DYNAMICALLY ALLOCATED ARRAYS

1. DYNAMICALLY ALLOCATED ARRAYS Chapter 2

2. DYNAMIC MEMORY ALLOCATION • Most often we face situations in programming where the data is dynamic in nature. • That is ,the no of data items keep changing during the execution of the program. • Eg: consider a program for processing a list of customers of a corporation. The list grows when names are added and shrinks when names are deleted. • When list grows we need to allocate more memory space to the list to accomadate additional data items. • Such situations handled using dynamic data structures, in conjunction with dynamic memory management.

3. Con.. • Dynamic data structures provide flexibility in adding ,deleting or rearranging data items at runtime. • Dynamic memory management techniques permit us to allocate additional memory space or to release unwanted space at runtime ,thus, optimizing the use of storage space. • C language requires the no of elements in an array to be specified at compile time. But we may not be able to do so always. • Our initial judgment of size, if it is wrong, may cause failure of the program or wastage of memory space.

4. Con.. • Many languages permit a programmer to specify an array’s size at runtime. • Such languages have the ability to calculate and assign, during execution ,the memory space required by the variables in a program. • The process of allocating memory at runtime is known as dynamic memory allocation. • Although C does not inherently have this facility, there are 4 library routines known as "memory management functions". That can be used for allocating and freeing memory during program execution.

5. Memory allocation functions • Malloc:Allocates requested size of bytes and returns a pointer to the first byte of the allocated space. • Calloc: Allocates space for an array of elements ,initializes them to zero and returns a pointer to the memory. • Free: frees previously allocated space. • Realloc:modifies the space of previously allocated space

6. One dimensional arrays • #define MAX_SIZE 100 • float sum(float [],int); • float input [max_size],answer; • Void main(void) • { • int i; • for(i= 0;i<max_size ; i++) • input[i] = i ; • Answer=sum(input , max_size) • Printf(“the sum is :%f \n”, answer); • }

7. Con.. • float sum(float list [],int n) • { • int i; • float tempsum = 0; • for( i= 0;i<n ;i ++) • Tempsum+=list[i]; • return tempsum; • }

8. Con.. • In the previous program we defined the constant MAX_SIZE to have the value 101. • As a result the program, the program can be used to sort a collection of up to 101 no. • If the user wishes more than 101 no ,we have to change the definition of MAX_SIZE using some larger value and recompile the program. • how large should this value be ? If we set this MAX_SIZE to a very larger no (say several million ) • We reduce the likelihood the program will fail at runtime because the input value of n is less likely to exceed this large value of MAX_SIZE

9. Con.. • When writing computer programs ,we often find ourselves in a situation were we cannot reliably determine how large an array to use • A good solution to this is to defer this decision to runtime and allocate the array when we have a good estimate of the required array size. • So for eg we could change the first few lines of the main program to:

10. Con.. • int i, n *list; • Printf(“enter size for generation”); • Scanf(“%d”,& n); • If (n < 1) • { • fprintf(stderr,”improper value of n \n”); • Exit(exit_failure); • } • Malloc(list,n*sizeof(int)); • Now ,the program fails only when n<1or we donot have sufficient memory to hold the list of nos that are to be stored

11. 2-dimensional arrays • In this representation ,a two dimensional array is represented as a one dimensional array in which each element is itself ,a one-dimensional array . • int x [3] [5] ; • We actually create a one –dimensional array x whose length is 3;each element of x is a one dimensional array whose length is 5. • C finds the element x[i] [j] by first accessing the pointer in x[i]. • This pointer gives us the address ,in memory of the zeroth element of the row i of the array .

12. Con.. • Then by adding j*sizeof(int) to this pointor, the address of the [j] th element of row i. • i.e element x[i] [j] is determined. • The following program gives a function that creates a two dimensional array at runtime. • int **myarray; • Myarray=make2darray(5,10); • Myarray[2] [4]=6; • The second line allocates memory for a 5 by 10 two-dimensional array of integers and the third line assigns the value 6 to the [2][4] element of this array.

13. Dynamically create a 2-d array • int ** make2darray(int rows, int cols) • {/*create a two dimensional rows x cols array */ • Int **x, i; • /*get memory for row pointors */ • Malloc(x, rows*sizeof(*x));; • /*get memory for each row */ • for(i=0;i< rows; i++) • Malloc(x[i],cols*sizeof(**x)); • return x; • }

14. calloc • The function calloc allocates a user-specfied amount of memory and initializes the allocated memory to 0. • (i.e all allocated bits are set to 0) a pointor to the start of the allocated memory is returned. • In case there is insufficient memory to make the allocation ,the returned value is null.so,for eg • Int *x; • X= calloc(n,sizeof(int)); • Could be used to declare 1D array of integers.

15. calloc • The function calloc allocates a user-specfied amount of memory and initializes the allocated memory to 0. • (i.e all allocated bits are set to 0) a pointor to the start of the allocated memory is returned. • In case there is insufficient memory to make the allocation ,the returned value is null.so,for eg • Int *x; • X= calloc(n,sizeof(int)); • Could be used to declare 1D array of integers.

16. calloc • The capacity of this array is n and x[0:n-1] are initially 0. • As was the case with malloc ,it is useful to define the macro calloc as below and use this to write a programs. • #define calloc(p,n,s) \ • If( !((p) =calloc(n,s))) { \ • Fprintf(stderr,”insufficient memory”); • Exit(exit_failure); \ • }

17. realloc • The function realloc resizes memory previously allocated by either malloc or calloc. • realloc(p, s) • Changes the size of the memory block pointed at by p to s. • The contents of the first min { s,oldsize} bytes of the block are unchanged as a result of this resizing. • when realloc is able to do the resizing,it returns a pointor to the start of the new block. • And when it is unable to do the resizing, the old block is unchanged and the function returns a null value.

18. Con.. • As with malloc and calloc,it is useful to define a macro REALLOC as below. • #define RELLOC(P,S) \ • If(!((p) =realloc(p,s))) { \ • fprintf(stderr,”insufficient memory”); \ • exit(exit_failure);\ • }