jose@shell (04/07/90)
There is an efficient way for using arrays that grow in size dynamically.
The following three programs and their execution times on a SPARCstation 1
make the point.
--------------------------------------
Program 1.
float old_array[10000000];
main () {
int i, j1, j2;
for (i=0; i<10000000; i++) old_array[i] = (float) i * 2. + 0.5;
}
-------------------------------------
Program 2.
/* declare an array of pointers to float; each element of array[] will be
pointing to an array of floating-point numbers. */
float *array[160];
/* SIZE is the size of the array of floating-point numbers. */
#define SIZE 64000
/* define the macro Array(n) to index into the appropriate array */
#define Array(n) *(array[n/SIZE] + (n%SIZE))
main () {
int i;
for (i=0; i<160; i++) array[i] = (float *) malloc (SIZE * sizeof(float));
for (i=0; i<10000000; i++) Array(i) = (float) i * 2. + 0.5;
}
--------------------------------------
Program 3.
/* declare an array of pointers to float; each element of array[] will be
pointing to an array of floating-point numbers. */
float *array[160];
/* SIZE is the size of the array of floating-point numbers. We'll select
SIZE to be a power of two, in order to avoid function calls. */
#define SIZE 64*1024
/* define the macro Array(n) to index into the appropriate array */
#define Array(n) *(array[n>>16] + (n&0xFFFF))
main () {
int i;
for (i=0; i<160; i++) array[i] = (float *) malloc (SIZE * sizeof(float));
for (i=0; i<10000000; i++) Array(i) = (float) i * 2. + 0.5;
}
---------------------------------------
The execution times measured with the C shell 'time' were in seconds:
Program user system wall-clock
1 29.3 12.2 118
2 113.3 12.4 167
3 28.3 9.9 106
---------------------------------------
Jose Barros
Staff Research Computer Scientist
Shell Development Co.
3737 Bellaire
Houston, Tx 77025
{bcm,rice}!shell!jose --- uucp; jose@shell.com --- internet
713-663-2581 --- FAX; 713-663-2660 --- Voice
Jose C. Barros
Senior Research Computer Scientist
Shell Development Company, Bellaire Research Center, Houston (713) 663-2660
..!{sun,psuvax,bcm,rice,cs.texas.edu,decwrl}!shell!jose jose@shell.uucp