Introduction to C Programming CE Lecture 18 Dynamic Memory Allocation and Ragged Arrays

Allocation of Static Memory A declaration such as char name[1000][21], allocates sufficient memory for 1000 names of 21 characters each. This storage of 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.

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

Example using malloc int size; char *s; printf(“\nhow many characters?\n”); scanf(“%d”, &size); s = malloc(size+1);// one extra for ‘\0’ printf(“type string\n”); gets(s); allocates only enough memory for the expected string.

Use of malloc for any data type malloc can be used to allocate memory for any. * ; // pointer to // some other code here = ( *)malloc(n * sizeof( )); allocates sufficient memory for n values of and returns a pointer which must be casted as the same pointer type as the.

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.

Ragged Arrays Declare 6 variable length names – array of string pointers char *name[6] = {“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.

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);

Main function int main(void) { char *list[1000]; // 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); }

print_strings function void print_strings(char *list[], int n) { int i; printf("\nalphabetical order is\n\n"); for (i=0; i<n; i++) { printf("%s\n", list[i]); }

void sort(char *a[], int n) {// array of n strings as pointers int i, j; char *temp;// string pointer for (j = n -1; j > 0; j--) {// each pass of j comparisons for (i=0; i < j; i++) {//each comparison if (strcmp(a[i], a[i+1]) > 0) {// swap the POINTERS temp = a[i]; a[i] = a[i+1]; a[i+1] = temp; }// else no swap }// end of pass }// end of all passes } // end sort

int read_strings(char *list[], int max) { int i = 0; char line[81];// store for each line printf("\ntype one string per line\n"); while (gets(line) != NULL && i < max) {// while not end of input list[i] = malloc (strlen(line)+1); // list[i] big enough // for line + 1 for ‘\0’ strcpy(list[i], line); // copy line into list[i] i++;// next element } return i; // actual number of strings }