calculating the value of a variable

Question

This feels like an embarrassingly basic question, but I can't find a clear answer (at least that I understand) in the I7 documentation.

I want to calculate the value of a variable from two other variables - e.g. Z = X - Y.

The first part seemed easy enough. I used:

X is a number that varies. X is initially -30.

Y is a number that varies. Y is initially 10.

Z is a number that varies.

Then I tried what to my untutored eye seemed the most intuitive way of assigning a value to Z:

Z is X minus Y.

But inform wouldn't compile. So after looking over a couple of examples in the Inform 7 Handbook I tried:

To decide which number is Z:  
    let Z be X minus Y;
    decide on Z.

Which compiled fine, but later on when I try to use Z in a description - something like:

"On the video-screen it says: the value of Z is [Z]."

Instead of returning what I expected, which is -40, it returns 0.

Obviously I'm doing something very stupid, so would appreciate any help.

Thanks in advance

Answer 1

I believe the main problem is that you still have the Z is a number that varies line left from the first attempt.

When you have both a variable and a "to decide" phrase with the same name, the variable "wins" and the phrase is effectively ignored. That's why it prints 0 because that's the default value of the variable. So removing that line should help.

After that you'll have to change the phrase slightly: you can't use a local variable that has the same name as the phrase.

To decide which number is Z:  
    let result be X minus Y;
    decide on result.

Or, since the local variable is redundant in this simple case,

To decide which number is Z:
    decide on X minus Y.

Comments

Ah! Of course that makes total sense. Thanks very much.

Answer 2

The intuitive way you tried first actually works, with two twists:

(1) Setting the value of a variable requires the use of "now":

Now Z is X minus Y.

(2) And the line has to appear somewhere within a rule or definition, it can't just stand on its own. This would work:, for example

When play begins:
     Now Z is X minus Y.