LilyPond Tankönyv: 3.3.4 Kontextusok tulajdonságainak módosítása

### 3.3.4 Kontextusok tulajdonságainak módosítása

Contexts are responsible for holding the values of a number of context properties. Many of them can be changed to influence the interpretation of the input and so change the appearance of the output. They are changed by the `\set` command. This takes the form

>     \set ContextName.propertyName = #value

Where the ContextName is usually `Score`, `Staff` or `Voice`. It may be omitted, in which case the current context (typically `Voice`) is assumed.

The names of context properties consist of words joined together with no hyphens or underscores, all except the first having a capital letter. Here are a few examples of some commonly used ones. There are many more.

propertyName | Type | Function | Example Value
--- | --- | --- | ---
extraNatural | Boolean | If true, set extra natural signs before accidentals | `#t`, `#f`
currentBarNumber | Integer | Set the current bar number | `50`
doubleSlurs | Boolean | If true, print slurs both above and below notes | `#t`, `#f`
instrumentName | Text | Set the name to be placed at the start of the staff | `"Cello I"`
fontSize | Real | Increase or decrease the font size | `2.4`
stanza | Text | Set the text to print before the start of a verse | `"2"`

where a Boolean is either True (`#t`) or False (`#f`), an Integer is a positive whole number, a Real is a positive or negative decimal number, and text is enclosed in double apostrophes. Note the occurrence of hash signs, (`#`), in two different places – as part of the Boolean value before the `t` or `f`, and before value in the `\set` statement. So when a Boolean is being entered you need to code two hash signs, e.g., `##t`.

Before we can set any of these properties we need to know in which context they operate. Sometimes this is obvious, but occasionally it can be tricky. If the wrong context is specified, no error message is produced, but the expected action will not take place. For example, the `instrumentName` clearly lives in the `Staff` context, since it is the staff that is to be named. In this example the first staff is labeled, but not the second, because we omitted the context name.

>     <<
>       \new Staff \relative {
>         \set Staff.instrumentName = "Soprano"
>         c''4 c
>      }
>       \new Staff \relative {
>       \set instrumentName = "Alto"  % Wrong!
>       d'4 d
>      }
>     >>

Remember the default context name is `Voice`, so the second `\set` command set the property `instrumentName` in the `Voice` context to „Alto”, but as LilyPond does not look for any such property in the `Voice` context, no further action took place. This is not an error, and no error message is logged in the log file.

Similarly, if the property name is mis-spelt no error message is produced, and clearly the expected action cannot be performed. In fact, you can set any (fictitious) ‘property’ using any name you like in any context that exists by using the `\set` command. But if the name is not known to LilyPond it will not cause any action to be taken. Some text editors with special support for LilyPond input files document property names with bullets when you hover them with the mouse, like JEdit with LilyPondTool, or highlight unknown property names differently, like ConTEXT. If you do not use an editor with such features, it is recommended to check the property name in the Internals Reference: see Tunable context properties, or Contexts.

The `instrumentName` property will take effect only if it is set in the `Staff` context, but some properties can be set in more than one context. For example, the property `extraNatural` is by default set to ##t (true) for all staves. If it is set to ##f (false) in one particular `Staff` context it applies just to the accidentals on that staff. If it is set to false in the `Score` context it applies to all staves.

So this turns off extra naturals in one staff:

>     <<
>       \new Staff \relative {
>         ais'4 aes
>       }
>       \new Staff \relative {
>         \set Staff.extraNatural = ##f
>         ais'4 aes
>       }
>     >>

and this turns them off in all staves:

>     <<
>       \new Staff \relative {
>         ais'4 aes
>       }
>       \new Staff \relative {
>         \set Score.extraNatural = ##f
>         ais'4 aes
>       }
>     >>

As another example, if `clefTransposition` is set in the `Score` context this immediately changes the value of the octavation in all current staves and sets a new default value which will be applied to all staves.

The opposite command, `\unset`, effectively removes the property from the context, which causes most properties to revert to their default value. Usually `\unset` is not required as a new `\set` command will achieve what is wanted.

The `\set` and `\unset` commands can appear anywhere in the input file and will take effect from the time they are encountered until the end of the score or until the property is `\set` or `\unset` again. Let’s try changing the font size, which affects the size of the note heads (among other things) several times. The change is from the default value, not the most recently set value.

>     c4
>     % make note heads smaller
>     \set fontSize = #-4
>     d e
>     % make note heads larger
>     \set fontSize = #2.5
>     f g
>     % return to default size
>     \unset fontSize
>     a b

We have now seen how to set the values of several different types of property. Note that integers and numbers are always preceded by a hash sign, `#`, while a true or false value is specified by `##t` and `##f`, with two hash signs. A text property should be enclosed in double quotation signs, as above, although we shall see later that text can actually be specified in a much more general way by using the very powerful `\markup` command.

#### Setting context properties with `\with`

The default value of context properties may be set at the time the context is created. Sometimes this is a clearer way of setting a property value if it is to remain fixed for the duration of the context. When a context is created with a `\new` command it may be followed immediately by a `\with { .. }` block in which the default property values are set. For example, if we wish to suppress the printing of extra naturals for the duration of a staff we would write:

>     \new Staff \with { extraNatural = ##f }

like this:

>     <<
>       \new Staff
>       \relative {
>         gis' ges aes ais
>       }
>       \new Staff \with { extraNatural = ##f }
>       \relative {
>         gis' ges aes ais
>       }
>     >>

Or, if the property override is to be applied to all staves within the score, it may be appended to an explicit `\new Score` command, like this:

>     \score {
>       \new Score \with { extraNatural = ##f } <<
>         \new Staff {
>           \relative {
>             gis' ges aes ais
>           }
>         }
>         \new Staff {
>           \relative {
>             gis' ges aes ais
>           }
>         }
>       >>
>     }

Properties set in this way may still be changed dynamically using `\set` and returned to the default value set in the `\with` block with `\unset`.

So if the `fontSize` property is set in a `\with` clause it sets the default value of the font size. If it is later changed with `\set`, this new default value may be restored with the `\unset fontSize` command.

#### Setting context properties with `\context`

The values of context properties may be set in all contexts of a particular type, such as all `Staff` contexts, with a single command. The context type is identified by using its type name, like `Staff`, prefixed by a back-slash: `\Staff`. The statement which sets the property value is the same as that in a `\with` block, introduced above. It is placed in a `\context` block within a `\layout` block. Each `\context` block will affect all contexts of the type specified throughout the `\score` or `\book` block in which the `\layout` block appears. Here is a example to show the format:

>     \score {
>       \new Staff {
>         \relative {
>           cis''4 e d ces
>         }
>       }
>       \layout {
>         \context {
>           \Staff
>           extraNatural = ##t
>         }
>       }
>     }

Context properties set in this way may be overridden for particular instances of contexts by statements in a `\with` block, and by `\set` commands embedded in music statements.

#### Lásd még

A kottaírás kézikönyve: Changing context default settings. The set command.

Internals Reference: Contexts, Tunable context properties.

#### LilyPond — Tankönyv v2.22.2 (stable-branch).