Introduction to C Programming CE003121

1
Introduction to C ProgrammingCE00312-1

• Lecture 18
• Dynamic Memory Allocation
• and Ragged Arrays

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Allocation of Static Memory

• A declaration such as char name100021,
  allocates sufficient memory for 1000 names of 21
  characters each.
• This storage of 21 000 characters is reserved at
  compilation time, i.e. before the program begins
  execution.
• This wastes space as most names are rather less
  than 20 letters.
• It also restricts names to a maximum of 20
  characters.
• We can resolve this wastage and restriction by
  allocation of memory as and when required, during
  execution of the program.

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Dynamic Allocation using malloc function

• A standard function, malloc(n) allocates n bytes
  (for n characters) in memory and returns a
  pointer to it.
• For example the following code
• Prompts the user for the size of the string
• Reads the size
• Allocates memory for a string of this size
• Reads the string

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Example using malloc

• int size
• char s
• printf(\nhow many characters?\n)
• scanf(d, size)
• s malloc(size1) // one extra for \0
• printf(type string\n)
• gets(s)
• allocates only enough memory for the expected
  string.

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Use of malloc for any data type

• malloc can be used to allocate memory for any
  ltdata typegt.
• ltdata typegt ltvariablegt // pointer to ltdata
  typegt
• // some other code here
• ltvariablegt (ltdata typegt )malloc(n
  sizeof(ltdata typegt))
• allocates sufficient memory for n values of
  ltdata typegt and returns a pointer which must be
  casted as the same pointer type as the
  ltvariablegt.

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Using malloc for an array of integers

• For example
• To allocate storage for 10 integers
• int p
• p (int ) malloc (10 sizeof(int))
• This is machine-independent, as sizeof returns
• size of an integer for any machine.

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Ragged Arrays

• Declare 6 variable length names array of string
  pointers
• char name6
• abdul, abraham, al , bill, fred,
  jean-pierre
• We can declare functions on ragged arrays
• void print_list(char table1 , int n)
• so that it can deal with strings of any length.

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Ragged array example

• Read a list of strings (one on each line) into an
  array where
• the size of each element is precisely the length
  of each
• string, ie the strings are of variable length - a
  ragged array.
• include "stdio.h"
• include "string.h"
• include "stdlib.h" // for malloc
• // prototypes
• void sort(char , int)
• int read_strings(char , int)
• void print_strings(char , int)

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Main function

• int main(void)
• char list1000
• // array of 1000 string pointers
• int n // actual number of strings
• n read_strings(list, 1000)
• // read n strings, max of 1000
• sort(list, n) // sort n strings
• print_strings(list, n) // print n strings
• return (0)

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print_strings function

• void print_strings(char list, int n)
• int i
• printf("\nalphabetical order is\n\n")
• for (i0 iltn i)
• printf("s\n", listi)

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void sort(char a, int n) // array of n
strings as pointers int i, j char temp //
string pointer for (j n -1 j gt 0 j--) //
each pass of j comparisons for (i0 i lt j
i) // each comparison if
(strcmp(ai, ai1) gt 0) // swap
the POINTERS temp ai
ai ai1 ai1
temp // else no swap //
end of pass // end of all passes // end
sort
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int read_strings(char list, int max) int i
0 char line81 // store for each
line printf("\ntype one string per
line\n") while (gets(line) ! NULL i lt max)
// while not end of input listi
malloc (strlen(line)1) // listi big
enough // for line 1 for \0
strcpy(listi, line) // copy line
into listi i // next element return
i // actual number of strings