To use all functions in this library you must:

`#include <stdlib.h>`

There are three basic categories of functions:

- Arithmetic
- Random Numbers
- String Conversion

The use of all the functions is relatively straightforward. We only consider them briefly in turn in this Chapter.

There are 4 basic integer functions:

int abs(int number); long int labs(long int number); div_t div(int numerator,int denominator); ldiv_t ldiv(long int numerator, long int denominator);

Essentially there are two functions with integer and long integer compatibility.

`abs`- functions return the absolute value of its
`number`arguments. For example, abs(2) returns 2 as does abs(-2). `div`- takes two arguments,
`numerator`and`denominator`and produces a quotient and a remainder of the integer division. The`div_t`structure is defined (in`stdlib.h`) as follows:typedef struct { int quot; /* quotient */ int rem; /* remainder */ } div_t;

(

`ldiv_t`is similarly defined).Thus:

#include <stdlib.h> .... int num = 8, den = 3; div_t ans; ans = div(num,den); printf("Answer:\n\t Quotient = %d\n\t Remainder = %d\n", \ ans.quot,ans.rem);

Produces the following output:

Answer: Quotient = 2 Remainder = 2

Random numbers are useful in programs that need to simulate random
events, such as games, simulations and experimentations. In practice no
functions produce truly random data -- they produce *pseudo-random*
numbers. These are computed form a given formula (different generators
use different formulae) and the number sequences they produce are
repeatable. A *seed* is usually set from which the sequence is
generated. Therefore is you set the same seed all the time the same set
will be be computed.

One common technique to introduce further randomness into a random number generator is to use the time of the day to set the seed, as this will always be changing. (We will study the standard library time functions later in Chapter 20).

There are many (pseudo) random number functions in the standard library. They all operate on the same basic idea but generate different number sequences (based on different generator functions) over different number ranges.

The simplest set of functions is:

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

srand( (unsigned int) time( NULL ));

The following program `card.c` illustrates the use of these functions
to simulate a pack of cards being shuffled:

/* ** Use random numbers to shuffle the "cards" in the deck. The second ** argument indicates the number of cards. The first time this ** function is called, srand is called to initialize the random ** number generator. */ #include <stdlib.h> #include <time.h> #define TRUE 1 #define FALSE 0 void shuffle( int *deck, int n_cards ) { int i; static int first_time = TRUE; /* ** Seed the random number generator with the current time ** of day if we haven't done so yet. */ if( first_time ){ first_time = FALSE; srand( (unsigned int)time( NULL ) ); } /* ** "Shuffle" by interchanging random pairs of cards. */ for( i = n_cards - 1; i > 0; i -= 1 ){ int where; int temp; where = rand() % i; temp = deck[ where ]; deck[ where ] = deck[ i ]; deck[ i ] = temp; } }

There are several other random number generators available in the standard library:

double drand48(void); double erand48(unsigned short xsubi[3]); long lrand48(void); long nrand48(unsigned short xsubi[3]); long mrand48(void); long jrand48(unsigned short xsubi[3]); void srand48(long seed); unsigned short *seed48(unsigned short seed[3]); void lcong48(unsigned short param[7]);

This family of functions generates uniformly distributed pseudo-random numbers.

Functions drand48() and erand48() return non-negative
double-precision floating-point values uniformly distributed
over the interval `[0.0, 1.0)`

.

Functions lrand48() and nrand48() return non-negative long
integers uniformly distributed over the interval `[0, 2**31)`

.

Functions mrand48() and jrand48() return signed long
integers uniformly distributed over the interval
`[-2**31, 2**31)`

.

Functions srand48(), seed48(), and lcong48() set the seeds for drand48(), lrand48(), or mrand48() and one of these should be called first.

Further examples of using these functions is given is Chapter 20.

There are a few functions that exist to convert strings to integer, long integer and float values. They are:

`double atof(char *string)`

-- Convert string to floating point value.
`int atoi(char *string)`

-- Convert string to an integer value
`int atol(char *string)`

-- Convert string to a long integer
value.
`double strtod(char *string, char *endptr)`

-- Convert string
to a floating point value.
`long strtol(char *string, char *endptr, int radix)`

--
Convert string to a long integer using a given radix.
`unsigned long strtoul(char *string, char *endptr, int radix)`

-- Convert string to unsigned long.

Most of these are fairly straightforward to use. For example:

char *str1 = "100"; char *str2 = "55.444"; char *str3 = " 1234"; char *str4 = "123four"; char *str5 = "invalid123"; int i; float f; i = atoi(str1); /* i = 100 */ f = atof(str2); /* f = 55.44 */ i = atoi(str3); /* i = 1234 */ i = atoi(str4); /* i = 123 */ i = atoi(str5); /* i = 0 */

**Note**:

- Leading blank characters are skipped.
- Trailing illegal characters are ignored.
- If conversion cannot be made zero is returned and
`errno`(See Chapter 17) is set with the value`ERANGE`.

The `qsort` standard library function is very useful function that is
designed to sort an array by a *key* value of *any type* into
ascending order, as long as the elements of the array are of fixed type.

qsort is prototyped (in `stdlib.h`):

void qsort(void *base, size_t num_elements, size_t element_size, int (*compare)(void const *, void const *));

Similarly, there is a binary search function, `bsearch()` which is
prototyped (in `stdlib.h`) as:

void *bsearch(const void *key, const void *base, size_t nel, size_t size, int (*compare)(const void *, const void *));

Using the same `Record` structure and `record_compare` function
as the `qsort()` example (in Chapter 11.3):

typedef struct { int key; struct other_data; } Record; int record\_compare(void const *a, void const *a) { return ( ((Record *)a)->key - ((Record *)b)->key ); }

Also, Assuming that we have an `array` of `array_length Record`s
suitably filled with date we can call `bsearch()` like this:

Record key; Record *ans; key.key = 3; /* index value to be searched for */ ans = bsearch(&key, array, arraylength, sizeof(Record), record_compare);

The function `bsearch()` return a pointer to the field whose key
filed is filled with the matched value of `NULL` if no match found.

Note that the type of the `key` argument **must** be the same as
the array elements (`Record` above), even though only the `
key.key` element is required to be set.

**Exercise 12534**

Write a program that simulates throwing a six sided die

**Exercise 12535**

Write a program that simulates the UK National lottery by selecting six different whole numbers in the range 1 - 49.

**Exercise 12536**

Write a program that read a number from command line input and generates a random floating point number in the range 0 - the input number.