Exercises: songs as data

These exercises practise this part on the real songs: the constants in library/song/, the demo they become, and the music they were read from. Each one says what is already there, so you extend it rather than build it again. Most are things to open in the app and listen to, or to work out with a pencil; some are pieces of Rust with a clear end; the ones called “Think about it” are questions, with no code to write.

Try the Rust ones on a branch of your own. Run cargo test after each change, and cargo fmt --all --check and cargo clippy --workspace --all-targets -- -D warnings before you call one finished: CI runs all three.

Songs as constants

1. A new song in its genre file (lineup-core, medium)

Add a public-domain tune to the demos. Song, Part, SONGS and the genre files all exist; you are adding an entry, not a feature (see A song is a constant).

1. Read two songs in the genre file you chose, such as folk.rs or blues.rs, and copy the shape of one, every field from name to about.
2. Write the tune with | between bars and a chart under it, and give the part a name such as “Verse” or “Tune”.
3. Make it pub(super) const, and add it to SONGS in song.rs, at the end of its genre’s block.
4. Let the tests find the mistakes: a bar that does not add up, a chord that cannot be read, a style in another meter, a note that is not in the key.

cargo test -p lineup-core library
cargo run --release -- --demos

It is done when the tests pass and the song is under File > Demos > Songs, in its genre, opens on the chord sheet and plays.

Practises: struct literals, const data, reading test failures.

2. Extend a song that is already there (lineup-core, medium)

Shenandoah plays its song twice after a pickup, and Danny Boy a verse and a chorus. Pick one and add a part the demo does not play yet: an introduction for the band alone, a last verse, or a coda. Write it bar by bar, give it a chart, and add it to the song’s form. Then open the demo and turn on the chord sheet: does the sheet show the chords you wrote, in the bars you meant?

Practises: the melody notation, form as a list of names.

3. A last chorus a semitone higher (lineup-core, easy)

Many pop songs lift their last chorus by a semitone. Add a copy of a song’s chorus with .in_key(...) one semitone above the song’s key, and put it last in the form. Which numerals of the chart have to change? (None: they are read in the part’s key.) Play the end: the demo’s last bar is on the home chord of the new key, as a_song_that_ends_after_a_modulation_ends_in_the_new_key checks.

Practises: builder methods on a const value.

4. A canon at the fifth (lineup-core, medium)

with_voices takes (voice, bar) pairs, so every voice of a round sings the tune at the same pitch. Many canons have the second voice a fifth higher or lower. Make each pair a triple, (voice, bar, semitones), and move each voice’s notes by its semitones where the demo builds the voices. Every round already written gets a 0. Then try Frère Jacques with its second voice 7 semitones up: which notes clash, and why does a round need its copies at the same pitch to fit the same chord?

Practises: changing a type and following the compiler’s errors.

5. Your song’s first notes (lineup-core, easy)

the_first_notes_are_right checks that Ode to Joy starts F♯ F♯ G A and Twinkle, Twinkle starts C C G G. Add a line for a song you know well, with its first four notes as MIDI numbers and their names in the message. Get one wrong on purpose first, and read what the failure says.

Practises: assert_eq! with a message, MIDI note numbers.

6. Lean on the highest note (lineup-core, medium)

perform in library/interpret.rs gives a note of a tune up to six velocity steps for how high it is in its phrase. Singers often lean on the single highest note of a phrase more than on the notes near it. Give that one note two steps more, and write the test first: in Twinkle, Twinkle, which note is the highest of the first phrase? Keep the test a_tune_gets_its_accents_but_stays_close_to_how_it_was_written passing.

Practises: Iterator::max_by_key, writing the test before the code.

7. Search by meter and key (the app, easy)

Open File > Demos and type 7/8 in the search box, then 5/4, then 2+2+3/8. Then try d minor. Which matches come first, and which are fainter? Why does fold keep / and + when it drops every other mark (see Search)?

8. Think about it: methods that take self

in_key and with_tempo are const fns that take self by value and return a new Part. Why does that let them be chained inside a constant, Part::new(...).in_key(...).with_tempo(...)? Why would a method taking &mut self not work there?

Reading the music

9. Which black key? (pencil, easy)

In C major, name the black key between F and G, with the rules of Spelling the notes: over a D7 chord; in the line G, ?, F; and over an A♭7 going to G. Then name the black key between A and B over Gm, and over F♯7.

10. A new note, or a new mode? (pencil and the app, easy)

Write the scales of E natural minor, E harmonic minor and E Dorian. Which note differs in each? Then play bars 21 to 23 of Balderrama. The tune sings C♯: over which chord? Is the music in E Dorian there, or is the C♯ a note on its way somewhere? And why is the song written in E natural minor, not E harmonic minor (see The tune and the key)?

11. Two ways to count six (the app, easy)

Clap six even claps, then group them 3+3 (ONE two three FOUR five six) and 2+2+2 (ONE two THREE four FIVE six). Play Acuarela del río, a polca in 6/8, and clap along with the bass, then with the drums. Which grouping does each play?

12. A pianist’s tempo (the playback trace, easy)

Trace Uno:

ferrous-lineup --trace "Uno" --out uno.tsv

and read its # tempo: lines for the first eight bars. Where does the pianist slow down, and where in the phrase does it happen? How many times does the tempo change in the whole song?

13. Roots a fifth apart (the app, easy)

In For Unto Us a Child Is Born, play bars 41 and 42 with only the Keys channel unmuted. Write down the chords from the second beat of bar 41 and their roots. How far is each root from the next? The note the Demos menu shows for the song names the same chain.

14. Another 4/2 chord (the app, easy)

Bar 28 of the Handel chorus has A7/G, a seventh chord with its seventh in the bass, figured 4/2 (see Figured bass). Bar 45 has another. Which chord is it, which chord follows it, and is the next bass note a step below?

15. The trout’s dominants (the app, easy)

Die Forelle is in D♭ major, and its third verse is the one that changes. Find the four bars of E♭7 on the chord sheet. What chord comes after them, and what is it the dominant of? And what is E♭7 the dominant of?

16. Why C minor needs B natural (the app, easy)

In Sheep May Safely Graze, find the middle section’s bars in C minor on the chord sheet. Which chord brings in B natural, and why does a minor key need it? (Compare the key’s scale, the harmonic minor, with C natural minor.)

17. Who plays Boléro (the app, easy)

Play Boléro and write down, for each statement of the tune, the instrument that plays it: the pattern names on the Song track tell you. How many statements are there before the music leaves C major, and which key does it go to? Then read Boléro’s parts in classical.rs: in “Horn, celesta and piccolos”, played_by gives the piccolos 19 and 28 semitones. Which intervals above the horn’s tune are those, and which notes of the harmonic series do they copy?

18. The Petrushka chord (pencil, easy)

Play C, E, G and F♯, A♯, C♯ together: two major triads a tritone apart, the Petrushka chord. Now play the octatonic scale C, D♭, E♭, E, F♯, G, A, B♭, which alternates half and whole steps. Which of the scale’s notes are not in the chord? Why can the chord not be in any one major or minor key?

19. A mode over a drone (the app, easy)

Open Petrushka, The Shrovetide Fair in the Evening, turn on 🔍 Find the key and play the Coachmen’s Dance, written in E Mixolydian. Which key does the finder show? E Mixolydian has the notes of A major: why does a key finder that compares how long each note sounds hear the key with the same notes? Then print the key-finding report:

cargo test --release -p lineup-core --test key_finding report -- --ignored --nocapture

and find the last tableau of Petrushka in it. Which keys does the score give, and which does the finder hear in their place?

20. A chord the key does not have (the app, easy)

Tea for Two is in A♭ major. Find the key marker where its second part starts. Which chord of A♭ is C major? (Count up from A♭: C is the third note, so C major is III, a chord the key of A♭ does not contain.)

21. From minor to major (the app, easy)

Volver’s verse is in G minor and its refrain in G major. Find the key marker where the refrain starts. Which two notes of G harmonic minor change in G major? Find them in the refrain’s first bars.

22. Two tempos and four chords (pencil and the playback trace, easy)

Trace The Big Bang Theory theme and find the first note of bar 5 and of bar 6. How long is a bar of 4/4 at 159 beats a minute? (4 × 60 / 159 = 1.509 seconds.) And at 130? Then:

• Name the four chords of the verse, IVadd9 V I vi7, in G and then in A.
• In bar 10 the tune alternates G and F♯ over G, then over Em7. Over G, F♯ is the chord’s major seventh; what is it over E minor?

23. A tango against its lead sheet (research, medium)

Pick one of the tangos in tango_played.rs, such as Patotero sentimental, and find a lead sheet of it. Transpose its chords to the pianist’s key, then compare one part bar by bar with the demo’s chart. Which chords did the notes alone name by a colour (a sixth, a ninth) rather than the chord the song is built on?

24. Think about it: a script in the app

The scripts that read MIDI files and engravings are not part of the app. What would it take to make File > Import MIDI file… write a demo constant instead of a project? List what a person decides now that the importer would have to decide: the key, where the parts begin, which track is the tune, which chords a lead sheet allows. Which of those does the app already know how to guess (see Hearing harmony)?