expression

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expression [2014/06/15 20:43] l [Logical operators] |
expression [2014/09/28 14:50] ateyourlembas |
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+ | <- [[display|Displaying A Passage Within Another]] --------- [[function|About Functions]]-> | ||

+ | |||

===== About Expressions ===== | ===== About Expressions ===== | ||

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<code> | <code> | ||

- | 2 + 2 | + | <<print 2 + 2>> |

</code> | </code> | ||

- | When a computer processes it, it results in the number 4. This process is called **evaluation**. This isn't algebra; everything you start with has to be a known quantity. You can do all the basic mathematical things you'd expect in an expression. | + | When Twine processes this [[<<print>>]] macro tag, it results in the number 4. This process is called **evaluation**. This isn't algebra; everything you start with has to be a known quantity. You can do all the basic mathematical things you'd expect in an expression. |

<code> | <code> | ||

- | (1 + 2) * 4 + (3 + 2) / 5 | + | <<print (1 + 2) * 4 + (3 + 2) / 5>> |

</code> | </code> | ||

- | This expression evaluates to the number 13. The computer follows the normal order of operations in mathematics: first multiplying and dividing, then adding and subtracting. You can group **subexpressions** together and force them to be evaluated first with parentheses. | + | This evaluates to the number 13. The computer follows the normal order of operations in mathematics: first multiplying and dividing, then adding and subtracting. You can group subexpressions together and force them to be evaluated first with parentheses. |

+ | | ||

+ | If you're not familiar with some of those symbols, here's a review: | ||

+ | ^ Operator ^ Function ^ Example ^ | ||

+ | | ''+'' | Addition. | ''5 + 5'' (is 10) | | ||

+ | | ''-'' | Subtraction. Can also be used to negate a number. | ''5 - -5'' (is 10) | | ||

+ | | ''*'' | Multiplication. | ''5 * 5'' (is 25) | | ||

+ | | ''/'' | Division. | ''5 / 5'' (is 1) | | ||

+ | | ''%'' | Modulo (remainder of a division). | ''5 % 26'' (is 1) | | ||

+ | | ''('' and '')'' | Brackets/parentheses (causes an expression to be evaluated earlier). | ''(5 + 10) * 2'' (is 30, not 25) | | ||

+ | | ||

+ | ==== A note about modulo ==== | ||

+ | | ||

+ | Modulo may seem somewhat obtuse an operator, but think of it like this: if you had a sequence of numbers: ''0, 1, 2, 3, 4, 5, 6...'', and you did ''% 3'' to each of them, they would become ''0, 1, 2, 0, 1, 2, 0...'' - that is, a constantly looping sequence. This ability to simplify rising sequences allows you to perform some otherwise complicated calculations easily. See the [[function#visited_string_string|visited()]] function for one such example. | ||

+ | | ||

+ | ==== Strings of text ==== | ||

You can also use **strings** in an expression. A string is a bunch of characters strung together, demarcated by matching pairs of either double or single quotes. You can use strings in expressions: | You can also use **strings** in an expression. A string is a bunch of characters strung together, demarcated by matching pairs of either double or single quotes. You can use strings in expressions: | ||

<code> | <code> | ||

- | "Hello" + ', ' + "sailor" | + | <<print "The" + ' former ' + "Prime Minister's">> |

</code> | </code> | ||

- | This expression pushes the strings together, and evaluates to "Hello, sailor". Notice that a space had to be added between the words; computers aren't smart enough to do that for us. Also, you can only add strings together. You can't subtract them, much less multiply or divide them. | + | This expression pushes the strings together, and evaluates to "The former Prime Minister's". Notice that spaces had to be added between the words in order to produce a properly spaced final string. Also, notice that you can only add strings together. You can't subtract them, much less multiply or divide them. Finally, notice that the apostrophe in "Prime Minister's" couldn't be included if that string had used single quotes, as they are the same character. (This is why both sets of quotes are permitted to build strings.) |

==== A note about strings and numbers ==== | ==== A note about strings and numbers ==== | ||

- | As you can infer from the above, strings and numbers are separate types of data. What happens if you try to add a number to a string? Put simply, the decimal number is changed into characters and added to the string. So, 2 + "2" produces "22", and 2 + 8 + "" + 4 produces "104". (The 2 and 8 are added, resulting in the number 10. Then the empty string is added, resulting in the string "10". Then the 4 is added to "10", resulting in "104".) | + | As you can infer from the above, strings and numbers are separate types of data. What happens if you try to add a number to a string? Put simply, the decimal number is changed into characters and added to the string. So, |

- | Strings can thus be considered a "contagious" data type - when they are added to other data types, the other data type becomes a string. | + | <code> |

+ | <<print 2 + "2">> | ||

+ | </code>produces the string "22", and | ||

+ | <code> | ||

+ | <<print 2 + 2 + "2" + 2>> | ||

+ | </code> | ||

+ | produces the string "422". (The 2 and 2 are added, resulting in the number 4. Then the string "2" is added, resulting in the string "42". Then the 2 is added to "42", resulting in "422".) | ||

+ | | ||

+ | Strings can thus be considered a "contagious" data type - when they are added to other data types, the other data type becomes a string. | ||

+ | | ||

+ | (One upshot of this is that you can **convert** a number to a string by simply adding the "empty string" to it - a string with zero characters, just two quotation marks with nothing between: | ||

+ | | ||

+ | <code> | ||

+ | <<set $twentyfour to 24 + "">> | ||

+ | </code> | ||

+ | Usually, though, there isn't much call for this - numbers are generally as useful as strings.) | ||

==== Functions ==== | ==== Functions ==== | ||

- | If they are all written into the story's code, expressions are not terribly interesting. However, there are devices called **[[function]]s** which can be used to obtain interesting values that can't be written down. | + | If they are all written into the story's code, expressions are not terribly interesting. After all, plain text already accomplishes what the <<print>> macros above can do. However, there are devices called **[[function]]s** which can be used to obtain interesting values that can't be written down. |

For instance, there exists a built-in function named ''either()'', which randomly picks one of the values given to it. Using this, you can have a gun with a random number of bullets in it: | For instance, there exists a built-in function named ''either()'', which randomly picks one of the values given to it. Using this, you can have a gun with a random number of bullets in it: | ||

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For a list of the most useful functions, see the [[function]]s article. | For a list of the most useful functions, see the [[function]]s article. | ||

+ | |||

==== Logical operators ==== | ==== Logical operators ==== | ||

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<code> | <code> | ||

- | ($master is 'Selator') and ($berries > 2) | + | <<if ($master is 'Selator') and ($berries > 2)>> |

</code> | </code> | ||

==== Use "is" instead of "=" ==== | ==== Use "is" instead of "=" ==== | ||

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An example: | An example: | ||

<code> | <code> | ||

- | $health > 2 and < 4 | + | <<if $health > 2 and < 4 >> |

</code> | </code> | ||

- | This expression is **invalid** because the > and < operators (and indeed, all expression operators except ''not'') require a distinct value to be on both sides of it. In a sense, the expression is interpreted as ''($health > 2) and ( < 4)'', which is obviously nonsensical. So, you must rewrite this as ''$health > 2 and $health < 4''. | + | This macro tag's expression is **invalid** because the > and < operators (and indeed, all expression operators except ''not'') require a distinct value to be on both sides of it. In a sense, the expression is interpreted as ''($health > 2) and ( < 4)'', which is obviously nonsensical. So, you must rewrite this as ''$health > 2 and $health < 4''. |

Another example: | Another example: | ||

<code> | <code> | ||

- | $name is "Perone" or "Pavone" | + | <<if $name is "Perone" or "Pavone" >> |

</code> | </code> | ||

- | This is also **invalid**, for a different reason: it will be interpreted as ''($name is "Perone") or ("Pavone")'' - which is to say, it is true if $name is "Perone", or if the string "Pavone" is not false. This, of course, means that it's always true - an undesirable outcome. You must rewrite it as ''$name is "Perone" or $name is "Pavone"''. | + | This macro tag's expression is also **invalid**, for a different reason: it will be interpreted as ''($name is "Perone") or ("Pavone")'' - which is to say, it is true if $name is "Perone", or if the string "Pavone" is not false. This, of course, means that it's always true - an undesirable outcome. You must rewrite it as ''$name is "Perone" or $name is "Pavone"''. |

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Some macros, such as [[<<if>>]], are designed to only accept conditions. But, if such macros receive an expression, then, depending on the value, it can be treated as if it was a condition in and of itself. The number 0, and a string with no characters in it "", are both treated as //false//. The other numbers, and every other string, are treated as //true//. This means that you can simplify a number of expressions: <<if $numberOfJavelins > 0>> can simply become <<if $numberOfJavelins>>. | Some macros, such as [[<<if>>]], are designed to only accept conditions. But, if such macros receive an expression, then, depending on the value, it can be treated as if it was a condition in and of itself. The number 0, and a string with no characters in it "", are both treated as //false//. The other numbers, and every other string, are treated as //true//. This means that you can simplify a number of expressions: <<if $numberOfJavelins > 0>> can simply become <<if $numberOfJavelins>>. | ||

+ | ====Summary of types of data values==== | ||

+ | |||

+ | To conclude, here is a summary of the types of values that you will most often encounter in macro expressions: | ||

+ | |||

+ | ^ Example of value ^ Description ^ | ||

+ | | "Some text", "2", "true", 'More text', '4', 'true' | **Strings**: snippets of text characters that can be [[<<print>>]]ed, joined up, or compared. Note that "2" and "true" resemble other types but are nonetheless strings. | | ||

+ | | 0, 2, 5, -11, 45.25, Infinity | **Numbers** that can be used in arithmetic calculations. ''Infinity'' isn't really a number, but has the special property that it's always ''>'' and ''>='' every actual number, so you can use it in that case. (The same applies to ''-Infinity'' and ''<''/''<='') | | ||

+ | | true, false | **Logical values**, created using the logical operators, and commonly used with to the [[<<if>>]] macro. True and false are the only values of this type. | ||

+ | <- [[display|Displaying A Passage Within Another]] --------- [[function|About Functions]]-> |

expression.txt · Last modified: 2017/10/09 20:39 (external edit)