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C Programming For Beginners - Part 13

written by: Noel Kalicharan•edited by: J. F. Amprimoz•updated: 10/7/2008

All programming languages define the concept of a 'token'. In this article, we explain what C tokens are. We also show how comments are written in C.

  • slide 1 of 3

    C tokens

    The tokens of a language are the basic building blocks which can be put together to construct programs. A token can be a reserved word (such as int or while), an identifier (such as b or sum), a constant (such as 25 or "Alice in Wonderland"), a delimiter (such as { or ;) or an operator (such as + or =).

    For example, consider the following portion of the program we met in this article:

    main() {int a, b, sum;a = 14;b = 25;sum = a + b;printf("%d + %d = %d\n", a, b, sum);
    }

    Starting from the beginning, we can list the tokens (in bold) in order:

    main - identifier

    ( - left bracket, delimiter

    ) - right bracket, delimiter

    { - left brace, delimiter

    int - reserved word

    a - identifier

    , - comma, delimiter

    b - identifier

    , - comma, delimiter

    sum - identifier

    ; - semicolon, delimiter

    a - identifier

    = - equals sign, operator

    14 - constant

    ; - semicolon, delimiter

    and so on. Thus we can think of a program as a ‘stream of tokens’, which is precisely how the compiler views it. So that, as far as the compiler is concerned, the above could have been written:

    main() { int a, b, sum;
    a = 14; b = 25; sum = a + b;
    printf("%d + %d = %d\n", a, b, sum); }

    The order of the tokens is exactly the same; to the compiler, it is the same program. To the computer, only the order of the tokens is important. However, layout and spacing are important to make the program more readable to human beings.

  • slide 2 of 3

    Comments in C

    All programming languages let you include comments in your programs. Comments can be used to remind yourself (and others) of what processing is taking place or what a particular variable is being used for. They can be used to explain or clarify any aspect of a program which may be difficult to understand by just reading the programming statements.

    This is very important since the easier it is to understand a program, the more confidence you will have that it is correct. It is worth adding anything which makes a program easier to understand.

    Remember that a comment (or lack of it) has absolutely no effect on how the program runs. If you remove all the comments from a program, it will run exactly the same way as with the comments.

    Each language has its own way of specifying how a comment must be written. In C, we write a comment by enclosing it within /* and */, for example:

    /* This program prints a greeting */

    A comment extends from /* to the next */ and may span one or more lines. The following is a valid comment:

    /* This program reads characters one at a time

    and counts the number of letters found */

    C also lets you use // to write one-line comments. The comment extends from // to the end of the line, for example:

    a = s * s; //calculate area; store in a

    In this series, we will use mainly one-line comments.