C programming notes - School of Physics

C programming notes
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36 of 40 19/03/2007 10:06 AM
of memory.
After freeing memory, it is good practice to set any pointers that referred to it to NULL, this will prevent you
from inadvertantly using deallocated memory.
The memory that malloc allocates can have random data in it. If you need it to be zeroed, either do this yourself
with a "for" loop of use the "calloc" function (see "man calloc").
Here is an example to get you started (mallocexample.c):
#include
#include
#include
/* Generates a two dimensional matrix, and fills it randomly with
zeroes and ones. */
int main(void) {
int xdim, ydim;
int i, j;
int *p, *q;
// Obtain the dimensions of the matrix from the user.
printf("x dimension of matrix? > ");
scanf("%d", &xdim);
printf("y dimension of matrix? > ");
scanf("%d", &ydim);
// malloc the memory needed for the matrix. Note the use of "sizeof(int)"
// which allows the program to run on machines with a variety of word sizes.
assert(NULL != (p = (int *)malloc(xdim * ydim * sizeof(int))));
// Fill the matrix with random 0s and 1s.
for (i = 0; i < xdim * ydim; i++) {
if (random() > RAND_MAX/2) {
*(p+i) = 1;
} else {
*(p+i) = 0;
}
}
// Now print the matrix out, showing the use of pointer arithmetic.
for (i = 0; i < ydim; i++) {
q = p + i * xdim;
for (j = 0; j < xdim; j++) {
printf("%1d", *(q++));
}
printf("\n");
}
// Free the memory we had allocated.
free((void *)p);
// And remove all information about where the memory used to be.
// This is overkill in this example, since the program immediately
// terminates, but it is good practice, in order to avoid
// accidentally using free'ed memory.
}
p = q = NULL;
return 0;
Generating random numbers
Random numbers are used in all sorts of ways in mathematics and physics. They are a fundamental tool that is often
useful when conducting numerical simulations of physical systems with a computer.
Writing a computer program to generate random numbers requires understanding some fairly subtle concepts.
There are several different algorithms for generating a new random number given an initial random number. One of the