Section 7.2. Assignment and Retrieval


7.2. Assignment and Retrieval

If a variable is a labelled shoebox (see "Variables" in Chapter 6), then to assign a value to a variable is to put something into the shoebox. If the variable already has a value, that value is replaced.

Assignment is performed with one of two commands: set or copy.

set

Syntax

 set variableName to value 

Description

Assigns value to variableName.

Example

 set x to 5 

There is a synonym using the word returning instead of set, with the parameters in reverse order, like this: 5 returning x. But I have never seen this synonym used.

copy

Syntax

 copy value to variableName 

Description

Assigns value to variableName.

Example

 copy 5 to x 

An abbreviation for copy is put; an abbreviation for to is into. Thus you could type put 5 into xalthough it would still come out as copy 5 to x. These abbreviations were designed to accommodate HyperCard users, who were habituated to this idiom.

There is no simple assignment operator, such as the equals sign (=). You cannot, for example, perform an assignment like this:

 x = 5 

That is a comparison, and returns a boolean result revealing whether x is already 5. That code is legal (and therefore does not cause a compile-time error) but is not an assignment (as any mildly experienced programmer would expect); this is a frequent cause of bugs in my scripts. See "The "English-likeness" Monster" in Chapter 4.

7.2.1. Set by Reference

As they both perform assignment, you might think set and copy must be completely interchangeable. In most cases, they are; but with regard to four types of valuelists, records, dates, and script objectsthey are not. With these data types, set sets by reference, meaning that you can end up with more than one name for the same value.

The reason why these four data types are singled out for special treatment is that they are the only kinds of value that can be mutated in place. Thus, after a set by reference, whatever mutation is performed upon such a value under one of its names applies to it under its other names as well. For example:

 set L to {1, 2, 3} set LL to L -- set by reference   set end of L to 4 -- mutate a list in place LL -- {1, 2, 3, 4}, because it is the same list as L 

For other datatypes, use whichever command you prefer; I habitually use set.

7.2.2. Multiple Assignment

In an assignment, variableName and value can optionally be listsof variable names and values, respectivelyallowing multiple assignments in one command. The first item in the value list is assigned to the first item in the variableName list, the second to the second, and so forth. If the value list is longer than the variableName list, the extra values are not assigned to anything; if the value list is shorter than the variableName list, there is a runtime error. This remarkably elegant feature is probably underutilized by beginners. (For a parallel construction involving assignment to a record, see "Record" in Chapter 13.) For example:

 set {x, y, z} to {1, 2, 3} z -- 3, and can you guess what x and y are? 

7.2.3. Retrieval

To retrieve the value of a variable (or fetch the value, or use the value, or whatever you want to call it), simply use the variable's name in code. As with most computer languages, there is no problem retrieving from and assigning to the same variable in a single statement:

 set x to x + 1 

There is no circularity, because the value of x is first retrieved, and 1 is added to that value; then the result of this addition is assigned to x, replacing the original value.

The result of a line consisting of just the name of a variable is the value of that variable. So, for example:

 set x to 5 x 

The result of that script is 5. This can be useful when you want to see the implicit result of a script for testing or debugging (see "Implicit Result" in Chapter 5).

It is possible to retrieve a variable's value by using the get command:

 set x to (get x) + 1 

But no one ever talks this way in real life, and as far as I know this use of get with a variable adds nothing. However, get with an object reference is another matter; see "Get" in Chapter 11.




AppleScript. The Definitive Guide
AppleScript: The Definitive Guide, 2nd Edition
ISBN: 0596102119
EAN: 2147483647
Year: 2006
Pages: 267
Authors: Matt Neuburg

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