Lesson 8.8. Random numbers in C

Standard C libraries

Most C compilers come with a collection of standard, standardized libraries containing common operations (display, math calculations, reading and writing to files ...). These libraries mainly contain functions which can be used by including the header of the library concerned. For example, to use the printf()`` function, you must include thestdio.h` library:

#include <stdio.h>

stdlib.h

The sdtlib.h library contains various basic functions (memory allocation, conversion of variables ...). The functions we'll focus one here are the srand () and rand () functions which generate random numbers (or more exactly pseudo-random numbers, but we will come back to that).

We will focus on the following elements of the stdlib.h library:

// Symbolic constant
#define RAND_MAX

// Functions
int rand(void);
void srand(unsigned int seed);

Random numbers

The rand () function generates a pseudo-random number between 0 and RAND_MAX. RAND_MAX is a symbolic constant (or macro) defined instdlib.h. This constant contains the maximum value returned by the rand () function.

Here is a basic example:

// Displays the maximum value of the pseudo-random generator
printf ("Maximum value: %d\n", RAND_MAX);

// Pick two random numbers
printf ("A random number: %d\n", rand());
printf ("Another random number: %d\n", rand());

If you run the above code multiple times, you will find that it always draw the same numbers.

Pseudorandom numbers

A processor is a deterministic machine which cannot generate real random numbers, moreover the general notion of randomness remains debatable. The numbers generated by the rand () function are [pseudo-random numbers] (https://en.wikipedia.org/wiki/Pseudorandom_number_generator) A pseudo-random algorithm generates a sequence of numbers which have certain properties, such as equiprobability. If the previous program still picks the same numbers, it is because they are the first of the sequence. Function srand () for seed random allows to define the seed generator and thus modify the starting point of the sequence. To avoid always picking the same random numbers, we conventionally use the current time as seed of the generator:

#include <stdlib.h>
#include <time.h>

// Initialize the seed of the pseudorandom generator
srand(time(NULL));

Now, on each run (at least one second apart from the previous one) the program displays different numbers.

Pick a random number in an interval

Most of the time, we need to pick numbers in a given interval. If we wish picking a number between 0 and max, the best solution is to use the rest of the integer division (modulo %):

// x is a pseudirandom number between 0 and max included
int x = rand() % (max+1);

If you want a lower bound, you have to shift the draw by adding the bound inferior :

// x is a pseudorandom number between min and max included
int x = min + rand() % (max + 1 - min);

Picking a random floating point number

It is also common to have to pick a real number. The trick is to divide the number generated by RAND_MAX, and thus obtain a result between 0 and 1:

// x is a random floating point number between 0 et 1
float x = (float)rand()/(float)(RAND_MAX);

Note the change of type (cast int ->float) in order to perform the division on floating. By dividing RAND_MAX by the boundmax, we can obtain the drawing a real number between 0 and max:

// x is a pseudorandom floating point number between 0 and max
float x = (float)rand() / ((float)RAND_MAX/max);

Finally, by shifting the interval, we get a floating point number in the interval [min; max] as seen previously:

// x is a pseudorandom floating point number between min and max
float x = min + (float)rand() / ((float)RAND_MAX/(max-min));

Exercises

Exercise 1

Write a program that simulates the draw of two 6-sided dice:

Dace 1 : 6
Dace 2 : 1

#include <stdio.h> // COMPLETE HERE int main(void) { // Initalize the pseudorandom generator // COMPLETE HERE // Draw of the two dice // COMPLETE HERE return 0; }

Exercise 2

To assess students, a teacher needs 24 grades in the range [0; 20]. Write a program that generates these 24 notes:

Grade #1 : 9.2
Grade #2 : 13.1
Grade #3 : 1.1
Grade #4 : 13.2
...
Grade #21 : 17.5
Grade #22 : 18.4
Grade #23 : 19.7
Grade #24 : 15.7

#include <stdio.h> #include <stdlib.h> #include <time.h> int main(void) { // Initalize the pseudorandom generator srand(time(NULL)); // Generate 24 grade between 0 and 20 // COMPLETE HERE return 0; }

Exercise 3

Write a function rangedom(int a, int b) which pick an integer in the interval [a; b]. Draw and display 100 random numbers between 8 and 12 included:

8 9 8 11 9 9 12 12 8 8 8 12 12 11 10 11 12 10 11 11 8 11 10 11 10 9 8 10 10 10 12 
10 8 9 8 9 10 10 8 12 10 10 9 9 8 11 10 10 10 10 8 10 8 8 11 12 11 8 11 8 12 10 11 
12 8 11 8 12 10 10 12 10 8 8 11 10 8 10 9 11 12 10 10 9 12 10 9 10 11 12 11 10 12 
9 10 9 9 10 9 12

#include <stdio.h> #include <stdlib.h> #include <time.h> // Return a random number between min and max int randMinMax(int a, int b); int main(void) { // Initalize the pseudorandom generator srand(time(NULL)); // Pick 100 random numbers between 8 and 12 // COMPLETE HERE printf("\n"); return 0; } // Return a random number between min and max // COMPLETE HERE

Exercise 4

Write a randFloat() function that pick a pseudo-random number between 0 and 1. Pick one million numbers and display the average. The average should be close to 0.5:

Average = 0.5003

#include <stdio.h> #include <stdlib.h> #include <time.h> #define N_TITRAGES 1e6 // Pick a random number between 0 and 1 // COMPLETE HERE int main(void) { double somme=0; // Initalize the pseudorandom generator srand(time(NULL)); // Pick 1 000 000 random numbers between 0 and 1 // and compute the sum iteratively // COMPLETE HERE // Displays the average // COMPLETE HERE printf ("\n"); return 0; } // Pick a random number between 0 and 1 // COMPLETER ICI

Quiz

Are the standard libraries supplied with all C compilers?

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Are the standard C libraries compatible from one compiler to another?

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What really does a pseudo-random generator?

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What is this line of code for?

srand(time(NULL));

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What does the following code do?

x = 5 + rand()%10;

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See also

• Learn C programming
• Lesson 1.1. History of C programming language
• Lesson 1.2. First program
• Lesson 1.3. Compilation
• Lesson 1.4. Preprocessor directives
• Lesson 1.5. Which compiler to choose?
• Lesson 1.6. Flowchart
• Lesson 2.1. Types and variables
• Lesson 2.2. Integers
• Lesson 2.3. Floating point numbers
• Lesson 2.4. Characters
• Lesson 2.5. Variables initialization
• Lesson 2.6. Ariane flight 501
• Lesson 3.1. Arithmetic operators
• Lesson 3.2. modulo
• Lesson 3.3. Operators and types
• Lesson 3.4. Type casting
• Lesson 3.5. Bitwise operators
• Lesson 3.6. Bitwise operators details
• Lesson 3.7. Shifting operators
• Lesson 3.8. Assignment operators
• Lesson 3.9. Increment and decrement operators
• Lesson 3.10. Comparison operators
• Lesson 3.11. Logical operators
• Lesson 3.12. Operator precedence
• Lesson 4.1. printf
• Lesson 4.2. scanf
• Lesson 4.3. putchar
• Lesson 5.1. Conditional branching (if...else)
• Lesson 5.2. Nested if and indentation
• Lesson 5.3. Checking intervals
• Lesson 5.4. Ternary Operator ?:
• Lesson 5.5. switch..case statement
• Lesson 5.6. Break keyword in switch..case
• Lesson 6.1. do..while loop in C
• Lesson 6.2. While loop in C
• Lesson 6.3. For loop in C
• Lesson 6.4. How to choose the right loop in C ?
• Lesson 6.5. Exercices on loops in C
• Lesson 7.1. Bit masking
• Lesson 7.2. Bit masking, how to clear a bit in C?
• Lesson 7.3. Bit masking, how to set a bit in C?
• Lesson 7.4. Bit masking, how to toggle a bit in C?
• Lesson 7.5. Bit masking, how to check a single bit in C?
• Lesson 7.6. Overview of bit masking
• Lesson 8.1. Basic syntax of C functions
• Lesson 8.2. Calling C functions
• Lesson 8.3. Void keyword
• Lesson 8.4. Return keyword
• Lesson 8.5. Variable scope
• Lesson 8.6. Global variables
• Lesson 8.7. Static variables
• Lesson 8.9. Mathematical functions in C
• Lesson 9.1. Syntax of C arrays
• Lesson 9.2. Array initialization
• Lesson 9.3. Multidimensional arrays
• Lesson 9.4. How C arrays are stored in memory?
• Lesson 9.5. Arrays and functions
• Lesson 9.6. Exercises on C arrays
• Lesson 10.1. Strings in C
• Lesson 10.2. Null terminated string
• Lesson 10.3. The string.h library
• Lesson 10.4. String and functions
• Lesson 11.1. Introduction to pointers in C
• Lesson 11.2. Syntax of C pointers
• Lesson 11.3. Dynamic memory allocation
• Lesson 11.4. Incrementing pointers
• Lesson 11.5. Pointers as function argument in C
• Lesson 12.1. Introduction to structures in C
• Lesson 12.2. Properties of structures in C
• Lesson 12.3. Structures and pointers
• Lesson 12.4. Structures and functions
• Lesson 13.1. Recursive functions in C
• Lesson 13.2. Depth of recursive functions
• Lesson 13.2. Mutual recursion
• Lesson 14.1. Additional exercises

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