How to use Eiffel functions?

Question

So I'm just starting to learn Eiffel. One of the first exercises in the book I'm using says to make a function that does base^exp without using ^. I've copied my code below.

class
    APPLICATION

inherit
    ARGUMENTS

create
    make

feature {NONE} -- Initialization

    make
            -- Run application.
        do
            create power(2;3)
            printf("2 to the power of 3 is " + answer)
        end

    power(base : REAL; exp : INTEGER) : REAL
        -- computers base raised to the bower of exp without using ^
        local
            remain : INTEGER
        do
            remain := exp
            if remain = 0 then
                result := 1
            else
                from
                until
                    remain = 0
                loop
                    result := result * result
                    remain := remain -1
                end
            end
        end
end

How do I use this? Do I need it on the same level as feature{NONE}'s make? I know how I'm calling it is wrong, and I can't find anything in the chapter I just read, or online on how to pass parameters into it or how to use it's results.

score 6 · Answer 1 · edited Jul 24 '15 at 23:22

There are several issues with the original code:

create is used to create an object, but you are not going to create anything, but to get a result of a computation of the function power by calling it. Therefore the keyword create is not needed.
You are using an entity answer to report the result of evaluation on a screen. However it is not declared anywhere. I believe the proper place would be a local variable declaration section.
The entity answer is not initialized to the result of the function power. This is usually done by an assignment instruction.
Feature arguments are separated by a comma, not by a semicolon.
From the original code it's unclear what is the type of the variable answer. Assuming it matches the type of the function power, before adding it to a string, it needs to be converted to a string. This is done by calling the feature out.
The standard feature for printing a string to a console is print, not printf.

Combining the critical points above, we get

make
        -- Run application.
    local
        answer: REAL
    do
        answer := power(2, 3)
        print ("2 to the power of 3 is " + answer.out)
    end

After that the code can be compiled. Now less critical points:

It is a good style to put features to a dedicated feature clauses, so I would add a line like feature -- Basic operations before the feature power.
The implementation of the feature power has at least two problems. I'm not going to detail them here, but would give two hints instead:
- by default numeric Result is initialized to 0, this needs to be taken into account for operations that use it without first assigning any other value
- even though an argument base is passed to the function power it remains unused in the original version of the code

How to use Eiffel functions?

1 Answers1