Transcript rand()
6.7 Case Study: Random Number
Generation
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• C++ Standard Library function rand
– Introduces the element of chance into computer applications
– Example
• i = rand();
– Generates an unsigned integer between 0 and RAND_MAX (a
symbolic constant defined in header file <cstdlib>)
– Function prototype for the rand function is in <cstdlib>
2008 Pearson Education, Inc. All rights reserved.
6.7 Case Study: Random Number
Generation (Cont.)
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• To produce integers in a specific range, use the modulus
operator (%) with rand
– Example
rand() % 6;
– Produces numbers in the range 0 to 5
– This is called scaling, 6 is the scaling factor
– Shifting can move the range to 1 to 6
1 + rand() % 6;
2008 Pearson Education, Inc. All rights reserved.
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// Fig. 6.8: fig06_08.cpp
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// Shifted and scaled random integers.
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#include <iostream>
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using std::cout;
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using std::endl;
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Outline
fig06_08.cpp
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#include <iomanip>
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using std::setw;
(1 of 2)
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10 #include <cstdlib> // contains function prototype for rand
11 using std::rand;
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#include and using for function rand
13 int main()
14 {
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// loop 20 times
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for ( int counter = 1; counter <= 20; counter++ )
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{
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// pick random number from 1 to 6 and output it
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cout << setw( 10 ) << ( 1 + rand() % 6 );
Calling function rand
2008 Pearson Education,
Inc. All rights reserved.
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// if counter is divisible by 5, start a new line of output
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if ( counter % 5 == 0 )
cout << endl;
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} // end for
fig06_08.cpp
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Outline
return 0; // indicates successful termination
(2 of 2)
27 } // end main
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2008 Pearson Education,
Inc. All rights reserved.
6.7 Case Study: Random Number
Generation (Cont.)
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• Function rand
– Generates pseudorandom numbers
– The same sequence of numbers repeats itself each time the
program executes
• Randomizing
– Conditioning a program to produce a different sequence of
random numbers for each execution
• C++ Standard Library function srand
– Takes an unsigned integer argument
– Seeds the rand function to produce a different sequence of
random numbers
2008 Pearson Education, Inc. All rights reserved.
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// Fig. 6.10: fig06_10.cpp
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// Randomizing die-rolling program.
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#include <iostream>
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using std::cout;
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using std::cin;
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using std::endl;
fig06_10.cpp
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#include <iomanip>
(1 of 2)
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using std::setw;
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Outline
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11 #include <cstdlib> // contains prototypes for functions srand and rand
12 using std::rand;
13 using std::srand;
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using statement for function srand
15 int main()
16 {
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unsigned seed; // stores the seed entered by the user
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Data type unsigned is short for unsigned int
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cout << "Enter seed: ";
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cin >> seed;
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srand( seed ); // seed random number generator
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Passing seed to srand to randomize the program
2008 Pearson Education,
Inc. All rights reserved.
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// loop 10 times
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for ( int counter = 1; counter <= 10; counter++ )
{
// pick random number from 1 to 6 and output it
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Outline
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cout << setw( 10 ) << ( 1 + rand() % 6 );
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// if counter is divisible by 5, start a new line of output
if ( counter % 5 == 0 )
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cout << endl;
} // end for
fig06_10.cpp
(2 of 2)
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return 0; // indicates successful termination
35 } // end main
Enter seed: 67
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Enter seed: 432
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Enter seed: 67
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Program outputs show that each
unique seed value produces a different
sequence of random numbers
2008 Pearson Education,
Inc. All rights reserved.
6.7 Case Study: Random Number
Generation (Cont.)
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• To randomize without having to enter a seed each
time
srand( time( 0 ) );
• This causes the computer to read its clock to obtain the seed
value
– Function time (with the argument 0)
• Returns the current time as the number of seconds since
January 1, 1970 at midnight Greenwich Mean Time (GMT)
• Function prototype for time is in <ctime>
2008 Pearson Education, Inc. All rights reserved.
6.7 Case Study: Random Number
Generation (Cont.)
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• Scaling and shifting random numbers
– To obtain random numbers in a desired range, use a
statement like
number = shiftingValue + rand() % scalingFactor;
• shiftingValue is equal to the first number in the desired range
of consecutive integers
• scalingFactor is equal to the width of the desired range of
consecutive integers
– number of consecutive integers in the range
2008 Pearson Education, Inc. All rights reserved.
1
// Fig. 7.10: fig07_10.cpp
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// Roll a six-sided die 6,000,000 times.
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#include <iostream>
using std::cout;
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using std::endl;
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#include <iomanip>
using std::setw;
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Outline
fig07_10.cpp
(1 of 2)
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10 #include <cstdlib>
11 using std::rand;
12 using std::srand;
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14 #include <ctime>
15 using std::time;
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17 int main()
18 {
Declare frequency as array of 7 ints
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const int arraySize = 7; // ignore element zero
int frequency[ arraySize ] = { 0 };
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srand( time( 0 ) ); // seed random number generator
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// roll die 6,000,000 times; use die value as frequency index
for ( int roll = 1; roll <= 6000000; roll++ )
frequency[ 1 + rand() % 6 ]++;
Generate 6000000 random
integers in range 1 to 6
Increment frequency values at the
index associated with the random number
2008 Pearson Education, Inc. All rights reserved.
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Outline
cout << "Face" << setw( 13 ) << "Frequency" << endl;
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// output each array element's value
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for ( int face = 1; face < arraySize; face++ )
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cout << setw( 4 ) << face << setw( 13 ) << frequency[ face ]
<< endl;
(2 of 2)
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fig07_10.cpp
return 0; // indicates successful termination
36 } // end main
Face
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Frequency
1000167
1000149
1000152
998748
999626
1001158
2008 Pearson Education, Inc. All rights reserved.