stub section

This subchapter is a stub section. It will be filled in with instructional material later. For now it serves the purpose of a place holder for the order of instruction.

Professors are invited to give feedback on both the proposed contents and the propsed order of this text book. Send commentary to Milo, PO Box 1361, Tustin, California, 92781, USA.

while loop

The next important repetition or loop structure is the DO WHILE loop.

Historically the first looping structure available was the iterative do loop. The DO WHILE loop was the form of loop used in the original proof of the concept of structured programming.

In the DO WHILE loop there is a condition test and the loop is repeated over and over again as long as the condition remains true.

We can use the DO WHILE loop to add flexibility to our average example. So far we have always had to have exactly five numbers to average. Using this method, we would need to write a whole new program for a list of six numbers to average and another program for a list of ten numbers to average. This would get ridicously out of hand in a hurry.

By using a DO WHILE loop, we can write one program for any size list of numbers to average:

    Initialize;
    BEGIN
        Subtotal := 0;
        Count := 0;
        IF there are no numbers available to average
            THEN REPORT an ERROR and STOP;
    END;
    DO WHILE there are still numbers available;
    BEGIN
        Add the next number to the Subtotal;
        Increment (add one to) the Count;
    END;
    Finish;
    BEGIN
        IF count NOT EQUAL zero (0)
            THEN average := subtotal / count;
        ELSE
            Report ERROR;
    END;

An important note regarding the DO WHILE loop (and loops in general) is that the loop needs to come to an end some time. If the loop never ends it is called an infinite loop and is one of the reasons that a program or computer freezes. The computer hasn’t actually stopped working (which is implied by the word freeze). Rather the program is stuck in a loop that it will never finish (until you turn the computer off, reboot, stop the program, etc.).

It is important to check for and avoid inifinite loops. Make sure that any loop you write will eventually come to an end.

C

Note that unlike most programming languages, C does not have an assignment statement, but rather has an assignment operator. This allows assignment to be compacted into other programming constructs, which can lead to confusion.

This feature can be used to make very terse (and confusing) code:

while (*p++ = *q++) ;

The above loop copies copies data from one string or block of data to another until a zero is encountered.

Stanford C essentials

Stanford CS Education Library This [the following section until marked as end of Stanford University items] is document #101, Essential C, in the Stanford CS Education Library. This and other educational materials are available for free at http://cslibrary.stanford.edu/. This article is free to be used, reproduced, excerpted, retransmitted, or sold so long as this notice is clearly reproduced at its beginning. Copyright 1996-2003, Nick Parlante, nick.parlante@cs.stanford.edu.

No Boolean -- Use int

C does not have a distinct boolean type-- int is used instead. The language treats integer 0 as false and all non-zero values as true. So the statement…

    i = 0;
    while (i - 10) {
       ...

will execute until the variable i takes on the value 10 at which time the expression (i - 10) will become false (i.e. 0).

While Loop

The while loop evaluates the test expression before every loop, so it can execute zero times if the condition is initially false. It requires the parenthesis like the if.

    while (<expression>) {
       <statement>
    }

Break

The break statement will move control outside a loop or switch statement. Stylistically speaking, break has the potential to be a bit vulgar. It’s preferable to use a straight while with a single test at the top if possible. Sometimes you are forced to use a break because the test can occur only somewhere in the midst of the statements in the loop body. To keep the code readable, be sure to make the break obvious -- forgetting to account for the action of a break is a traditional source of bugs in loop behavior.

Stanford CS Education Library This [the above section] is document #101, Essential C, in the Stanford CS Education Library. This and other educational materials are available for free at http://cslibrary.stanford.edu/. This article is free to be used, reproduced, excerpted, retransmitted, or sold so long as this notice is clearly reproduced at its beginning. Copyright 1996-2003, Nick Parlante, nick.parlante@cs.stanford.edu.

end of Stanford C essentials

Stanford Perl essentials

This [the following section until marked as end of Stanford University items] is document #108 [Essential Perl] in the Stanford CS Education Library --see http://cslibrary.stanford.edu/108/. This document is free to be used, reproduced, or sold so long as this paragraph and the copyright are clearly. Copyright 2000-2002, Nick Parlante, nick.parlante@cs.stanford.edu.

6. If/While Syntax

Perl’s control syntax looks like C’s control syntax. Blocks of statements are surrounded by curly braces { }. Statements are terminated with semicolons (;). The parenthesis and curly braces are required in if/while/for forms. There is not a distinct “boolean” type, and there are no “true” or “false” keywords in the language. Instead, the empty string, the empty array, the number 0 and undef all evaluate to false, and everything else is true. The logical operators &&, ||, ! work as in C. There are also keyword equivalents (and, or, not) which are almost the same, but have lower precedence.

Loops

These work just as in C…

    while (expr ) {
      stmt;
      stmt;
    }

The “next” operator forces the loop to the next iteration. The “last” operator breaks out of the loop like break in C. This is one case where Perl (last) does not use the same keyword name as C (break).

This [the above section] is document #108 [Essential Perl] in the Stanford CS Education Library --see http://cslibrary.stanford.edu/108/. This document is free to be used, reproduced, or sold so long as this paragraph and the copyright are clearly. Copyright 2000-2002, Nick Parlante, nick.parlante@cs.stanford.edu.

end of Stanford Perl essentials

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Coding example: I am making heavily documented and explained open source code for a method to play music for free — almost any song, no subscription fees, no download costs, no advertisements, all completely legal. This is done by building a front-end to YouTube (which checks the copyright permissions for you).

View music player in action: www.musicinpublic.com/.

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while loop

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while loop

C

Stanford C essentials

No Boolean -- Use int

While Loop

Break

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Stanford Perl essentials

6. If/While Syntax

Loops

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while loop

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while loop

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Stanford C essentials

No Boolean -- Use int

While Loop

Break

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Stanford Perl essentials

6. If/While Syntax

Loops

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