Sinclair QL Beginner's Manual page 120

red black blue magenta green cyan yellow white
reveals the problem. We compare black with red and decrease p to the value, 1. We come round
again and try to compare black with a variable colour$(p-1) which is colour$(0) which does not exist.
This is a well-known problem in computing and the solution is to "post a sentinel" on the end of the
array. Just before entering the REPeat loop we need:
LET colour$(0) = comp$
Fortunately SuperBASIC allows zero subscripts, otherwise the problem would have to be solved at
the expense of readability.
MODIFIED PROGRAM
100 REM Insertion Sort
110 DIM colour$(8,7)
120 FOR item = 1 TO 8 : READ colour$(item)
130 FOR item = 2 TO 8
140 LET p=item
150 LET comp$ = colour$(p)
160 LET colour$(0) = comp$
170 REPeat compare
180 IF comp$ >= colour$(p-1) : EXIT compare
190 LET colour$(p) = colour$(p-1)
200 LET p = p-1
210 END REPeat compare
220 LET colour$(p) = comp$
230 END FOR item
240 PRINT"Sorted..." ! colour$
250 DATA "black","blue","magenta","red"
260 DATA "green","cyan","yellow","white"
COMMENT
1. The program works well. It has been tested with awkward data:
A A A A A A A
B A B A B A B
A B A B A B A
B C D E F G H
G F E D C B A
2. An insertion sort is not particularly fast, but it can be useful for adding a few items to an already
sorted list. It is sometimes convenient to allow modest amounts of time frequently to keep items in
order rather than a substantial amount of time less frequently to do a complete re-sorting.
You now have enough background knowledge to follow a development of the handling of the file of
seven names and telephone numbers.
SORTING A MICRODRIVE FILE
In order to sort the file 'phone' into alphabetical order of names we must read it into an internal array,
sort it, and then create a new file which will be in alphabetical order of names.
It is never good practice to delete a file before its replacement is clearly established and proven
correct. You should therefore copy the file first, as security using a different name. The required
processes are as follows:
1. Copy the file 'phone' to 'phone_temp'
2. Read the file 'phone' into an array