Computer Programming with GNU Smalltalk - Free

50 Computer Programming with GNU Smalltalk
This structure starts from number aNumber and evaluates the aBlock object until it reaches
anotherNumber. It increases one at every loop. aBlock object also includes a block argument so that
programmer can use the current index in expressions of aBlock.
We mentioned that whileTrue: message is usually used with indefinite cases. to:do: on the
contrary, is used when it is known how many loops we needed. That's why it is sometimes called
definite loop.
For experienced programmers:
Being a definite loop, you might guess that this message is used in Smalltalk instead of for loop of C
based languages.
Now let's look at this structure via an example:
"5_lines.st"
"A program which prints 5 lines to demonstrate the usage of to:do: message."
1 to: 5 do: [:x |
Transcript show: 'This is the ', x printString, '. line.'; cr.
]
This is the 1. line.
This is the 2. line.
This is the 3. line.
This is the 4. line.
This is the 5. line.
This program prints five lines to the output each indicating their own index. Note that we used a block
argument, x, for getting the index of each loop. If we didn't write an argument for block object then we
would get an error.
to:by:do:
Sometimes we might not want to increase the index of to:do: message one by one. There is an
alternative message called to:by:do: for this purpose. Its general structure is like this:
aNumber to: anotherNumber by: step do: aBlock
It allows us to reach the number anotherNumber in steps of the number defined by step. For
example, write the same example above, this time by using this alternative message to increase the
index two by two. Also count until ten, this time, so that the loop does not end very quickly:
"tobydo.st"
"A program to demonstrate the usage of to:by:do: message."
1 to: 10 by: 2 do: [:x |
Transcript show: 'This is the ', x printString, '. line.'; cr.