Why is the 'if' statement considered evil?

Question

I just came from Simple Design and Testing Conference. In one of the session we were talking about evil keywords in programming languages. Corey Haines, who proposed the subject, was convinced that if statement is absolute evil. His alternative was to create functions with predicates. Can you please explain to me why if is evil.

I understand that you can write very ugly code abusing if. But I don't believe that it's that bad.

Answer 1

The if statement is rarely considered as "evil" as goto or mutable global variables -- and even the latter are actually not universally and absolutely evil. I would suggest taking the claim as a bit hyperbolic.

It also largely depends on your programming language and environment. In languages which support pattern matching, you will have great tools for replacing if at your disposal. But if you're programming a low-level microcontroller in C, replacing ifs with function pointers will be a step in the wrong direction. So, I will mostly consider replacing ifs in OOP programming, because in functional languages, if is not idiomatic anyway, while in purely procedural languages you don't have many other options to begin with.

Nevertheless, conditional clauses sometimes result in code which is harder to manage. This does not only include the if statement, but even more commonly the switch statement, which usually includes more branches than a corresponding if would.

There are cases where it's perfectly reasonable to use an if

When you are writing utility methods, extensions or specific library functions, it's likely that you won't be able to avoid ifs (and you shouldn't). There isn't a better way to code this little function, nor make it more self-documented than it is:

// this is a good "if" use-case
int Min(int a, int b)
{
    if (a < b) 
       return a;
    else
       return b;
}

// or, if you prefer the ternary operator
int Min(int a, int b)
{
    return (a < b) ? a : b;
}

Branching over a "type code" is a code smell

On the other hand, if you encounter code which tests for some sort of a type code, or tests if a variable is of a certain type, then this is most likely a good candidate for refactoring, namely replacing the conditional with polymorphism.

The reason for this is that by allowing your callers to branch on a certain type code, you are creating a possibility to end up with numerous checks scattered all over your code, making extensions and maintenance much more complex. Polymorphism on the other hand allows you to bring this branching decision as closer to the root of your program as possible.

Consider:

// this is called branching on a "type code",
// and screams for refactoring
void RunVehicle(Vehicle vehicle)
{
    // how the hell do I even test this?
    if (vehicle.Type == CAR)
        Drive(vehicle);
    else if (vehicle.Type == PLANE)
        Fly(vehicle);
    else
        Sail(vehicle);
}

By placing common but type-specific (i.e. class-specific) functionality into separate classes and exposing it through a virtual method (or an interface), you allow the internal parts of your program to delegate this decision to someone higher in the call hierarchy (potentially at a single place in code), allowing much easier testing (mocking), extensibility and maintenance:

// adding a new vehicle is gonna be a piece of cake
interface IVehicle
{
    void Run();
}

// your method now doesn't care about which vehicle 
// it got as a parameter
void RunVehicle(IVehicle vehicle)
{
    vehicle.Run();
}

And you can now easily test if your RunVehicle method works as it should:

// you can now create test (mock) implementations
// since you're passing it as an interface
var mock = new Mock<IVehicle>();

// run the client method
something.RunVehicle(mock.Object);

// check if Run() was invoked
mock.Verify(m => m.Run(), Times.Once());

Patterns which only differ in their if conditions can be reused

Regarding the argument about replacing if with a "predicate" in your question, Haines probably wanted to mention that sometimes similar patterns exist over your code, which differ only in their conditional expressions. Conditional expressions do emerge in conjunction with ifs, but the whole idea is to extract a repeating pattern into a separate method, leaving the expression as a parameter. This is what LINQ already does, usually resulting in cleaner code compared to an alternative foreach:

Consider these two very similar methods:

// average male age
public double AverageMaleAge(List<Person> people)
{
    double sum = 0.0;
    int count = 0;
    foreach (var person in people)
    {
       if (person.Gender == Gender.Male)
       {
           sum += person.Age;
           count++;
       }
    }
    return sum / count; // not checking for zero div. for simplicity
}

// average female age
public double AverageFemaleAge(List<Person> people)
{
    double sum = 0.0;
    int count = 0;
    foreach (var person in people)
    {
       if (person.Gender == Gender.Female) // <-- only the expression
       {                                   //     is different
           sum += person.Age;
           count++;
       }
    }
    return sum / count;
}

This indicates that you can extract the condition into a predicate, leaving you with a single method for these two cases (and many other future cases):

// average age for all people matched by the predicate
public double AverageAge(List<Person> people, Predicate<Person> match)
{
    double sum = 0.0;
    int count = 0;
    foreach (var person in people)
    {
       if (match(person))       // <-- the decision to match
       {                        //     is now delegated to callers
           sum += person.Age;
           count++;
       }
    }
    return sum / count;
}

var males = AverageAge(people, p => p.Gender == Gender.Male);
var females = AverageAge(people, p => p.Gender == Gender.Female);

And since LINQ already has a bunch of handy extension methods like this, you actually don't even need to write your own methods:

// replace everything we've written above with these two lines
var males = list.Where(p => p.Gender == Gender.Male).Average(p => p.Age);
var females = list.Where(p => p.Gender == Gender.Female).Average(p => p.Age);

In this last LINQ version the if statement has "disappeared" completely, although:

1. to be honest the problem wasn't in the if by itself, but in the entire code pattern (simply because it was duplicated), and
2. the if still actually exists, but it's written inside the LINQ Where extension method, which has been tested and closed for modification. Having less of your own code is always a good thing: less things to test, less things to go wrong, and the code is simpler to follow, analyze and maintain.

Huge runs of nested if/else statements

When you see a function spanning 1000 lines and having dozens of nested if blocks, there is an enormous chance it can be rewritten to

1. use a better data structure and organize the input data in a more appropriate manner (e.g. a hashtable, which will map one input value to another in a single call),
2. use a formula, a loop, or sometimes just an existing function which performs the same logic in 10 lines or less (e.g. this notorious example comes to my mind, but the general idea applies to other cases),
3. use guard clauses to prevent nesting (guard clauses give more confidence into the state of variables throughout the function, because they get rid of exceptional cases as soon as possible),
4. at least replace with a switch statement where appropriate.

Refactor when you feel it's a code smell, but don't over-engineer

Having said all this, you should not spend sleepless nights over having a couple of conditionals now and there. While these answers can provide some general rules of thumb, the best way to be able to detect constructs which need refactoring is through experience. Over time, some patterns emerge that result in modifying the same clauses over and over again.

Answer 2

Sometimes it's necessary to take an extreme position to make your point. I'm sure this person uses if -- but every time you use an if, it's worth having a little think about whether a different pattern would make the code clearer.

Preferring polymorphism to if is at the core of this. Rather than:

if(animaltype = bird) {
     squawk();
} else if(animaltype = dog) {
     bark();
}

... use:

animal.makeSound();

But that supposes that you've got an Animal class/interface -- so really what the if is telling you, is that you need to create that interface.

So in the real world, what sort of ifs do we see that lead us to a polymorphism solution?

if(logging) {
      log.write("Did something");
}

That's really irritating to see throughout your code. How about, instead, having two (or more) implementations of Logger?

this.logger = new NullLogger();    // logger.log() does nothing
this.logger = new StdOutLogger();  // logger.log() writes to stdout

That leads us to the Strategy Pattern.

Instead of:

if(user.getCreditRisk() > 50) {
     decision = thoroughCreditCheck();
} else if(user.getCreditRisk() > 20) {
     decision = mediumCreditCheck();
} else {
     decision = cursoryCreditCheck();
}

... you could have ...

decision = getCreditCheckStrategy(user.getCreditRisk()).decide();

Of course getCreditCheckStrategy() might contain an if -- and that might well be appropriate. You've pushed it into a neat place where it belongs.

Answer 3

If is not evil! Consider ...

int sum(int a, int b) {
    return a + b;
}

Boring, eh? Now with an added if ...

int sum(int a, int b) {
    if (a == 0 && b == 0) {
        return 0;
    }
    return a + b;
}

... your code creation productivity (measured in LOC) is doubled.

Also code readability has improved much, for now you can see in the blink of an eye what the result is when both argument are zero. You couldn't do that in the code above, could you?

Moreover you supported the testteam for they now can push their code coverage test tools use up more to the limits.

Furthermore the code now is better prepared for future enhancements. Let's guess, for example, the sum should be zero if one of the arguments is zero (don't laugh and don't blame me, silly customer requirements, you know, and the customer is always right). Because of the if in the first place only a slight code change is needed.

int sum(int a, int b) {
    if (a == 0 || b == 0) {
        return 0;
    }
    return a + b;
}

How much more code change would have been needed if you hadn't invented the if right from the start.

Thankfulness will be yours on all sides.

Conclusion: There's never enough if's.

There you go. To.

Answer 4

Perhaps with quantum computing it will be a sensible strategy to not use IF statements but to let each leg of the computation proceed and only have the function 'collapse' at termination to a useful result.

Answer 5

A good quote from Code Complete:

Code as if whoever maintains your program is a violent psychopath who knows where you live.
— Anonymous

IOW, keep it simple. If the readability of your application will be enhanced by using a predicate in a particular area, use it. Otherwise, use the 'if' and move on.

Answer 6

I think If statements are evil, but If expressions are not. What I mean by an if expression in this case can be something like the C# ternary operator (condition ? trueExpression : falseExpression). This is not evil because it is a pure function (in a mathematical sense). It evaluates to a new value, but it has no effects on anything else. Because of this, it works in a substitution model.

Imperative If statements are evil because they force you to create side-effects when you don't need to. For an If statement to be meaningful, you have to produce different "effects" depending on the condition expression. These effects can be things like IO, graphic rendering or database transactions, which change things outside of the program. Or, it could be assignment statements that mutate the state of the existing variables. It is usually better to minimize these effects and separate them from the actual logic. But, because of the If statements, we can freely add these "conditionally executed effects" everywhere in the code. I think that's bad.

Answer 7

Good that in ruby we have unless ;)

But seriously probably if is the next goto, that even if most of the people think it is evil in some cases is simplifying/speeding up the things (and in some cases like low level highly optimized code it's a must).

Answer 8

IMO: I suspect he was trying to provoke a debate and make people think about the misuse of 'if'. No one would seriously suggest such a fundamental construction of programming syntax was to be completely avoided would they?

Answer 9

Just like in the bible verse about money, if statements are not evil -- the LOVE of if statements is evil. A program without if statements is a ridiculous idea, and using them as necessary is essential. But a program that has 100 if-else if blocks in a row (which, sadly, I have seen) is definitely evil.

Answer 10

if is not evil(I also hold that assigning morality to code-writing practices is asinine...).

Mr. Haines is being silly and should be laughed at.

Answer 11

It probably comes down to a desire to keep code cyclomatic complexity down, and to reduce the number of branch points in a function. If a function is simple to decompose into a number of smaller functions, each of which can be tested, you can reduce the complexity and make code more easily testable.

Answer 12

Predicates come from logical/declarative programming languages, like PROLOG. For certain classes of problems, like constraint solving, they are arguably superior to a lot of drawn out step-by-step if-this-do-that-then-do-this crap. Problems that would be long and complex to solve in imperative languages can be done in just a few lines in PROLOG.

There's also the issue of scalable programming (due to the move towards multicore, the web, etc.). If statements and imperative programming in general tend to be in step-by-step order, and not scaleable. Logical declarations and lambda calculus though, describe how a problem can be solved, and what pieces it can be broken down into. As a result, the interpreter/processor executing that code can efficiently break the code into pieces, and distribute it across multiple CPUs/cores/threads/servers.

Definitely not useful everywhere; I'd hate to try writing a device driver with predicates instead of if statements. But yes, I think the main point is probably sound, and worth at least getting familiar with, if not using all the time.

Answer 13

I have to say that I recently have begun to view if statements as a code smell: especially when you find yourself repeating the same condition several times. But there's something you need to understand about code smells: they don't necessarily mean that the code is bad. They just mean that there's a good chance the code is bad.

For instance, comments are listed as a code smell by Martin Fowler, but I wouldn't take anyone seriously who says "comments are evil; don't use them".

Generally though, I prefer to use polymorphism instead of if statements where possible. That just makes for so much less room for error. I tend to find that a lot of the time, using conditionals leads to a lot of tramp arguments as well (because you have to pass the data needed to form the conditional on to the appropriate method).

Answer 14

There is the Anti-If campaign which started earlier in the year. The main premise being that many nested if statements often can often be replaced with polymorphism.

I would be interested to see an example of using the Predicate instead. Is this more along the lines of functional programming?

Answer 15

There is another sense in which if can be evil: when it comes instead of polymorphism.

E.g.

 if (animal.isFrog()) croak(animal)
 else if (animal.isDog()) bark(animal)
 else if (animal.isLion()) roar(animal)

instead of

 animal.emitSound()

But basically if is a perfectly acceptable tool for what it does. It can be abused and misused of course, but it is nowhere near the status of goto.

Answer 16

I think it depends on what you're doing to be honest.

If you have a simple if..else statement, why use a predicate?

If you can, use a switch for larger if replacements, and then if the option to use a predicate for large operations (where it makes sense, otherwise your code will be a nightmare to maintain), use it.

This guy seems to have been a bit pedantic for my liking. Replacing all if's with Predicates is just crazy talk.

Answer 17

I'll agree with you; he was wrong. You can go too far with things like that, too clever for your own good.

Code created with predicates instead of ifs would be horrendous to maintain and test.

Answer 18

The only problem with a predicates (in terms of replacing if statements) is that you still need to test them:

function void Test(Predicate<int> pr, int num) 
{
    if (pr(num))
    { /* do something */ }
    else
    { /* do something else */ }
}

You could of course use the terniary operator (?:), but that's just an if statement in disguise...