41 — Records: a small address book

A list holds values in order. But what you want from an address book is not "the third person" — it is "where Mina lives". That is what a record is for: every value in it has a name of its own.

#Steps

1. Make a record and put something in it. Putting takes a name and a value together:

   set book to an empty record
   put Mina at Seoul in book
   put Sana at Busan in book
   

   copyrun it →

   It is the same word as a list's put; when the name holds a record it takes the two things instead of one.

2. Read a value by its name. You never need to know where it is:

   set book to an empty record
   put Mina at Seoul in book
   show Mina in book
   

   copyrun it →

   You get Seoul. That is the whole difference between a list and a record.

3. Asking how many is the same sentence as for a list:

   set book to an empty record
   put Mina at Seoul in book
   put Sana at Busan in book
   show how many book
   

   copyrun it →

   You get 2.

4. Going through a record gives you the names. Read each value back with the name it gave you:

   set book to an empty record
   put Mina at Seoul in book
   put Sana at Busan in book
   for each name in book
       show name
       show name in book
   end
   

   copyrun it →

   Mina, Seoul, Sana, Busan.

5. Asking whether something is there, and taking it out — both are the list sentences again:

   set book to an empty record
   put Mina at Seoul in book
   if book contains Mina
       show Mina in book
   end
   remove Mina from book
   show how many book
   

   copyrun it →

   Seoul, then 0.

6. The whole thing:

   set book to an empty record
   put Mina at Seoul in book
   put Sana at Busan in book
   put Ada at Daegu in book
   show how many book
   for each name in book
       show name
       show name in book
   end
   if book contains Sana
       show found Sana
       show Sana in book
   end
   remove Mina from book
   show how many book
   

   copyrun it →

#Try it yourself

Add sort book and run it — it is refused. A record has no order: there is no first and no last, only names to look things up by. Put things you want in order in a list, and things you want to look up by name in a record.

#What you learned

• set <name> to an empty record makes one. Every value in it has a name.
• put <name> at <value> in <record> puts one in; <name> in <record> reads it back.
• how many, contains, for each and remove are the same sentences as for a list. What the name holds decides what they mean.
• Going through a record gives you the names; read each value with its name.
• A record has no order, so sort and the first are not for it.