music

# relation expressions

## summary

This subchapter looks at relation expressions.

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# relation expressions

This subchapter looks at relation expressions.

When we use a decision control structure, we are making a decision based on something. This something is whether or not something is true (or false).

This something being tested for our decision is some kind of relation expression.

The most basic is a simple test of truth. “Is this true?”

With numbers, we can have a simple test of whether two numbers are equal or not equal. If the two numbers are not equal, we can test to see which is larger or which is smaller. And we can combine these two tests with (greater than or equal) or (less than or equal) -- parenthesis added for clarity.

We can also use functions for simple testing. The function does some kind of evaluation and returns a result of either TRUE or FLASE and we use that result for our decision.

## 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.

### Relational Operators

These operate on integer or floating point values and return a 0 or 1 boolean value.

 == Equal != Not Equal > Greater Than < Less Than >= Greater or Equal <= Less or Equal

To see if x equals three, write something like:

if (x == 3) …

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.

### Pitfall = ≠ ==

An absolutely classic pitfall is to write assignment (=) when you mean comparison (==). This would not be such a problem, except the incorrect assignment version compiles fine because the compiler assumes you mean to use the value returned by the assignment. This is rarely what you want

if (x = 3) …

This does not test if x is 3. This sets x to the value 3, and then returns the 3 to the if for testing. 3 is not 0, so it counts as “true” every time. This is probably the single most common error made by beginning C programmers. The problem is that the compiler is no help -- it thinks both forms are fine, so the only defense is extreme vigilance when coding. Or write “= ≠ ==” in big letters on the back of your hand before coding. This mistake is an absolute classic and it’s a bear to debug. Watch Out! And need I say: “Professional Programmer’s Language.”

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.

## JOVIAL

The following material is from the unclassified Computer Programming Manual for the JOVIAL (J73) Language, RADC-TR-81-143, Final Technical Report of June 1981.

The JOVIAL operators are summarized in the following table:

Type          Operators       Operation

Relational    <   >   =       less than, greater than, equal
<=  >=  <>      less than or equal,
greater or equal, not equal

A relational operator compares any two values of the same type
and produces a Boolean value as its result.  A logical operator
takes bit-string values and also produces a Boolean result.  (A
Boolean value is a one-bit bit-string, representing "true" or
"false", depending on whether it is one or zero.)

The JOVIAL operators are described in detail in Chapter 11,
where, for example, you will find the rules for operations on
fixed values and for the comparison of such objects as
character-strings and pointers.

Chapter 1 Introduction, page 8

# 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).

## view text bookHTML file

Because I no longer have the computer and software to make PDFs, the book is available as an HTML file, which you can convert into a PDF.

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