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C programming notes file:///F:/my_docs/web_phys2020/Cprogrammingnotes.html 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.
C programming notes file:///F:/my_docs/web_phys2020/Cprogrammingnotes.html 25 of 40 19/03/2007 10:06 AM Minimum field width: the output field will be at least this wide, and wider if necessary. "." separates the field width from the precision. Precision: its meaning depends on the type of object being printed: Character: the maximum number of characters to be printed. Integer: the minimum number of digits to be printed. Floating point: the number of digits after the decimal point. Exponential format: the number of significant digits. Length modifer: "h" means short, or unsigned short; "l" means long or unsigned long; "L" means long double. Conversion character: a single character specifying the type of object being printed, and the manner in which it will be printed, according to the following table: Character Type Result d,i int signed decimal integer o int unsigned octal (no leading zero) x, X int unsigned hex (no leading 0x or 0X) u int unsigned decimal integer c int single character s char * characters from a string f double floating point [-]dddd.pppp e, E double exponential [-]dddd.pppp e[=/-]xx g, G double floating if the exponent less than -4 or >= precision else exponential p void * pointer n int * the number of characters written so far by printf is stored into the argument (i.e., not printed) % print % Here is an example program (printfexample.c) to show some of these effects (try running the program yourself): #include int main(void) { int i = 123; double f = 3.14159265358979323846; printf("i = %i\n", i); printf("i = %o\n", i); printf("i = %x\n", i); printf("i = %X\n", i); printf("i = %+i\n", i); printf("i = %8i\n", i); printf("i = %08i\n", i); printf("i = %+08i\n", i); printf("f = %f\n", f); printf("f = %10.3f\n", f); printf("f = %+10.3f\n", f); printf("f = %g\n", f); printf("f = %10.6g\n", f); printf("f = %10.6e\n", f); } return 0; Notes: There are differences between the various implementations of "printf". The above should be a subset of the available options. Consult the manual (e.g., "info gcc") for your particular C compiler to be sure. "printf" is actually an integer function, which returns the number of characters written (or a negative number if an error occurred). Formatted input: scanf overview Input in C is similar to output: the same conversion characters are used. The main difference is that you use a routine called "scanf", and you must pass the addresses of the variables you want to change, not their values. For example: scanf("%d", &i); /* reads an integer into `i' */ scanf("%i", &i); /* reads an integer (or octal, or hex) into `i' */ scanf("%f %i", &f, &i); /* reads a double followed by an integer */ scanf is actually an integer function, which returns the number of input items assigned (or the integer constant EOF (defined in stdio.h) if the end-of-file is reached or an error occurred).
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C programming notes - School of Physics

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