Library Functions stdlib'h

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Library Functions stdlib.h

• This library contains many useful general purpose
  functions
• These functions can be broken down into six
  logical groupings
• String conversion
• Random number
• Memory allocation
• Communication with the environment
• Searching and sorting utilities
• Integer arithmetic

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String Conversion Functions

• These functions are used to convert strings to
  different data types
• This is useful when data is read into a string
  buffer and must be converted to a numeric value
  so that a calculation can be performed on it
• double atof(const char s)
• int atoi(const char s)
• long int atol(const char s)

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atoi and atof

• These functions are used to convert a string to
  an integer and a double value
• Leading blank characters are skipped
• Trailing illegal characters are ignored
• If conversion cannot be made zero is returned

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Example

• This example illustrates the function atoi

• 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

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Random Number Functions

• Random numbers are often needed in programs to
  simulate random events
• It is important to understand that these
  functions do not produce truly random numbers
• They produce pseudo-random numbers which are
  generated by a formula and are repeatable
• Some random number functions in this C library
  are
• int rand(void)
• void srand(unsigned int seed)

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int rand(void)

• This function returns a pseudo-random number in
  the range 0 to RAND_MAX which is at least 32767
  (215 1)

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void srand(unsigned int seed)

• It is possible to improve the randomness of a
  variable by changing the starting point of the
  random function
• The starting point is called a seed
• If the seed is set to something like a time
  varying variable such as time of day we can see
  how this could really improve the unrepeatability
  of the function
• The function srand is used to set the initial
  seed for the rand function
• If srand is not used the initial seed is 1

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Example

• This example illustrates the functions rand and
  srand

• include ltstdlib.hgt
• include ltstdio.hgt
• include lttime.hgt
• int main(void)
• int random
• srand((unsigned int)time(NULL))
• random rand()
• printf("i\n", random)
• system("PAUSE")

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Communication with the Environment

• These functions are used to control how the
  program interacts with the computer environment
  and include
• void abort(void)
• void exit(int status)
• int system(const char string)

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exit and abort

• These two functions cause the program to
  terminate
• The exit function terminates the program
  gracefully in other words it terminates the
  program and completes all the necessary
  termination procedures
• The abort function causes abnormal program
  termination

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int system(const char string)

• The system function passes its string argument to
  the operating systems command processor for
  execution

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Searching and Sorting Utilities

• These functions are used to preform a binary
  search function and a simple data sort
• They include
• void bsearch(const void key, const void base,
  size_t n, size_t size, int (cmp)()const void
  keyval, const void datum))
• void qsort(void base, size_t n size_t size, int
  (cmp)(const void, const void))

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qsort

• This function is used to arrange objects in an
  array base in ascending order according to the
  comparison function cmp
• base contains n objects each of size size
• cmp returns a
• Negative value if first argument is less than
  the second
• Zero if they are equal
• Positive value if it is greater

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bsearch

• This function searches an ordered array base (of
  n objects each of size size) for an item matching
  key according to a comparison function cmp
• cmp returns a
• Negative value if first argument is less than
  the second
• Zero if they are equal
• Positive value if it is greater
• Items in base are assumed to be sorted in
  ascending order
• It returns a pointer to an item matching key or
  NULL if none are found

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Integer Arithmetic

• This library also contains some basic arithmetic
  functions. For more mathematic operations use
  math.h
• These include
• int abs(int j)
• div_t div(int numer, int denom)
• long int labs(long int j)
• ldiv_t ldiv(long int numer, long int denom)

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abs and labs Functions

• The abs and labs function return the absolute
  value of their number arguments.
• The abs function works on integer values while
  the labs function works on long values

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Example

• This example illustrates the functions abs

• include ltstdlib.hgt
• include ltstdio.hgt
• int main(void)
• signed int num, absnum
• printf(Please enter an integer )
• scanf(i, num)
• absnum abs(num)
• printf(The absolute value of the integer is
  i\n,absnum)
• system(PAUSE)

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div and ldiv Functions

• These functions take two arguments
• numer which is the numerator
• demon which is the denominator
• They return a structure which holds two values
• The quotient quot
• The reminder rem
• div works on integer values while ldiv works on
  long values

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Example

• This example illustrates the functions div

• include ltstdlib.hgt
• include ltstdio.hgt
• int main(void)
• int num, denon
• div_t ans
• num 80
• denon 4
• ans div(num,denon)
• printf(The answer is Quotient i and Remainder
  i,ans.quot, ans.rem)
• system(PAUSE)