I'm studying APL from here.
Why am I getting this syntax error?
'computer' [ 1 2 3 ] ← 'COM'
SYNTAX ERROR
'computer'[1 2 3]←'COM'
^
But if I save 'computer' in a variable I don't get the error:
T ← 'computer'
T
computer
T[1 2 3] ← 'COM'
T
COMputer
What am I doing wrong?
'computer' is a constant, and you can't change the value of a constant itself, only the current value of a variable.
Think about it: If you could assign to 'computer', then next time you wrote 'computer', would you expect the result to be COMputer? How about 2←3? Clearly, this doesn't make any sense.
However, you can amend a value without assigning it to a name, using the relatively new @ "at" operator (it isn't included in Mastering Dyalog APL, but the documentation is available online).
'COM'@1 2 3⊢'computer'
COMputer
You can read this as put the letters 'COM' at indices 1 2 3 of the word 'computer'. The ⊢ here only serves to separate 1 2 3 from 'computer so it is clear to @ what constitutes the indices and what is the array to be amended.
Run it on TryAPL!
That bracket notation is made specifically for modifying variables. The return value of T[1 2 3] ← 'COM' is 'COM', so if the expression didn't modify a variable, it would be pointless (or, almost identical to ⊢).
To get a modified array, not modify a variable, use the operator @:
('COM'@1 2 3) 'computer'
Try it online!
