Lab 4: Blocks and OrderedCollections

What is a block?

A block is a sequence of statements. It can be passed to other statements as an argument. A block may have parameters too. Refer to page 516-517 of Coad/Nicola book.

What am I going to do?

First, create a subclass of OrderedCollection, and call it MyList. This class has only three instance methods:

        initialize
                1 to: 15 do: [:i | self add: i]


        doBlock: aBlock
                aBlock value: self.


        eachDo: aBlock
                1 to: self size do: [:index |
                        self at: index put: (
                        aBlock value: (self at: index))]

Then, in the Workspace, write:

        |list sum result|


        list := MyList new initialize.
        sum := [:aList |
                | tem |
                tem := 0.
                aList do:[:v | tem := tem + v].
                result := tem.
                ].


        list doBlock: sum.
        Transcript show: result printString; cr.

Here, sum is a block. It takes a collection as parameter, and get the sum of the value of that collection. After the execution of the above, you should get 120 as result.

Now add one more variable inc1 in the variable list. And append the code below in the workspace.

        inc1 := [:each | each + 1].
        list eachDo: inc1.
        list doBlock: sum.
        Transcript show: result printString; cr.

The block inc1 is used to increase one element of the collection by one. After ``do it'', you should get 135.

Now, here comes your job. You should make the following blocks:

average

Similar to sum, but get the average value as result.

deviation

It gets the deviation from average as the result. The deviation from average of a collection [Macro error: Can't compile this script because of a syntax error.] is defined as

        (   (x1 - average)2
                + (x2 - average)2
                + ...
                + (xn - average)2 ) / (n - 1)

(note the square operations).

squ

It sets the value of each element of the collection to the square of itself.

Then, your Workspace should looks like:

        |list sum result inc1 average deviation squ|


        list := MyList new initialize.
        sum := [:aList |
                | tem |
                tem := 0.
                aList do:[:v | tem := tem + v].
                result := tem.
                ].


        list doBlock: sum.
        Transcript show: result printString; cr.


        average := []. "<-- Your job"

        list doBlock: average.
        Transcript show: result printString; cr.

        deviation := []. "<-- Your job"

        list doBlock: deviation.
        Transcript show: result printString; cr.

        inc1 := [:each | each + 1].

        list eachDo: inc1.

        list doBlock: sum.
        Transcript show: result printString; cr.

        list doBlock: average.
        Transcript show: result printString; cr.

        list doBlock: deviation.
        Transcript show: result printString; cr.

        squ := []. "<-- Your job"

        list eachDo: squ.

        list doBlock: sum.
        Transcript show: result printString; cr.

        list doBlock: average.
        Transcript show: result printString; cr.

        list doBlock: deviation.
        Transcript show: result printString; cr.

Finally ...

After you have done those blocks and got correct results, make a class method demo, copy everything on your Workspace and paste in that method. Also, at the beginning of that method, write down the results you got as comments. For example,

        demo
                "My results (in order): 120, 135, ..."


                |list sum result inc1 average deviation squ|
                ... ...

Turning in

Save the class you made, and file it out. Use the command:

        cat filename.st | elm -s "TI,Lab4," cs2390@prism.gatech.edu

And, quit your VisualWorks environment.