© 1991 Brian R. Page

Blast Off With BASIC

Chapter Eleven: Keep Going!

Now the real fun begins. It is time to take your ideas and turn them into programs. Each chapter of this book introduced new BASIC commands and programming techniques. You learned about displaying output with PRINT, controlling program flow with GOSUB-RETURN and FOR-NEXT, and, of course, how to use each of the variable types. With these skills, you can write your own programs. Think carefully about all the steps required to accomplish something, a game for example, and then write BASIC code to do each step. And don't expect your programs to work right the first time. They almost never do!

The best way to leap into programming is to return to the programs we developed in the earlier chapters and make them better or different. This will help you learn BASIC. As you become more comfortable with the commands, you will naturally start to write your own programs. Here are some ideas to get you started.

If you have color display screen, then immediately go back and throw color into all the programs. Be creative. In text mode, every line of the screen can be a different color! Use the colors to help the human. Colors make the displays more readable. You might even try mixing text and graphics mode. The NIM game, for instance, could easily run in graphics mode. Put the pebbles on the screen. Draw seventeen circles to start, and subtract the circles as you and the computer take turns. Better yet, have the circles change color as they are subtracted. Circles chosen by the computer should turn a different color than circles subtracted by the human. Improvements such as these could be added with subroutines that barely change the original program.

The UGUESS number guessing game could also be improved. The program could beep each time the player made a guess. A high frequency SOUND means the guess was too high. A low frequency sound means the guess was too low. Another possibility is to give the player a hint. Some code like this could set the frequency of a SOUND depending on the secret number:

800 FREQ% = 900
810 IF NUMBER% < 90 THEN FREQ% = 800
820 IF NUMBER% < 80 THEN FREQ% = 700
830 IF NUMBER% < 70 THEN FREQ% = 600
840 IF NUMBER% < 60 THEN FREQ% = 500

and so on...

The FREQ% variable is changed by every statement until NUMBER% is greater than the condition in the IF-THEN test. For example, the FREQ% for a secret number of 74 would first be set by line 800 and then changed by lines 810 and 820. It would then fail every test beginning with line 830. Since line 820 was the last successful test, FREQ% would remain equal to 700 Hz.

Once you learned the tones, you would have an advantage over someone who did not know the program.

The sound effects programs of chapter four are other good places to experiment with BASIC. Imagine a random number subroutine that creates numbers between 100 and 13,000. Use each number as a frequency for a short SOUND statement. What would that sound like? Maybe we could get the screen to change colors as the tones are sounded?

Create new sounds. How about a railroad train? Can you make an airplane crash? Combine graphics with sounds. Use the CIRCLE statement along with a high pitched squeal to blow up a balloon that finally explodes and disappears.


Improving old programs is just a way to get started programming on your own. To really master BASIC, write some programs from scratch. Here are a few ideas.


Travel overland by wagon from St. Louis to Portland, Oregon. The year is 1850. The game begins in St. Louis, gateway to the West. You have $1,000. With this money you must buy a wagon, horses, gun powder, and food. Will you live to see the Pacific Ocean? Random numbers can be used to throw obstacles in your path -- Indian attacks, snow storms in the mountains, or losing your wagon while crossing a raging stream. A program such as this combines story screens with random numbers. A screen might look like this:

  *                                                                 *
  *   Congratulations!  You have reached Ft. Laramie in the         *
  *   Wyoming Territory.                                            *
  *                                                                 *
  *   You may buy supplies at the fort.  Do you want to stop?       *
  *                                                                 *
  *        1.  Stop.                                                *
  *        2.  No, keep going.                                      *
  *                                                                 *
  *                                                                 *
  *                                                                 *
  *   Supplies:                                                     *
  *        food       = 100 pounds                                  *
  *        gun powder = 10 pounds                                   *
  *        horses     = 2                                           *
  *        money left = $ 300                                       *
  *        Date:  July 29                                           *

When deer and bears cross your path you will have the chance to go hunting. Random numbers decide if your hunt is successful. Then extra pounds of food may be added to your supplies. Of course, gun powder must be subtracted as well. Each of these tasks can be placed in subroutines.


Writing a program to play the game of tic-tac-toe may be harder than you would imagine. The program must have a way to see that three squares in a row have been taken. Perhaps assign each square a variable name. When a square is taken by a player, set the variable equal to some number. Then use a series of IF-THEN statements to check if one player has three squares in a row.

For example, say that a square chosen by the human causes the number three to be assigned to its variable. Suppose also that the top three square are known as A%, B%, and C%. An IF-THEN test could see if A% + B% + C% is equal to nine. If so, then the human wins! When the computer choses a square, assign the variable a number such as seven. Three squares in a row belonging to the computer would equal twenty-one. Eight IF-THEN tests must be built to check for all possible winning rows. If you are clever you could put these eight in a subroutine and use the same statements to check for both the human and the computer.


Adventure is like Oregon Trail. Instead of a journey across the plains, you could be trapped inside an Egyptian pyramid in a maze of twisty passages. Random numbers determine the treasures you find and the various goblins and spooks which haunt you.


Imagine yourself as captain of a starship. Are enemy spaceships after you? Do you have enough energy for your shields and blaster? Can you make it back to star base for supplies?


Turn your computer keyboard into a piano keyboard! Use a loop with an INPUT statement to accept letters which then SOUND certain piano notes.


Programs can be practical too. Write a program to convert temperature in Fahrenheit to Celsius. Here is the algorithm:

       CELSIUS! = .5555 * (FAHRENHEIT! - 32)

To convert in the other direction, from Celsius to Fahrenheit, use this formula:

       FAHRENHEIT! = (1.8 * CELSIUS!) + 32


Another useful program can help with conversions between the metric system of measurements and the English system. When we need to go from centimeters to inches, we can let BASIC do the work. Here is a table of conversions which you can put in a program. List a menu of conversions and then write a subroutine for each one.

      FROM            TO             MULTIPLY BY

      centimeters     inches         0.3937
      inches          centimeters    2.54
      meters          feet           3.281
      feet            meters         0.3048
      meters          yards          1.0936
      yards           meters         0.9144
      kilometers      miles          0.6214
      miles           kilometers     1.6093
      liters          gallons        0.2642
      gallons         liters         3.785


A final suggestion is a program to compute gas mileage. This might be fun to use after a long family car trip. Record the number of miles traveled with each tank of gasoline along with the number of gallons needed at each fill up. Then use a BASIC program to to divide the number of miles (MILES!) by the number of gallons (GALLONS!). The data could come either from INPUT commands or even from READ-DATA statements.


Once you decide to write a program, what do you do next? The first step is to completely understand the task that you want to program. For example, to write a BASIC program to play a game, first learn all of the rules. These rules must be turned into lines of BASIC. Next, write down a list of all of the little tasks which must be accomplished in the program. Use this list to draw a flowchart. The flowchart shows how the routines work: the order in which they run and what they must do. Be sure that the organization is logical. Also, at this point, when you have a flowchart, keep in mind that humans cannot be trusted. Add routines that catch cheating and mistakes.

Flowcharts do not have to be beautiful. Use them as working drawings. When you discover problems or think of a better way to do something, just scratch out some blocks and change it. The flowchart is not so much a map which tells you how to write a program as it is a sketch pad for thinking about a program. Good work on a flowchart saves you work later.

When the flowchart is finished, it is time to start writing BASIC code. Use subroutine building blocks. Begin with the hardest or most important routines. Think in terms of input and output. Each routine accepts some input from the rest of the program. Input will be the variables supplied to the subroutine. After some processing, the routine either changes variables or creates new ones. This is the output.

Write and test subroutines separately. Add lines which do nothing more than create some input. Then let the subroutine do its work. Print the output. In the real program, the output may never be printed. However, during testing you will certainly want to see the output of each subroutine. We used this technique during the development of the binary search algorithm for the IGUESS program in chapter eight.

As the BASIC statements are entered at the keyboard, use the RENUM command often to keep plenty of space between the line numbers. The RENUM command, by itself, simply numbers the lines by tens, beginning with ten. Sometimes, while writing a program, you will need to insert lines of BASIC code between statements which already have line numbers. For example, say you need to insert a GOSUB between lines 435 and 436. This is a problem. When you get in this pinch, issue RENUM and BASIC will do all the work for you. BASIC renumbers all of the lines in the program and changes all the GOTO and GOSUB line numbers. This makes it quick, simple, and safe to add lines. The original lines 435 and 436 end up with new line numbers that are ten apart. Now you can add that GOSUB in between.

The RENUM command is also useful when combining subroutines. You may write a subroutine as a separate program and then merge it with the main program. To do this, first renumber the subroutine using the line numbers it will need when combined. Issue the RENUM command along with the number of the first line number. For example, suppose you wrote a subroutine beginning with a line number 10. When the subroutine is added to the main program, it must begin with line number 400. No problem. Just issue the command:

and BASIC will change line ten of the subroutine into line 400. The other lines will be numbered 410, 420, 430, and so on. Now that the routine is properly numbered, another step is necessary. This step uses a new feature of the SAVE command.

As you recall from chapter two, the SAVE command is followed by a file path which includes the disk drive, directory, and file name. The file path is used by BASIC to create a disk file containing a BASIC program. When BASIC stores a program on disk, it squeezes, or compresses, all the characters so the file will not occupy too much disk space. This compression also allows BASIC to load the file quickly into memory. To see what a compressed file looks like, issue the DOS TYPE command.


The SAVE command can also be used to create an a file of ordinary characters. With this option, blank spaces are not squeezed out. They are left as real spaces. To see how this works, first you must use the BASIC interpreter to LOAD a BASIC program. Any program will do. Then issue this command:

The ,A following the file name tells BASIC to store the file using ordinary characters. Now issue the SYSTEM command to exit from BASIC and try the DOS TYPE command again.
Saving a file in ordinary characters is necessary in order to merge BASIC programs. Using our example above, where a subroutine has been renumbered with lines numbers beginning at 400, that program must be saved with the ,A option. Next, LOAD the main program. Then issue the BASIC MERGE command to bring in the subroutine.
If any line numbers are duplicated in both programs, the original line will be replaced by the duplicate in the merged program. This is why the renumbering must take place before the subroutine is saved.

Here are the steps for merging BASIC programs:

  1. RENUM the subroutine using line numbers that are not duplicated in the main program.
  2. SAVE the subroutine using the ,A option.
  3. LOAD the main program.
  4. MERGE the subroutine.


BUGS are problems. Sometimes the bugs are easy to kill. Your program crashes with an error message and you discover, for instance, that you have some mismatched parentheses. At other times, the bugs are harder. The program runs from beginning to end, but the output is not correct. Bugs are just a fact of life. If you are the kind of person who must have everything perfect and cannot stand puzzles, then programming is not for you. On the other hand, if you like mysteries and solving problems then you will probably enjoy programming. A good programmer is not necessarily someone who makes few mistakes. A good programmer is often someone who is simply very good at rooting out and solving problems.

Remember that computers do exactly what you ask them to do. They are like simple children. If you accidentally include a variable name within the quotation marks of a PRINT statement, then BASIC is going to display the variable name rather than the contents of the variable. The key to debugging is learning to think like a computer. Study programs one line at a time. Take the branches along with the GOTOs and GOSUBs. Make no assumptions. The computer only knows about variables, BASIC commands, and line numbers. It does not know what you want to accomplish.

The best guard against bugs is thorough testing. Design the subroutines so you know exactly what input is required, that is, which variables are needed by the subroutine, along with the the output which must be produced. Then, when the bug happens, slip extra PRINT commands into the beginnings and endings of the subroutines. This way it is possible to see where the problem lies. Is the subroutine getting the wrong input? If so, then chase the problem to where the input was created. On the other hand, if the input is correct and the output is not, then the bug lies in the subroutine itself. This is the time to think like a computer and discover why the subroutine is not behaving. The problem may be as simple as a misspelled variable name.

Misspelled variable names can, indeed, be troublesome. BASIC has no way to tell you that you have misspelled a variable. It cannot see that the names are similar. BASIC just handles the misspelled name as if it were another variable. You might think that a misspelled variable would be empty. Unfortunately, this is not the case for integer and single precision variables. These variable which hold numbers contain a value of zero until your program changes them. String variable, on the other hand, are empty. However, they may still cause problems. For example, consider this error:

330 IF FIRSTNANE$ = "Bonzo" THEN GOSUB 110

GOSUB 110 will never be executed no matter what is entered to fill the variable on line 320. The misspelled variable FIRSTNANE$ on line 330 is empty and will never hold anything.

If you have cornered the bug in one subroutine and you still cannot fix it, consider rewriting the entire subroutine. Most tasks can be solved in more than one way. For a tough bug, it might be time to try another way. Replace the whole routine with one that works. This is another advantage to using subroutine building blocks.

When a program ends in an error, it is often helpful to look at what the variables contain. This is especially true if a program goes into a loop that must be stopped with CTRL-Break. Once the program has stopped running, issue the BASIC PRINT command followed by one or more variable names. Do not add a line number to the command. The line number causes BASIC to use its editor to add the line to the program. A PRINT without a line number is executed immediately by BASIC. For example, suppose you have a FOR-NEXT loop using the variable X% as a counter. Unfortunately, when you run this program it goes into an endless loop. After issuing a CTRL-Break, type in PRINT X% to check the value in the counter. If the value is not what it should be, then you know right away that something in your program is changing X%. Perhaps you have nested FOR-NEXT loops using the same variable name, X%, as a counter.

Hopefully, these debugging tips have not scared you away from programming. Solving bugs can be almost as much fun as programming. Don't let the bugs get in your way. Dream up a program. Write it in BASIC. Crush the bugs.


Blast Off With BASIC just gets you off the ground. In this book, you have learned around thirty BASIC statements. This is plenty to start writing your own programs. However, these thirty are just an introduction. BASIC has over 150 commands, statements, and functions. Many of these are seldom used. However, others would help you right now.

To expand your knowledge of BASIC, see the reference manuals describing the BASIC supplied with your personal computer. Unfortunately, reference manuals may not be good places to learn. The job of a reference manual is to completely describe each BASIC command. While it will show all of the options which may be used with a command, it may not tell you much about how or why you would use it. For this information, consider other How To books. Many exist. Check libraries and book stores. A careful study of this book has prepared you for one of these more detailed texts.

Another way to learn more is to share programs with friends. The old saying, two heads are better than one is often true in programming. Sometimes it is easier to find a bug in a program written by a friend than it is in your own programs. It is easier to think like a computer.

The best way to learn about programming is to write programs. Even if you have nothing more than a reference manual, you can discover how the BASIC commands work. Write little programs just to exercise the new statements you learn. You can use these as examples later.

Most importantly, sit down at the keyboard and experiment. Explore the commands and files. Change programs. Make them do what you want. Flush out the bugs. Experiment. Think of new ideas and turn them into programs. Try it.



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