10  LET
BUTTER = 75 |
And now at last you shall write a computer program. Turn the computer off & on, just to make sure that it is clear. Now type
10 LET BUTTER=75 (& NEWLINE)
and the screen will look like this:
| 10 LET
BUTTER = 75 |
This is different from what happened with EGGS in chapter 6; if you type
PRINT BUTTER
you will see (from the report 2) that the variable BUTTER has not been set up. (Press NEWLINE again & the screen should go back to looking like the picture.)
Because the LET statement had a number, 10, in front of it, the computer did not execute it straight away, but saved it for later. 10 is its line number, & is used to refer to it rather in the same way that names are used to refer to variables. A set of these stored statements is called a program. Now type
20 PRINT BUTTER
and the screen should look like this:
| 10 LET BUTTER = 75 20 PRINT
BUTTER |
This is a listing of your program. To have the program carried out (or executed or run), type
RUN (don't forget the NEWLINE)
& the answer 75 will appear in the top left-hand corner of the screen. At the bottom left-hand corner you will see the report 0/20. 0, as you know, means 'OK, no problems', & 20 is the number of the line where it finished. Press NEWLINE, & the listing will come back.
Note that the statements were executed in the order of their line numbers.
Now suppose you suddenly remember that you also need to record the price of yeast. Type
15 LET YEAST=40
& in it goes. This would have been much harder if the first two lines had been numbered 1 & 2 instead of 10 & 20 (line numbers must be whole numbers between 1 & 9999), so that is why, when first typing in a program, it is good practice to leave gaps in the line numbers.
Now you need to change line 20 to
20 PRINT BUTTER,YEAST
You could type out the replacement in full, but there is a way
to use what is there already. You see that little by line 15? This is the
program cursor, & the line it points to is the current line.
Press the 
key
(shifted 6), & it will move down to line 20. (
moves it up again.) Now
press the EDIT key (shifted 1), & a copy of line 20
will be displayed at the bottom of the screen. Press the 
key 7 times so that
the 
cursor moves
to the end of the line, & then type
,YEAST (without NEWLINE)
The line at the bottom should now read
20 PRINT BUTTER, YEAST
Press NEWLINE & it will replace the old line 20. The screen will now look like this:
| 10 LET BUTTER = 75 15 PRINT
BUTTER, YEAST 20 PRINT BUTTER |
RUN this program & both prices will be displayed.
(Here is a useful trick involving EDIT, to use when you want to clear the bottom part of the screen altogether. Press EDIT, & the current line will be brought down from the program, replacing what you wanted deleting. If you now press NEWLINE, the line will be put back in the program, making no difference to it, and the bottom part of the screen will be cleared leaving just the cursor.)
Now type - in a fit of absent-mindedness -
12 LET YEAST=40
This will go up into the program and you will realise your mistake. To delete this unnecessary line, type
12 (with NEWLINE, of course)
You will notice with surprise that the program cursor has
gone. You should imagine it as being hidden in between lines 10
and 15, so if you press it will move up to line 10, while if you press it will move down to line
15.
Last, type
LIST 15
You will now see on the screen
15LET
YEAST=40
20 PRINT
BUTTER,YEAST
Line 10 has vanished from the screen, but it is still in your program - which you can prove by pressing NEWLINE again. The only effect of LIST 15 are to produce a listing that starts at line 15, and to put the program cursor at line 15.
LIST
on its own makes the listing start at the beginning of the program.
Programs
Editing programs using
Statements: RUN, LIST
1. Modify the program so that it displays not only the two prices, but also messages to show which is which.
2. Use the EDIT key to change the price of butter.
3. Run the program and then type
PRINT BUTTER,YEAST
The variables are still there, even though the program has finished.
4. Type
12 (and NEWLINE)
Again, the program cursor will be hidden between lines 10 and 15. Now press EDIT, and line 15 will come down: when the program cursor is hidden between two lines, EDIT brings down the second one. Type NEWLINE to clear the bottom part of the screen.
Now type
30
This time, the program cursor is hidden after the end of the program; and if you press EDIT, then line 20 will be brought down.
5. Put a LIST statement in the program so that when you run it, it lists itself.