| Chapter 11: Phrases |

| 11.13. Let and temporary variables

A variable, as we have seen, is a name for a value which changes, though always remaining of the same kind. For instance, if "target" is a number variable (or "number that varies") then it may change value from 2 to 4, but not from 2 to "fishknife".

To make complicated decisions, phrases often need to remember values on a temporary basis. We have already seen this for the counter in a "repeat" loop, which exists only inside that loop, and then is no longer needed. Now we define named values which exist only inside the current phrase. For instance:

> let outer bull be 25;
> let the current appearance be "reddish brown";
> let the special room be Marley Wood;

creates temporary variables "outer bull", "current appearance" and "special room". The kinds of these are deduced from the values given, so that, for instance,

> say "The outer bull scores [the outer bull in words] when you practice archery in [special room]."

produces

> The outer bull scores twenty-five when you practice archery in Marley Wood.

Temporary variables made by "let" are only temporarily in existence while a phrase is being carried out. Their values often change: we could say

> let x be 10;
> change x to 11;

for instance, or indeed we could "let x be 10" and then "let x be 11". But although we are allowed to change the value, we are not allowed to change the kind of value. The name "x" must always have the same kind of value throughout the phrase to which it belongs, so the following will not be allowed:

> let x be 45;
> change x to "Norway";

(The difference between "let" and "change" is that "let" can create a new temporary variable, whereas "change" can only alter one which already exists: on the other hand, "change" can change many other things as well, whereas "let" applies only to temporary variables.)

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| Example A pushable box More on using kinds of value inside phrases, this time to simulate something new: a box which, although it cannot be picked up, can be pushed around between three different places on the floor.

146
| Example Tinted wallpaper A worked example using a new kind of value, "colour", within phrases to talk about tinted wallpaper.

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