Debugging

If you write and run your own program, it probably won’t work.

Your first reaction will be to blame the computer. Don’t!

It’s easy to write a program that’s nearly correct but hard to find the little bug fouling it up. Most time you spend at the computer will be devoted to debugging.

Debugging can be fun. Hunting for the bug is like going on a treasure hunt – or solving a murder mystery. Pretend you’re Sherlock Holmes. Your mission: to find the bug and squish it! When you squish it, have fun: yell out, "Squish!"

How can you tell when a roomful of programmers is happy? Answer: when you hear continual cries of "Squish!"

To find a bug, use three techniques:

Inspect the program.

Trace the computer’s thinking.

Shorten the program.

Here are the details.…

Inspect the program

Take a good, hard look at the program. If you stare hard enough, maybe you’ll see the bug.

Maybe you typed the letter O instead of zero? Zero instead of the letter O?

Typed I instead of 1? Typed 1 instead of I?

Pressed the SHIFT key when you weren’t supposed to? Forgot to press it?

Typed an extra letter? Omitted a letter?

Typed a line you thought you hadn’t? Omitted a line?

1. You forgot to insert the word END or EXIT DO into your program.

2. Into a DO loop or FOR loop, you inserted a PRINT line that should be outside the loop.

3. When you started typing the program, you forgot to choose New from the file menu; so the computer is including part of the previous program.

Trace the computer’s thinking

10 PRINT "I'm at line 10. The values are"; b; c; x$

20 PRINT "I'm at line 20. The values are"; b; c; x$

Then run the program. Those extra lines tell you what the computer is thinking about b, c, and x$ and also tell you how many times the computer reached lines 10 and 20. For example, if the computer prints what you expect in line 10 but prints strange values in line 20 (or doesn’t even get to line 20), you know the bug occurs after line 10 but before line 20.

Here’s a good strategy. Halfway down your program, insert a line that says to print all the values. Then run your program. If the line you inserted prints the correct values, you know the bug lies underneath that line; but if the line prints wrong values (or if the computer never reaches that line), you know the bug lies above that line. In either case, you know which half of your program contains the bug. In that half of the program, insert more lines, until you finally zero in on the line that contains the bug.

Shorten the program

When all else fails, shorten the program.

Hunting for a bug in a program is like hunting for a needle in a haystack: the job is easier if the haystack is smaller. So make your program shorter: delete the last half of your program. Then run the shortened version. That way, you’ll find out whether the first half of your program is working the way it’s supposed to. When you’ve perfected the first half of your program, tack the second half back on.

To write a long, correct program easily, write a short program first and debug it, then add a few more lines and debug them, add a few more lines and debug them, etc. So start with a small program, perfect it, then gradually add perfected extras so you gradually build a perfected masterpiece. If you try to compose a long program all at once – instead of building it from perfected pieces – you’ll have nothing more than a mastermess – full of bugs.

Moral: to build a large masterpiece, start with a small masterpiece. To build a program so big that it’s a skyscraper, begin by laying a good foundation; double-check the foundation before you start adding the program’s walls and roof.

Error messages

Syntax errors

If you say "prind" instead of "print", the computer will say:

Syntax error

That means the computer hasn’t the faintest idea of what you’re talking about!

If the computer says you have a syntax error, it’s usually because you spelled a word wrong, or forgot a word, or used a word the computer doesn’t understand. It can also result from wrong punctuation: check your commas, semicolons, and colons. It can also mean your DATA statement contains a string but your READ statement says to read a number instead; to fix that problem, change the READ statement by putting a dollar sign at the end of the variable’s name.

If you try to say PRINT 5 + 2 but forget to type the 2, the computer will say:

Expected: expression

If you type a left parenthesis but forget to type the right parenthesis that matches it, the computer will say:

Expected: )

If you accidentally type extra characters (or an unintelligible word) at the end of the line, the computer will say:

Expected: end-of-statement

Numeric errors

If the answer to a calculation is a bigger number than the computer can handle, the computer will say:

Overflow

To help the computer handle bigger numbers, remember to put a decimal point in any problem whose answer might be bigger than 32 thousand.

If you try to divide by zero, the computer will say:

Division by zero

If you feed the computer a number that’s inappropriate, the computer will say:

Illegal function call

That’s what the computer will say if you try saying WIDTH 50 instead of WIDTH 40, or you try saying LPRINT CHR$(1200) instead of LPRINT CHR$(12).

Printer errors

If your printer runs out of paper, the computer will say:

Out of paper

If your printer isn’t communicating well with the computer, the computer will say:

Device fault

That means printer’s cable to the computer is unplugged or loose or defective or plugged into the wrong socket, or the printer is turned off, or the printer is off-line (because you pressed a button that turned off the printer’s ON LINE light), or the paper is jammed, or there’s no more ink left, or the printer broke.

Logic errors

Some commands come in pairs.

DO without LOOP

If you say LOOP but no line says DO, the computer will say:

LOOP without DO

The words FOR and NEXT form another pair. If part of the pair is missing, the computer will say –

FOR without NEXT

or:

NEXT without FOR

If you say SELECT but no line says END SELECT, the computer will say:

SELECT without END SELECT

If you say END SELECT but no line’s first word is SELECT, the computer will say:

END SELECT without SELECT

Between the SELECT and END SELECT lines, you’re supposed to have several lines saying CASE. If you say CASE but no line’s first word is SELECT, the computer will say:

CASE without SELECT

Label not defined

ELSE without IF

END IF without block IF

Block IF without END IF

Out of DATA

Type mismatch

When you feed the computer a string, you must put the string in quotation marks, and put a dollar sign after the string’s variable. If you forget to type the string’s quotation marks or dollar sign, the computer won’t realize it’s a string; the computer will think you’re trying to type a number instead; and if a number would be inappropriate, the computer will say "Type mismatch". So when the computer says "Type mismatch", it usually means you forgot a quotation mark or a dollar sign.

PAUSE key

Magicians often say, "The hand is quicker than the eye." The computer’s the ultimate magician: the computer can print information on the screen much faster than you can read it.

Then the computer will pause, to let you read what’s on the screen.

When you’ve finished reading what’s on the screen and want the computer to stop pausing, press the ENTER key. Then the computer will continue printing rapidly, where it left off.

If your eyes are as slow as mine, you’ll need to use the PAUSE key often! You’ll want the computer to pause while you’re running a program containing many PRINT statements (or a PRINT statement in a loop).

F keys

F6 (immediate window)

Usually you type in the program window. That’s where you type your programs.

Any command you type in the immediate window will be obeyed by the computer immediately. (The computer will not wait for you press SHIFT F5.)

For example, in the immediate window type this:

PRINT 4 + 2

When you press the ENTER key at the end of that line, the computer obeys that line immediately; the computer immediately makes the screen turn black and prints the answer (6) near the bottom of the black screen. When you finish admiring the answer, press the F4 key to switch back to the blue screen.

The cursor will still be in the immediate window. Go ahead: type another PRINT line in the immediate window. When you press the ENTER key at the end of the line, the computer will print the answer on the black screen. Press F4 again, so you see the blue screen again.

You can use the immediate window to quickly PRINT the answers to calculations (so you can discard your calculator) and to explore advanced aspects of the PRINT command.

When you get tired of using the immediate window, press the F6 key again, which moves the cursor back up to the program window.

Try this experiment: in the program window, write a program and run it (by pressing SHIFT F5). When the program finishes running (or gets interrupted by an error), go down to the immediate window (by pressing F6) and tell the computer to PRINT the program’s variables. For example, if the program mentioned a variable called x, say this in the program window:

PRINT x

That makes the computer print the number that x stands for. That information will help you debug the program.

F9 (breakpoint)

So when pointing at a line, pressing F9 makes the line turn red; pressing F9 again makes the line turn back to blue.

Try this experiment: type a long program (containing 5 lines or more), and make two of the lines turn red. Make the red lines be in the middle of the program, instead of being the top or bottom lines.

While the computer pauses at the red line, do whatever you wish! For example:

You can peek at the black screen (by pressing F4 to see the black screen, then pressing F4 again to return to the blue screen).

You can use the immediate window (by pressing F6, then typing a PRINT command in the immediate window, then pressing F6 to return to the program window).

You can edit your program.

Pressing F5 makes the computer continue where it left off. Pressing SHIFT with F5 makes the computer run the program from the beginning instead.

If you’ve turned many lines red, here’s how to get rid of all the redness: choose "Clear All Breakpoints" from the Debug menu (by pressing Alt then D then C).

F7 (run to here)

To have fun, make sure none of the lines in your program is red; then point at a line in the middle of your program (by using the arrow keys), and press F7.

The computer will temporarily turn the line red (so the line becomes a breakpoint). Then the computer will run the program up to that line. Then the computer will get rid of the red.

So if you point at a line and then press F7, the computer will run just the program’s beginning, up to that line.

Notice the contrast:

Pressing SHIFT with F5 runs the whole program.

Pressing F7 runs just the program’s beginning.

F8 (single step)

Pressing F8 makes the computer run one more line, then show you the blue screen again. At the blue screen, do whatever you wish: you can press F4 (to peek at the black screen), or press F6 (to move to the immediate window), or edit your program, or press F8 again (to run the next line).

The first time you press F8, the computer typically runs the program’s first line; but if the computer was in the middle of running your program and was interrupted (by a red breakpoint line or a line saying STOP), pressing F8 makes the computer continue where it left off and do one more line.

Apostrophe

'This program is another dumb example, written by Russy-poo, a stupid jerk.

'It was written on Halloween, under a full moon, shaded by his fangs.

CLS

c = 40 'because Russ has 40 computers

h = 23 'because 23 of his computers are haunted

PRINT c - h 'That is how many computers are unhaunted and safe for kids.

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