The first step in writing a program, whether it will eventually be programmed in BASIC or Assembler, is to express your problem in terms of simple steps that the computer can understand.
The Atom could be put to an immense number of different uses; anything from solving mathematical problems, controlling external equipment, playing games, accounting and book-keeping, waveform processing, document preparation...etc. The list is endless. Obviously all these applications cannot be included in a computer's repertoire of operations. Instead what is provided is a versatile set of more fundamental operations and functions which, in combination, can be used to solve such problems.
It is therefore,up to you to become familiar with the fundamental operations that are provided, and learn how to solve problems by combining these operations into programs.
Programming is rather like trying to explain to a novice cook, who understands little more than the meanings of the operations 'stir', 'boil’, etc, how to bake a cake. The recipe corresponds to the program; it consists of a list of simple operations 'stir', 'bake', with certain objects such as 'flour', 'eggs':
Recipe 1. Sponge Cake
1. Mix together 4 oz. sugar and 4 oz. butter.
2. Stir in 2 eggs.
3. Stir in 4 oz. flour.
4. Put into tins.
5. Bake for 20 mins. at Mark 4.
6. Remove from oven and eat.
7. END
The recipe is written so that, provided all the ingredients are already to hand, the cook can follow each command in turn without having to look ahead and worry about what is to come. Similarly, a computer only executes one operation at a time, and cannot look ahead at what is to come.
Before writing a program in BASIC or Assembler it is a good idea to draw a 'flowchart' indicating the operations required, and the order in which they should be performed. The generally accepted standard is for operations to be drawn inside rectangular boxes, with lines linking these boxes to show the flow of control. A simple flowchart for the program to bake a cake might be drawn as follows:
Many recipes do not just contain a sequence of steps to be performed, but contain conditions under which several different courses of action should be taken. For example, for a perfect cake line 5 would be better written:
5. Bake until golden brown.
It would then be necessary to open the oven door every five minutes and make a decision about the colour of the cake. Decisions are represented in flowcharts by diamond-shaped boxes, with multiple exits labelled with the possible outcomes of the decision. The new flowchart would then be:
The action 'bake for 5 mins.' is repeatedly performed until the test 'is it brown?' gives the answer 'yes'. Of course the program would go dramatically wrong if the oven were not switched on; the program would remain trapped in a loop.
With these two simple concepts, the action and the decision, almost anything can be flowcharted. Part of the trick in flowcharting programs is to decide how much detail to put into the flowchart. For example, in the cake program it would be possible to add the test 'is butter and sugar mixed?' and if not, loop back to the operation 'mix butter and sugar'. Usually flowcharts should be kept as short as possible so that the logic of the program is not obscured by a lot of unnecessary fine detail.
Recipes sometimes require a particular series of operations to be performed a fixed number of times. The following recipe for puff pastry illustrates this:
Recipe 2. Puff Pastry
1. Mix 6 oz. flour with 2 oz. butter.
2. Roll out pastry.
3. Spread with 2 oz. butter.
4. Fold in half.
5. Repeat steps 2 to 4 a further 3 times.
6. END
In this recipe the cook has to perform operations a total of 4 times. A cook would probably keep a mental note of how many times he has performed these operations; for the sake of the flowchart it is convenient to give the number of operations a label, such as T. The flowchart of the puff pastry recipe would then be:
The loop consisting of statements 2 to 4 is performed 4 times; the test at the end gives the answer 'no' for T=1, 2, and 3, and the answer 'yes' for T=4.
To perform an operation several times in a BASIC or Assembler program an identical method can be used; a counter, such as T, is used to count the number of operations and the counter is tested each time to determine whether enough operations have been completed.
A recipe may include a reference to another recipe. For example, a typical recipe for apple tart might be as follows:
Recipe 3. Apple Tart
1. Peel and core 6 cooking apples.
2. Make pastry as in recipe 2.
3. Line tart tin with pastry.
4. Put in apple.
5. Bake for 40 mins. mark 4.
6. END
To perform step 2 it is necessary to insert a marker in the book at the place of the original recipe, find the new recipe and follow it, and then return to the original recipe and carry on at the next statement.
In computer programming a reference to a separate routine is termed a 'subroutine call'. The main recipe, for apple tart, is the main routine; one of its statements calls the recipe for puff pastry, the subroutine. Note that the subroutine could be referred to many times throughout the recipe book; in the recipe for steak and kidney pie, for example. One reason for giving it separately is to save space; otherwise it would have to be reproduced for every recipe that needed it.
Note that, in order not to lose his place, the cook needed a marker to insert in the recipe book so that he should know where to return to at the end of the subroutine. In BASIC or assembler programs the computer keeps a record of where you were when you call a subroutine, and returns you there automatically at the end of the subroutine. In other respects, the process of executing a subroutine on a computer is just like this analogy.
Before writing a program in BASIC or Assembler it is a good idea to express the problem in one of the forms used in this chapter:
1. As a list of numbered steps describing, in words, exactly what to do at each step.
2. As a flowchart using the following symbols:
|
for actions | |
 |
for decisions | |
|
start of program | |
|
end of program |
Having done this, the job of writing the program in BASIC or Assembler will be relatively easy.