C programming notes - School of Physics

C programming notes
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24 of 40 19/03/2007 10:06 AM
}
break;
default: printf("i was outside the range 0 to 2\n");
Here is an example with multiple switch values (switch.c)
Assert as a tool for improving program reliability
By now you will be aware that it is very easy to write buggy programs in C, and it may not always be obvious at
runtime that there is a problem. Checking for all possible error conditions in a program can be tedious. A useful tool to
improve program reliability is the "assert" construct defined in the system include-file "assert.h". An example
(assertexample.c) will clarify this:
#include
#include
int main(void) {
int i;
}
printf("enter a positive integer > ");
assert(1 == scanf("%i", &i));
assert(i > 0);
printf("you successfully entered %i\n", i);
return 0;
If the expression inside parentheses after "assert" is true, the program continues as normal, if the expression is false (i.e,
zero), then the program aborts with an error message.
This may sound like a trivial concept, but by using "assert" extensively, you will be surprised at how your
programming improves.
It is particularly useful to use "assert" to check for conditions that should never arise in your program, e.g., suppose you
write a function that should only ever be called with an argument between -1.0 and 1.0, then, use "assert" in the
function to check this, e.g.,
double myAsin(double x) {
assert(fabs(x) < 1.0);
...
}
Also, you can use assert to check for unexpected conditions such as failure to open a file, or failure to allocate memory,
e.g.,
assert(NULL != (fp = fopen("data.dat", "r")));
assert(NULL != (p = malloc(1024));
Formatted output: printf description
The format string given to the "printf" function may contain both ordinary characters (which are simply printed out)
and conversion characters (beginning with a percent symbol, %, these define how the value of an internal variable is to
be converted into a character string for output).
Here is the syntax of a conversion specification:
%{flags: - + space 0 #}{minimum field width}{.}{precision}{length modifier}{conversion character}
Flags: "-" means left-justify (default is right-justify), "+" means that a sign will always be used, " " prefix a
space, "0" pad to the field width with zeroes, "#" specifies an alternate form (for details, see the manual!). Flags
can be concatenated in any order, or left off altogether.