how to write this without a if then

Roelof,

One technique to eliminating the use of #ifTrue:ifFalse: is to use double dispatching. There are some good examples of using double dispatching in Ralph Johnson's paper "ARITHMETIC AND DOUBLE DISPATCHING IN SMALLTALK-80"[1].  you should be able to get the basic idea by skimming the smalltalk code examples.

I used double dispatching fairly extensively in Metacello ...

In practice you may not eliminate all use of #ifTrue:ifFalse:, but double dispatching works quite well in places where you are tempted to do type checking...

Dale

Hello,

Yesterday I had a talk with luc frabresse about using if then.
He said if I understand it right, Its the best to not using a if then or a ifTrue/ifFalse.

Can anyone help me figure out how to rewrite this project so I will not use the ifTrue in the basement function.

my code so far can be found here : https://github.com/RoelofWobben/AOC2015

Roelof

DRAT!  What (genius negated) designed gmail's interface?

If I am wrong that the canHandleInput: class method of

IllegalMoveSanta should return false, not true (because

the order of the elements of #subclasses is not defined,

so that IllegalMoveSanta might *always* be selected),

then that is further evidence that avoiding "if" made

the code LESS readable.

You are a beginner at this, doing all the right things

for a beginner to do.  One of the things you have to do

is to develop a sense of "smell" for code.  Each class

ought to "pull its weight".  Four classes -- which it

really does not make sense to instantiate, just put the

instance methods on the class side and drop the #new --

just to avoid a couple of very clear "ifs"?  I don't

think so.  IF THESE CLASSES NEEDED TO EXIST FOR SOME

OTHER REASON, if they had real work to do, sure.  But

they don't.  They only bloat and obscure the code.

On Wed, 28 Nov 2018 at 20:54, Richard O'Keefe <[hidden email]> wrote:

In Advent-of-Code 2015, the first problem is

really quite simple.  There are at least two

ways to think about it.

"CS101":

   set a counter to 0

   for each character of the string

     if it is '(' increment the counter

     if it is ')' decrement the counter

   report the counter

"Higher level":

   report the difference between

     (the number of '(' characters in the string) and

     (the number of ')' characters in the string).

Expressed in Smalltalk this looks something like

  Transcript print:

    (s occurrencesOf: $() - (s occurrencesOf: $)); cr.

Make no mistake: you *cannot* tell the difference

between $( and $) using class-based dispatch because

they belong to the same class.  There has to be an

"if" somewhere, the question is not whether but where.

In this case, counting the number of occurrences of an

object in a collection is has been a standard Collection

method for nearly 40 years; it's one of the basic

operations you need to learn.

The "higher level" approach can be less efficient that

the "CS101" approach, but in a case like this we really

do not care.  We want the code to be *clear*.

What about the second part of the problem?

Not having submitted any answers, I can't actually

see the second part on the AOC site, but luckily you

have included it in your program.

We want to

   [find the first place] where

      [the cumulative sum] of (c=$()

      [minus]

      [the cumulative sum] of (c=$))

      equals -1.

The "CS101" approach is

   n := i := 0.

   while n >= 0 and i < size(s) do

      i +:= 1

      if s[i] = $( then n := n + 1

      if s[i] = $) then n := n - 1

   report n

The "higher level" approach looks something like

  n := (s cumCount: [:each | each = $(]) -

       (s cumCount: [:each | each = $)])

       indexOf: -1.

-- although it gives 0 instead of s size + 1 when

-1 is never reached.

Here #indexOf: is standard, #- is defined on sequences

in Squeak and Pharo, but #cumCount: does not

exist.  So we need something like

    cumCount: aBlock       

      |c a|       

      a := Array new: self size.

      c := 0.

      self withIndexDo: [:each :i |

        (aBlock value: each) ifTrue: [c := c + 1].

        a at: i put: c].

      ^a

This is *not* coupled to the particular use we have

in mind for it; it is in no way tied to characters

or strings.  It's quite general.

Note: we do not need any new classes, except maybe

a place to put one problem-specific method.

While this answer, with no loop and no if in the

problem-specific code, is quite pretty, it has a

problem.  Suppose the string to have M characters

and the desired step to be number K.  The CS101

approach takes O(K) time and no allocations, but

the higher level approach takes O(M) time and

allocates three M-element Arrays.  (In a non-strict

functional language like Haskell, the higher level

version *also* takes O(K) time, and with a good

enough "deforesting" compiler should allocate no

data structures.)

For a problem like this, I really don't care about

the efficiency aspect.  If I *do* care about that,

then starting from a higher level version gives me

something to test a lower level version against.

To get an efficient answer to the second part,

we still don't need a new semantic class, just

some place to put the code.

Day1

  class methods:

    indexOfFirstBasementTime: steps

      |floor|

      floor := 0.

      steps keysAndValuesDo: [:i :each |

        each = $( ifTrue: [floor := floor + 1].

        each = $) ifTrue: [floor := floor - 1].

        floor = -1 ifTrue: [^i]].

      ^0 "same convention as #indexOf:"

Does this contain "if"?  Why yes, it does.

Is there any problem with that?  Why no, there isn't.

You need to treat members of the same class (left

parenthesis, right parenthesis, others) differently.

You need to treat members of the same class (minus

one, all other integers) differently.

Would there be any gain in clarity or maintainability

if these ifs were somehow eliminated?  Certainly NOT.

Quite the reverse, in fact.

input2 withIndexDo: [ :element :index | |action| 

action:= SantaAction getActionFor: element. floor := action  doMovementFor: floor .

self hasReachedBasement 

ifTrue: [^ index]].

    ^ '-2'.

There is only one word for this: obfuscated.

I was initially puzzled by your returning -2

instead of the conventional 0 if the basement

is not reached, and then *deeply confused* by

the fact that you are returning a *string* in

this case.

Looking at your code, I was further confused

by variables called 'aSymbol' whose value is

always and only a Character, never a Symbol.

And if I am wrong that

IllegalMoveSanta class>> canHandleInput:

in Smalltalk is to return 0 when something is

      

approach

We want to find the first place where something

becomes true.  There are again at least to approaches. 

On Wed, 28 Nov 2018 at 05:41, Roelof Wobben <[hidden email]> wrote:

Hello,

Yesterday I had a talk with luc frabresse about using if then.
He said if I understand it right, Its the best to not using a if then or
a ifTrue/ifFalse.

Can anyone help me figure out how to rewrite this project so I will not
use the ifTrue in the basement function.

my code so far can be found here : https://github.com/RoelofWobben/AOC2015

Roelof