C programming notes - School of Physics

C programming notes
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28 of 40 19/03/2007 10:06 AM
first element is "table[0][0][0]", the next element is "table[0][0][1]", and the last element is "table[1][2][3]").
When writing programs that use large arrays (say, more than a few megabytes), you should be very careful to ensure
that array references in your program are as close as possible to being consecutive (otherwise you may get severe
swapping problems, i.e., the memory is swapped to hard disk, resulting in more than a factor of 1000 slow-down).
Initialisation of multidimensional arrays
This is best demonstrated with an example:
int mat[3][4] = {
}
Operations on arrays
{ 0, 1, 2, 23},
{ 3, 4, 5, 24},
{ 6, 7, 8, 25}
If you wish to, e.g., add two arrays together, you have to do it element-by-element yourself using a "for" loop. For
example:
int a0[10], a1[10], a2[10];
for (int i = 0; i < 10; i++) {
a0[i] = a1[i] + a2[i];
}
Even something as simple as setting one array to have the same value as another has to be done with a "for" loop. E.g. :
int a0[10], a1[10];
for (int i = 0; i < 10; i++) {
a0[i] = a1[i];
}
Character arrays
Arrays can be of any type. In C, you manipulate character strings as arrays of characters, and operations on the strings
(such as concatenation, searching) are done by calling special libary functions (e.g., strcat, strcmp).
Note that when calling a string-manipulation function, the end of the string is taken as the position of the first NUL
character (which has a numerical value of zero) in the string.
Character arrays can be initialised in the following ways:
char str[] = {'a', 'b', 'c'};
char prompt[] = "please enter a number";
In the example, "str" has length 3 bytes, and "prompt" has length 22 bytes, which is one more than the number of
characters in "please enter a number", the extra byte is used to store a NUL character (zero) as an indication of the end
of the string. "str" does not having a trailing NUL.
Functions
A function call allows a commonly used piece of code to be made available through a single calling statement. For
instance, to calculate the square of a number we could use the following (floating point) function:
float sqr(inval) /* square a float, return a float */
float inval;
{
float square;
square = inval * inval;
return(square);