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.

nested if

I previsously mentionedcompund testing (testing for more than one condition at a time). Another variation is the nested test. In the nested IF, we make a test and then in either the THEN or the ELSE part (or both), we follow up with an additional test. The nesting can be continued to as many levels as needed.

As an example, say we were searching through a list of people for either a blue-eyed man or a brown-eyed woman. At the top level fo the IF-THEN-ELSE we would test for man or woman. Then we would nest additional testing. If the THEN part was for the condition of the person being a woman, the nested test would be for brown eyes, and in the ELSE part (the person is a man, assuming only two choices) we would test for blue eyes.

    IF person is a WOMAN
        THEN IF person has brown eyes
            THEN successful match (woman with brown eyes)
            ELSE unsuccessful match (woman without brown eyes)
        ELSE (must be a MAN) IF person has blue eyes
            THEN successful match (man with blue eyes)
            ELSE unsuccessful match (man without blue eyes)

Nested IFs can become very complicated and difficult to read and prone to mismatch errors.

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.

IF

    if (expr) {         ## basic if -- ( ) and { } required
      stmt;
      stmt;
    }

    if (expr) {         ## if + elsif + else
      stmt;
      stmt;
    }
    elsif (expr) {      ## note the strange spelling of "elsif"
      stmt;
      stmt;
    }
    else {
      stmt;
      stmt;
    }

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

Python

Blocks of code are indicated by indentation in Python. The language does not use such denotations as a pair of braces {} or a begin/end pair.

    if expression
        statement
        statement
    elif expression
        statement
        statement
    elif expression
        statement
        statement
    else expression
        statement
        statement
    statement
    statement

Blocks of code may use any combination of tabs and space characters to indicate a block, but Python does an exact match. Because tabs can vary by operating system and even software program, it is strongly recommended to use the Python convention of four space characters for each level of indentation.

A block of code is terminated by ending its indentation.

A series of blocks can be terminated simultaneously by a change in indentation.

    if expression
        statement
        statement
        if
            statement
            statement
        statement
        statement
    elif expression
        statement
        statement
        elif expression
            statement
            statement
    else expression
        statement
        statement
    statement
    statement

Notice that the use of indentation solves the problem of matching up else if and else statements when there are also nested ifs.

free music player coding example

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/.

Create your own copy from the original source code/ (presented for learning programming).

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†UNIX used as a generic term unless specifically used as a trademark (such as in the phrase “UNIX certified”). UNIX is a registered trademark in the United States and other countries, licensed exclusively through X/Open Company Ltd.

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nested if

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nested if

Stanford Perl essentials

6. If/While Syntax

IF

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Python

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Stanford Perl essentials

6. If/While Syntax

IF

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