size(); ++face )
28         cout << setw( 4 ) << face << setw( 13 ) << frequency[ face ]
29            << endl;
30   } // end main

Face    Frequency
   1      1000167
   2      1000149
   3      1000152
   4       998748
   5       999626
   6      1001158

Fig. 7.10. Die-rolling program using an array instead of switch.

Figure 7.10 uses the array frequency (line 18) to count the occurrences of each side of the die. The single statement in line 22 of this program replaces the switch statement in lines 23–45 of Fig. 6.8. Line 22 uses a random value to determine which frequency element to increment during each iteration of the loop. The calculation in line 22 produces a random subscript from 1 to 6, so array frequency must be large enough to store six counters. However, we use a seven-element array in which we ignore frequency[0]—it’s clearer to have the die face value 1 increment frequency[1] than frequency[0]. Thus, each face value is used directly as a subscript for array frequency. We also replace lines 49–54 of Fig. 6.8 by looping through array frequency to output the results (Fig. 7.10, lines 27–29).

..................Content has been hidden....................

You can't read the all page of ebook, please click here login for view all page.
Reset
3.129.217.194