C Basic, commments
Character Set
The characters that can be used to from words, numbers and expressions depend upon the computer on which the program is run. However, a subset of characters is available that can be used on most personal, micro mini and mainframe computers to form a standard program. The characters in C are grouped into the following categories :
- Letters
- Digits
- Special characters
- White spaces
A complete table of character set
Letters:
A to Z in uppercase or capital letters.
a to z in lowercase or small letters.
Digits :
0, 1, 2, 3, 4, 5, 6, 7, 8, 9
Special Characters :
| Character | Description | Character | Description |
|---|---|---|---|
| , | comma | & | ampersand |
| . | period | ^ | caret |
| ; | semicolon | * | asterisk |
| : | colon | - | minus sign |
| ? | question mark | + | plus sign |
| ' | apostrophe | < | opening angular bracket or less than sign |
| " | quotation mark | > | closing angular bracket or greater than sign |
| ! | exclamation mark | ( | left parenthesis |
| | | vertical bar | ) | right parenthesis |
| / | slash | [ | left square bracket |
| \ | backslash | ] | right square bracket |
| ~ | tiled | { | left brace |
| _ | under score | } | right brace |
| $ | doller sign | % | per cent sign |
| # | number sign |
White Spaces:
Space, line-feed, tab, form-feed, carriage-return, vertical-tab, and new-line characters are called "white-space characters" which are not visible on the screen.
White-space characters enhance readability of the program files and do same job as spaces between words and lines in the source code. C compiler ignores white-space characters at the time of reading the source code unless you use the white-space characters in string literals.
Trigraph Characters
The source character set of C source programs is contained within seven-bit ASCII character set, but is a superset of the ISO 646-1983 Invariant Code Set.
Trigraphs are a sequence of three characters starting with two consecutive question marks(??) followed by another character and allow the compiler to replace with their corresponding punctuation characters. Trigraph sequences are used in C program files in some keyboards which do not support some characters mentioned above table (Special Characters). The following table shows the list of trigraph sequences followed by an example.
| Trigraph sequence | Punctuation character | Description |
|---|---|---|
| ??= | # | comma |
| ??( | [ | period |
| ??) | ] | semicolon |
| ??< | { | colon |
| ??> | } | question mark |
| ??! | | | apostrophe |
| ??/ | \ | quotation mark |
| ??' | ^ | exclamation mark |
| ??- | ~ | vertical bar |
For example, if you attempt to print the string ??<character??> with this printf statement
#include <stdio.h>
main()
{
printf( "??< Character ??>" );
}
the string printed is { Character } because ??< and ??> is trigraph sequences that are replaced with the "{" and "}" character.
Comments
Comments are a good way to write notations to explain what a program does. Comments can appear anywhere in a program. C compiler ignores comments at the time of reading the source code
C language supports the following two different ways of commenting.
Starts with a "/*" and ends with "*/" at the end of the comment. Comments in C language can occupy more than one line but cannot be nested. Comments can appear anywhere in a C program. This following example is a comment accepted by the compiler:
/* Comments in C language can
occupy more than one lines */
printf( "Welcome to C programming" );
Comments can appear in the same line after the statement:
printf( "Welcome to C programming" ); /* Comments can come in same line after the statement */
Comments in C language cannot contain nested comments. The example bellow causes an error:
/* This is the outer comments
/*Start to write statements*/
printf( "Welcome to C programming" );
*/
Some compilers also support single-line comment preceded by two forward slashes (//). Here is an example.
// This is a single line comment.
printf( "Welcome to C programming" );
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