© 1991 Brian R. Page

Blast Off With BASIC

Chapter Nine: Next Stop -- Mars!

Imagine bouncing around on the Moon weighing only one-sixth of what you weigh here on Earth. Does this sound like fun? Think of the high jumps and the baseball games! How far could you smack a ball? Imagine the ballet leaps possible on a lunar stage! Someday we may tune our televisions to a live performance of the Ballet Company of Tranquillity Base.

Why stop with the Moon? Even on Mars you weigh only one-third of your ordinary body weight. Traveling around the solar system is going to be fun. While we are waiting for our rockets, we can write a BASIC program that shows our weight on the nine planets. Along the way, we can learn some new BASIC commands.

Our program begins by asking a person's weight. Then it multiplies their weight by a factor for each planet. For example, on the Moon, multiply the Earth weight by .17. Take a 100 pound person. Multiply 100 by .17. Their lunar weight is only 17 pounds! Finally, the program displays the result for each planet. The program has four steps:

  1. Input Earth weight
  2. Prepare the output screen
  3. Multiply the weight by the factor for each planet
  4. Print the results

Since the last two steps are repeated for each planet, perhaps a processing loop could be used. This would help keep the program simple. The same algorithm is used for each planet. Only the factor needs to change. A FOR-NEXT loop could handle this nicely.

Before tackling the weight calculation routine, let us prepare the screen for output. Here is the subroutine:

600 REM *****  PREPARE THE SCREEN  *****
610 CLS
630 PRINT "  Here is how much you will weigh on various planets"
640 PRINT "  as you travel around the solar system."
660 PRINT ,"Planet",,"Weight"

Notice on line 660 that we are using commas again with the PRINT statement. This trick was introduced in chapter four. Now it is time to explore this more.

BASIC divides the display screen into zones that are fourteen characters wide. When a comma is used in a PRINT statement, and that comma is not contained within quotation marks, BASIC shifts the text over to the beginning of the next zone. This gets complicated when the text is wider than fourteen spaces. Some examples will help. Make sure the commas are not within the quotation marks.

10 PRINT "1","2","3","4","5"
20 PRINT "1","Here is some long text","2"

Line 10 prints a number in the first position of each zone. The next line puts the numeral 1 in the first zone, then uses a comma to shift the text into the second zone. However, since the text has 22 characters, it takes up the sec- ond and third zones. The comma following the text causes the numeral 2 to be placed in zone four. Your screen should look like this:

1              2              3              4              5
1              Here is some long text        2

As you can see, using commas with PRINT statements is a good way to create columns of output. That is exactly what we will do with our program to calculate outer space weights.

Look again at line 660. The first comma causes BASIC to skip the first zone. Then it prints the word Planet. Next, two commas cause BASIC to skip to zone three and then to zone four. The word Weight is placed in zone four. Since Planet did not completely fill up zone two, the first of the two commas following it only cause PRINT to skip to the beginning of the next zone. Compare line 20 of the little example program with line 660. Each print something in zone four but they use a different number of commas to get there. Experiment with commas and different lengths of text.


We already found that our program will use a simple formula to calculate weights on other planets. It will multiply weight on Earth by a factor for each planet. The same formula can be used each time. Only the factor needs to change. For example, the factor for the Moon is .17. For Mars it is .36. To keep the FOR-NEXT processing loop simple, we must supply the formula with a different factor each time through the loop. The BASIC READ-DATA commands provide a good solution.

The READ statement works much like INPUT. Both match some input to a variable. INPUT accepts typed input from the keyboard and assigns it to either a string, single precision, or integer variable. READ does the same, except the input does not come from the keyboard. READ works together with a DATA statement. The DATA statement supplies the input which READ assigns to variable. A program may contain many DATA statements. Each DATA is followed on the same line by some text or numbers. When a READ is executed, the information following the first DATA is assigned to a variable. When another READ is executed, the second DATA is used. The next READ uses the third DATA. Experiment with this example:

110 END
120 DATA First DATA statement
130 DATA Second DATA statement
140 DATA "This one, has a comma in the middle"
150 DATA "That's why it needs quotation marks"
160 DATA Here is the last DATA statement

The READ on line 10 places the text on line 120 into the string variable MESSAGE$. The PRINT moves MESSAGE$ to the screen. Then, another READ on line 30 gets the next DATA, this time on line 130, and places it in the same variable. In this example, five READ commands are used with five PRINT statements to process all of the DATA statements. We could write a smaller program using a FOR-NEXT loop.

10  FOR X% = 1 TO 5
40  NEXT X%
50  END
120 DATA First DATA statement
130 DATA Second DATA statement
140 DATA "This one, has a comma in the middle"
150 DATA That's why it needs quotation marks
160 DATA Here is the last DATA statement

Each time the READ executes within the FOR-NEXT loop, it picks up the next DATA statement. Only one set of READ and PRINT commands are used to process all five DATA statements.

The DATA statements can also supply more that one variable with each READ. To do this, separate each piece of data by a comma. The READ must then use more than one variable name. Try this program:

10 FOR X% = 1 TO 3
40 NEXT X%
50 END
60 DATA Call,me,1
70 DATA Call,me,2
80 DATA Call,me,3

Each piece of data in a DATA statement is called a FIELD. The fields are separated by commas. When they are read, the first field is placed in the first variable name of the READ statement. The second field is placed in the second variable, and so on. The variable type must match the type of data. For example, the integer variable THIRD% in the READ on line 20 can only be used to hold whole numbers. An error message is issued if you try to place letters into an integer variable. This works just like INPUT.

If a field in a DATA statement contains commas, semicolons (;), or many blank spaces, that field must be enclosed in quotation marks.

Each time a READ statement is executed it gathers the input from the next DATA statement. Therefore, one DATA statement must exist for each time READ executes. If a READ is executed after all DATA statements have been processed, BASIC issues the error message Out of Data and the program ends.


To see how READ-DATA is used in our weight calculating program, here are the remaining BASIC statements.

10  REM *********************************************************
20  REM *  WEIGHT                                               *
30  REM *  Program to calculate your weight on various planets. *
40  REM *********************************************************
50  CLS
60  PRINT "How much do you weigh on Earth?"
80  GOSUB 600                'PREPARE THE SCREEN
90  FOR X% = 1 TO 10
130 NEXT X%
140 END
410 DATA Mercury,.38
420 DATA Venus,.89
430 DATA Earth,1
440 DATA Moon,.17
450 DATA Mars,.36
460 DATA Jupiter,2.5
470 DATA Saturn,1.05
480 DATA Uranus,.89
490 DATA Neptune,1.18
500 DATA Pluto,.03

As you can see, this program is so short that most of the processing is performed in the mainline routine. Only a single subroutine is used. That subroutine prepares the screen for output.

Notice the ten DATA statements beginning at line 410. Each has two fields. The first is the planet name. The second field is the weight factor. These DATA statements are matched to variable names by the READ statement on line 100. This READ is part of a FOR-NEXT loop which executes ten times, once for each DATA statement. If you were to add more DATA statements, the FOR-NEXT loop would have to be changed.

The READ statement assigns the planet name in each DATA statement to a string variable called PLANET$. The weight factor is assigned to a single precision variable called FACTOR!. A single precision variable is needed because the weight factors are not whole numbers. Indeed, for several planets, the factors are numbers less than one. Single precision variables are allowed to hold fractional numbers. The INPUT statement on line 70 also uses a single precision variable to hold the person's weight on Earth. This means that BASIC will permit the user to enter something like 125.8765 pounds.

Now look at line 110. ETWEIGHT% is the integer variable holding the person's weight on another planet. ET stands for extra terrestrial. ETWEIGHT% is an integer variable that is created by multiplying together two single precision variables! When BASIC creates ETWEIGHT% it rounds the sum from the multiplication. ETWEIGHT% is printed as output on line 120. An integer variable is used to make the output look nice. If you want to know the weight more exactly, change ETWEIGHT% from an integer variable to a single precision variable on lines 110 and 120.

To check out the operation of the program, reply 100 to the INPUT statement. Incidentally, the weight does not have to be in pounds. The multiplication works for kilograms, ounces, or any other unit of weight. The output is the same unit of measure as the input. For 100 pounds, your program should produce a display like this:

            Here is how much you will weigh on various planets
            as you travel around the solar system.

                Planet                      Weight

                Mercury                      38
                Venus                        89
                Earth                        100
                Moon                         17
                Mars                         36
                Jupiter                      250
                Saturn                       105
                Uranus                       89
                Neptune                      118
                Pluto                        3

If any of your weights differ from these, check the DATA statements. Make sure that the factors are correct.

Each line of output in the table was created by line 120. A single PRINT statement inside of a FOR-NEXT loop executed ten times. Each time it added a line to the screen. For each line, the same two variables were used. The PLANET$ variable is changed by the READ statement in line 100 within the FOR-NEXT loop. ETWEIGHT% is also created by the FOR- NEXT loop. Aside from setting up the screen, the real work in this program is done by just three lines. The three lines inside of the FOR-NEXT loop READ the DATA, calculate ETWEIGHT%, and PRINT the output. This routine can handle any number of DATA statements. Just change the upper limit on line 90 to match the number of DATA statements.

When humans colonize the rest of the solar system, the planets will not be the only places people live. Some of the best sites will be among the asteroids. Asteroids are great chunks of rock, and possibly ice, that orbit the Sun between Mars and Jupiter. Thousands of asteroids exist. Future astronauts will probably tunnel into these giant boulders to carve out tiny worlds where humans can live. The largest asteroid is named Ceres. It is 477 miles in diameter. If you stood on the surface of Ceres, your weight would be only 1/40 of your Earth weight. Let's add Ceres to our table. Add a DATA statement for Ceres with a factor of .025 and make the necessary change to the FOR-NEXT loop on line 90.

To gain experience using commas to position output in a PRINT statement and with the READ-DATA statements, several improvements can be made to this program. First of all, this program lists a person's weight at the surface of the nine planets. However, some planets, like Jupiter and Saturn, do not have solid surfaces. These planets are gas giants. Their outer-most layers are simply thin clouds. Deeper into the atmosphere the clouds just get thicker and thicker. You cannot stand on the surface of Jupiter. Beyond Saturn lie the icy worlds of Uranus, Neptune, and Pluto. Although they, like Jupiter and Saturn, are covered in clouds, these outer planets probably have icy surfaces. It might be possible to stand.

Our program should note these differences. Let's add a short message to the output table. To do this, add another string variable to each DATA statement. Then enter a short description like Gas giant or Icy world to each planet. For Earth, enter Home Sweet Home. Then change the READ state- ment to assign the new field to a string variable. Call it MESSAGE$. Finally, include MESSAGE$ in the PRINT statement after ETWEIGHT%. Your output might look like this:

            Here is how much you will weigh on various planets
            as you travel around the solar system.

                Planet        Weight

                Mercury        38           Hot!
                Venus          89           Always Cloudy
                Earth          100          Home Sweet Home
                Moon           17           We've been here
                Mars           36           Good place to live
                Ceres          3            Asteroid
                Jupiter        250          Gas giant
                Saturn         105          Gas giant
                Uranus         89           Icy world
                Neptune        118          Icy world
                Pluto          3            Far Out!

Now let's get fancy. Who cares how much we weigh on Mars? Let's find out how high we can jump! Add another question after line 70. Ask, "How high can you jump on Earth?" Store the reply in a single precision variable called JUMP!. Then create another new single precision variable called JUMPFACTOR! and use it to add another column to our output. Add this algorithm into the FOR-NEXT loop.


Next, in the PRINT statement, slip ETJUMP% in between WEIGHT% and MESSAGE$. Of course, you must also change the column headings in line 660. For a 100 pound person who can jump three feet on Earth, your output should look like this:

            Here is how much you will weigh on various planets
            as you travel around the solar system.

                Planet        Weight        High Jump

                Mercury        38            8            Hot!
                Venus          89            3            Always Cloudy
                Earth          100           3            Home Sweet Home
                Moon           17            18           We've been here
                Mars           36            8            Good place to live
                Ceres          3             100          Asteroid
                Jupiter        250           1            Gas giant
                Saturn         105           3            Gas giant
                Uranus         89            3            Icy world
                Neptune        118           3            Icy world
                Pluto          3             100          Far Out!

To calculate JUMPFACTOR! we divide a single precision variable by an integer variable. Since this mixes variable types we do not use integer division. Use the ordinary slash symbol (/) for division, not the backslash symbol (\) used in integer division.

Some final improvements can be made by using colors. Add COLOR statements at lines 605, 655, and 665. Let the first COLOR statement set a background and a text color. The other two should simply change the text color. See "Text Mode Graphics" on page 78 to review using color in text mode.

Now that the program is complete, have some fun with it. Find out how high you can really jump on Earth. Then use that measurement in this program. Answer this question. On Earth, basketball goals are mounted at ten feet above the floor. Since just about everyone can slam dunk a ten foot goal on the Moon or Mars, how high should the goal be mounted?



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