Can you write any algorithm without an if statement?

This site开发者_运维技巧 tickled my sense of humour - http://www.antiifcampaign.com/ but can polymorphism work in every case where you would use an if statement?

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Smalltalk, which is considered as a "truly" object oriented language, has no "if" statement, and it has no "for" statement, no "while" statement. There are other examples (like Haskell) but this is a good one.

Quoting Smalltalk has no “if” statement:

Some of the audience may be thinking that this is evidence confirming their suspicions that Smalltalk is weird, but what I’m going to tell you is this:

An “if” statement is an abomination in an Object Oriented language.

Why? Well, an OO language is composed of classes, objects and methods, and an “if” statement is inescapably none of those. You can’t write “if” in an OO way. It shouldn’t exist. Conditional execution, like everything else, should be a method. A method of what? Boolean.

Now, funnily enough, in Smalltalk, Boolean has a method called ifTrue:ifFalse: (that name will look pretty odd now, but pass over it for now). It’s abstract in Boolean, but Boolean has two subclasses: True and False. The method is passed two blocks of code. In True, the method simply runs the code for the true case. In False, it runs the code for the false case. Here’s an example that hopefully explains:

(x >= 0) ifTrue: [
'Positive'
] ifFalse: [
'Negative'
]

You should be able to see ifTrue: and ifFalse: in there. Don’t worry that they’re not together.

The expression (x >= 0) evaluates to true or false. Say it’s true, then we have:

true ifTrue: [
'Positive'
] ifFalse: [
'Negative'
]

I hope that it’s fairly obvious that that will produce ‘Positive’.

If it was false, we’d have:

false ifTrue: [
'Positive'
] ifFalse: [
'Negative'
]

That produces ‘Negative’.

OK, that’s how it’s done. What’s so great about it? Well, in what other language can you do this? More seriously, the answer is that there aren’t any special cases in this language. Everything can be done in an OO way, and everything is done in an OO way.

I definitely recommend reading the whole post and Code is an object from the same author as well.

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That website is against using if statements for checking if an object has a specific type. This is completely different from if (foo == 5). It's bad to use ifs like if (foo instanceof pickle). The alternative, using polymorphism instead, promotes encapsulation, making code infinitely easier to debug, maintain, and extend.

Being against ifs in general (doing a certain thing based on a condition) will gain you nothing. Notice how all the other answers here still make decisions, so what's really the difference?

Explanation of the why behind polymorphism:

Take this situation:

void draw(Shape s) {
    if (s instanceof Rectangle)
        //treat s as rectangle
    if (s instanceof Circle)
        //treat s as circle
}

It's much better if you don't have to worry about the specific type of an object, generalizing how objects are processed:

void draw(Shape s) {
    s.draw();
}

This moves the logic of how to draw a shape into the shape class itself, so we can now treat all shapes the same. This way if we want to add a new type of shape, all we have to do is write the class and give it a draw method instead of modifying every conditional list in the whole program.

This idea is everywhere in programming today, the whole concept of interfaces is all about polymorphism. (Shape is an interface defining a certain behavior, allowing us to process any type that implements the Shape interface in our method.) Dynamic programming languages take this even further, allowing us to pass any type that supports the necessary actions into a method. Which looks better to you? (Python-style pseudo-code)

def multiply(a,b):
    if (a is string and b is int):
        //repeat a b times.
    if (a is int and b is int):
        //multiply a and b

or using polymorphism:

def multiply(a,b):
    return a*b

You can now use any 2 types that support the * operator, allowing you to use the method with types that haven't event been created yet.

See polymorphism and what is polymorhism.

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Though not OOP-related: In Prolog, the only way to write your whole application is without if statements.

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Yes actually, you can have a turing-complete language that has no "if" per se and only allows "while" statements:

http://cseweb.ucsd.edu/classes/fa08/cse200/while.html

As for OO design, it makes sense to use an inheritance pattern rather than switches based on a type field in certain cases... That's not always feasible or necessarily desirable though.

@ennuikiller: conditionals would just be a matter of syntactic sugar:

if (test) body;     is equivalent to    x=test; while (x) {x=nil; body;}

if-then-else is a little more verbose:

if (test) ifBody; else elseBody;

is equivalent to

x = test; y = true;
while (x) {x = nil; y = nil; ifBody;}
while (y) {y = nil; elseBody;}

the primitive data structure is a list of lists. you could say 2 scalars are equal if they are lists of the same length. you would loop over them simultaneously using the head/tail operators and see if they stop at the same point.

of course that could all be wrapped up in macros.

The simplest turing complete language is probably iota. It contains only 2 symbols ('i' and '*').

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Yep. if statements imply branches which can be very costly on a lot of modern processors - particularly PowerPC. Many modern PCs do a lot of pipeline re-ordering and so branch mis-predictions can cost an order of >30 cycles per branch miss.
On console programming it's sometimes faster to just execute the code and ignore it than check if you should execute it!

Simple branch avoidance in C:

if (++i >= 15)
{
    i = 0;
)

can be re-written as

 i = (i + 1) & 15;  

However, if you want to see some real anti-if fu then read this

Oh and on the OOP question - I'll replace a branch mis-prediction with a virtual function call? No thanks....

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The reasoning behind the "anti-if" campaign is similar to what Kent Beck said:

Good code invariably has small methods and small objects. Only by factoring the system into many small pieces of state and function can you hope to satisfy the “once and only once” rule. I get lots of resistance to this idea, especially from experienced developers, but no one thing I do to systems provides as much help as breaking it into more pieces.

If you don't know how to factor a program with composition and inheritance, then your classes and methods will tend to grow bigger over time. When you need to make a change, the easiest thing will be to add an IF somewhere. Add too many IFs, and your program will become less and less maintainable, and still the easiest thing will be to add more IFs.

You don't have to turn every IF into an object collaboration; but it's a very good thing when you know how to :-)

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You can define True and False with objects (in a pseudo-python):

class True:
    def if(then,else):
        return then
    def or(a):
        return True()
    def and(a):
        return a
    def not():
        return False()

class False:
    def if(then,else):
        return false
    def or(a):
        return a
    def and(a):
        return False()
    def not():
        return True()

I think it is an elegant way to construct booleans, and it proves that you can replace every if by polymorphism, but that's not the point of the anti-if campaign. The goal is to avoid writing things such as (in a pathfinding algorithm) :

if type == Block or type == Player:
    # You can't pass through this
else:
    # You can

But rather call a is_traversable method on each object. In a sense, that's exactly the inverse of pattern matching. "if" is useful, but in some cases, it is not the best solution.

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I assume you are actually asking about replacing if statements that check types, as opposed to replacing all if statements.

To replace an if with polymorphism requires a method in a common supertype you can use for dispatching, either by overriding it directly, or by reusing overridden methods as in the visitor pattern.

But what if there is no such method, and you can't add one to a common supertype because the super types are not maintained by you? Would you really go to the lengths of introducing a new supertype along with subtypes just to get rid of a single if? That would be taking purity a bit far in my opinion.

Also, both approaches (direct overriding and the visitor pattern) have their disadvantages: Overriding the method directly requires that you implement your method in the classes you want to switch on, which might not help cohesion. On the other hand, the visitor pattern is awkward if several cases share the same code. With an if you can do:

if (o instanceof OneType || o instanceof AnotherType) {
    // complicated logic goes here
}

How would you share the code with the visitor pattern? Call a common method? Where would you put that method?

So no, I don't think replacing such if statements is always an improvement. It often is, but not always.

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I used to write code a lot as the recommend in the anti-if campaign, using either callbacks in a delegate dictionary or polymorphism.

It's quite a beguiling argument, especially if you are dealing with messy code bases but to be honest, although it's great for a plugin model or simplifying large nested if statements, it does make navigating and readability a bit of a pain.

For example F12 (Go To Definition) in visual studio will take you to an abstract class (or, in my case an interface definition).

It also makes quick visual scanning of a class very cumbersome, and adds an overhead in setting up the delegates and lookup hashes.

Using the recommendations put forward in the anti-if campaign as much as they appear to be recommending looks like 'ooh, new shiny thing' programming to me.

As for the other constructs put forward in this thread, albeit it has been done in the spirit of a fun challenge, are just substitutes for an if statement, and don't really address what the underlying beliefs of the anti-if campaign.

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You can avoid ifs in your business logic code if you keep them in your construction code (Factories, builders, Providers etc.). Your business logic code would be much more readable, easier to understand or easier to maintain or extend. See: http://www.youtube.com/watch?v=4F72VULWFvc

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Haskell doesn't even have if statements, being pure functional. ;D

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You can do it without if per se, but you can't do it without a mechanism that allows you to make a decision based on some condition.

In assembly, there's no if statement. There are conditional jumps.

In Haskell for instance, there's no explicit if, instead, you define a function multiple times, I forgot the exact syntax, but it's something like this:

pseudo-haskell:

def posNeg(x < 0):
    return "negative"

def posNeg(x == 0):    
    return "zero"

def posNeg(x):
    return "positive"

When you call posNeg(a), the interpreter will look at the value of a, if it's < 0 then it will choose the first definition, if it's == 0 then it will choose the second definition, otherwise it will default to the third definition.

So while languages like Haskell and SmallTalk don't have the usual C-style if statement, they have other means of allowing you to make decisions.

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This is actually a coding game I like to play with programming languages. It's called "if we had no if" which has its origins at: http://wiki.tcl.tk/4821

Basically, if we disallow the use of conditional constructs in the language: no if, no while, no for, no unless, no switch etc.. can we recreate our own IF function. The answer depends on the language and what language features we can exploit (remember using regular conditional constructs is cheating co no ternary operators!)

For example, in tcl, a function name is just a string and any string (including the empty string) is allowed for anything (function names, variable names etc.). So, exploiting this we can do:

proc 0 {true false} {uplevel 1 $false; # execute false code block, ignore true}
proc 1 {true false} {uplevel 1 $true;  # execute true code block, ignore flase}

proc _IF {boolean true false} {
    $boolean $true $false
}

#usage:
_IF [expr {1<2}] {
    puts "this is true"
} {
  #else:
    puts "this is false"
}

or in javascript we can abuse the loose typing and the fact that almost anything can be cast into a string and combine that with its functional nature:

function fail (discard,execute) {execute()}
function pass (execute,discard) {execute()}
var truth_table = {
    'false' : fail,
    'true' : pass
}
function _IF (expr) {
  return truth_table[!!expr];
}

//usage:
_IF(3==2)(
    function(){alert('this is true')},
//else
    function(){alert('this is false')}
);

Not all languages can do this sort of thing. But languages I like tend to be able to.

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The idea of polymorphism is to call an object without to first verify the class of that object.
That doesn't mean the if statement should not be used at all; you should avoid to write

if (object.isArray()) {
  // Code to execute when the object is an array.
} else if (object.inString()) {
  // Code to execute if the object is a string.
}

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It depends on the language.

Statically typed languages should be able to handle all of the type checking by sharing common interfaces and overloading functions/methods.

Dynamically typed languages might need to approach the problem differently since type is not checked when a message is passed, only when an object is being accessed (more or less). Using common interfaces is still good practice and can eliminate many of the type checking if statements.

While some constructs are usually a sign of code smell, I am hesitant to eliminate any approach to a problem apriori. There may be times when type checking via if is the expedient solution.

Note: Others have suggested using switch instead, but that is just a clever way of writing more legible if statements.

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Well, if you're writing in Perl, it's easy!

Instead of

if (x) {
    # ...
}

you can use

unless (!x){
    # ... 
}

;-)

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In answer to the question, and as suggested by the last respondent, you need some if statements to detect state in a factory. At that point you then instantiate a set of collaborating classes that solve the state specific problem. Of course, other conditionals would be required as needed, but they would be minimized.

What would be removed of course would be the endless procedural state checking rife in so much service based code.

Interesting smalltalk is mentioned, as that's the language I used before being dragged across into Java. I don't get home as early as I used to.

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I thought about adding my two cents: you can optimize away ifs in many languages where the second part of a boolean expression is not evaluated when it won't affect the result.

With the and operator, if the first operand evaluates to false, then there is no need to evaluate the second one. With the or operator, it's the opposite - there's no need to evaluate the second operand if the first one is true. Some languages always behave like that, others offer an alternative syntax.

Here's an if - elseif - else code made in JavaScript by only using operators and anonymous functions.

document.getElementById("myinput").addEventListener("change", function(e) {

  (e.target.value == 1 && !function() {
    alert('if 1');
  }()) || (e.target.value == 2 && !function() {
    alert('else if 2');
  }()) || (e.target.value == 3 && !function() {
    alert('else if 3');
  }()) || (function() {
    alert('else');
  }());

});

<input type="text" id="myinput" />

This makes me want to try defining an esoteric language where blocks implicitly behave like self-executing anonymous functions and return true, so that you would write it like this:

(condition && {
  action
}) || (condition && {
  action
}) || {
  action
}