Building a program
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With these seven basic things, and their related code, you can build pretty much every program in existence. We just add them together. For example, can you work out which bubble the following code makes? Flick back to the descriptions if you need to. Click on the bubble you think is made.
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Algorithms and Comments
The usual starting point for a program is an "Algorithm" – an English-language (or other) description of how a program is going to work. Most programs start with the coder writing down a sequence of steps the program will go through to do a job, and this then acts as a recipe for the program to be built around. For example, an algorithm for the above might be:
# When space key pressed
# Set 'x' to 'Hello World' -- make sure you make x first somewhere
# Check this happened ok with an if
# Provided it has, print 'x' three times in a loop.
# Otherwise, print the current year.
The key thing is that an algorithm allows us to logically break down the problem and think whether it will work before we get tied up in coding. Here, for example, we note that we need to set up the 'x' variable in Scratch's variable settings before we try to write this code.
When code is typed, algorithms are quite often added to the code text files as "Comments". Comments are bits of code that
are notes for people looking at the code, rather than for the computer to try to understand. Most languages have
some way of adding notes and signalling that the computer shouldn't compile them. The above algorithm is written as Python comments, for example. The computer
wouldn't try to understand them as Python code; it would ignore them because they start with #, which shows they are comments.
A lot of languages use // in the same way. With Scratch you can add comments by right-clicking on
blocks or the background area.
Writing the algorithm as comments helps to structure the code before you start building it.
The final thing to say is that code should be written and fixed a line at a time. The secret is to think how you can test each thing you do to make sure it works. This is usually as simple as putting in a few print/say instructions to test things are as you think they are. The code above could just set x up and test it by printing it once, for example. You can always delete testing code once you've checked stuff is working.
If you write one or two lines and test they work, if there is a new bug you know it is in the lines you've just written. If you write a massive amount of code and then try to run it, you'll get a lot of bugs showing. Not only is this very disheartening, but the size of new code makes it hard to find them.
Before you start to make a program, you write out a list of how it is going to work, step by step. This is called an "algorithm". It is like a recipe. You say what bits of code you are going to need, and what order to put them in.
Writing this helps you think how you will make the program work before you write the code.
With most languages you can put your algorithm into the file your code is in as a "comment". Comments are writing in the code that the computer ignores. They are just for you, to help you say how the code works.
If you put your algorithm in your code, you can then write each bit of it as code, one line at a time.
Usually there is a special letter you put at the
start of something you write to say it is a comment not code, for
example, # or //.
# This is a comment.
To add a comment to Scratch, click the right mouse button on a block or on the block background and choose "add comment".
The best way to write code is to write one or two lines, then test they work. That way, if there is a new bug, you know where it is.
The worst way to write code is to write large programs and then test them, as you'll have lots of bugs and not know where they are.
So, given all what we know, Scratch seems like a good starting point for most people. We'll look at some
Scratch resources, but then we'll look at some other languages as well.