When to use 'if…else if' and when to use

Question

1) I know how if…else if statements work, but in the next example both methods are identical as far as the resulting value is concerned. So does it matter which of the two methods I use or should I always go for one that is semantically closest to what the code is trying to do ( here I’m guessing that semantically the two methods are quite different )? So which method would you use and why?

protected string GetNumberDescription(int value)
{
    if (value >= veryBigNumber)
        return veryBigNumberDescription;
    else if (value >= bigNumber)
        return bigNumberDescription;
    else if (value >= smallNumber)
        return smallNumberDescription;
    else
        return "";
}

protected string GetNumberDescription(int value)
{
    if (value >= veryBigNumber)
        return veryBigNumberDescription;
    if (value >= bigNumber)
        return bigNumberDescription;
    if (value >= smallNumber)
        return smallNumberDescription;
    else
        return "";
}

2) I noticed losts of code uses the following format when writting if ... else if statements:

if ...
else if ...
else ...

But isn't ( at least conceptually ) more correct way:

if ...
else
  if ...
  else ...

Answer 1

They are semantically identical. I doubt there is any difference in performance, so the choice is about style and readability. If you're curious, compile the above code into an assembly and use Reflector to see if there is any difference in the IL. My guess is there would be little difference if any.

Answer 2

1. You should probably use the first. It's more accepted, and more logical. (You don't care about anything afterwards if this condition is true... indicate that in your code.)
2. It's generally more accepted, and more compact and readable, to use the first way (else if). Python even has a specific keyword for it (elif).

Answer 3

protected string GetNumberDescription(int value) 
{ 
    string ret = "";

    if (value >= veryBigNumber) 
        ret = veryBigNumberDescription; 
    else if (value >= bigNumber) 
        ret = bigNumberDescription; 
    else if (value >= smallNumber) 
        ret =  smallNumberDescription; 

    return ret; 
} 

The question is already answered, but I added this because:

1. There are advantages to having a single exit from a function.
2. The same variable is being tested repeatedly, so the if/elseif is most appropriate.

Edited to add:

I'm usually a big fan of exiting a function early in cases like this:

if (conditionOne)
{
    if (conditionTwo)
    {
        if (conditionThree)
        {
            interesting_stuff();
        }
    }
    else
        boring();
}
return;

I'm much happier seeing this:

if (!conditionOne)
    return;

if (!conditionTwo)
{
    boring();
    return;
}

if (!conditionThree)
    return;

interesting_stuff();
return;

But when there are exits at many different nesting levels, it's too easy to miss one as you're reading over the code.

Also, as mentioned in the comments: a single exit means only one place to put a breakpoint.

Answer 4

It's mostly a matter of preference. when using return statements, you don't need the elses in my opinion, since it's clear that the function ends right there. It's all very situation and style dependent, so there is no clear 'best' way to do it.

Answer 5

I would use the first option, as it shows in code the comparisons are "grouped" some. For 2 I think they end up in exactly the same IL code.

I always make sure the most common senario is first in the if..else statement, as that is best for performance.

Answer 6

I would suggest the former, as you are making a logical choice between four possibilities. You are grouping the decision in one if/else if block.

If you separate out the decisions like in your second function, they appear to not be connected at all. Clearly they are, as the value variable is repeated, but it might not necessarily be obvious.

Additionally, when you've got nothing to return, please return null, and not a blank string. In an application I maintain, it is still necessary to use if (value == null || value != "").

Answer 7

I personally would use

protected string GetNumberDescription(int value)
   {
       if (value >= veryBigNumber)
           return veryBigNumberDescription;
       if (value >= bigNumber)
           return bigNumberDescription;
       if (value >= smallNumber)
           return smallNumberDescription;
       return string.Empty;
   }

It really all depends on what your function is doing. If its simple like this then keep it simple. If you might need to do something with the result before you return it then I'd use Egrunin's solution

Answer 8

1) The first one is more prevalent and in common practice. So it would be wise to stick to the standard practice.

2) Generally when we have more than one conditional statement, else if is used in the intermediate conditional outputs and else is used in the last conditional output. Moreover it might be more efficient as I believe it does not have to check the same condition everytime unlike the if-else-if-else... type.

Its more like a choice provided to you where you find your own out of many paths going to the same destination. But the choice that you hear or see often is the one that is popular because of experience of many users and experts.

Answer 9

If the conditions are mutually exclusive then I think an else if pattern is best. It will be harder to accidentally break as the code evolves and becomes more complex, because your control flow more accurately represents the mutual exclusiveness of the logic.

Consider if someone re-factored your first example to be similar to egrunin's example such that there is only one return, but consider they forgot to add the "else" to each if:

protected string GetNumberDescription(int value) 
{ 
    string ret = "";

    if (value >= veryBigNumber) 
        ret = veryBigNumberDescription; 
    if (value >= bigNumber) 
        ret = bigNumberDescription; 
    if (value >= smallNumber) 
        ret =  smallNumberDescription; 

    return ret; 
} 

They have just introduced a bug. They may not have realized that the returns were part of the logic being expressed. While the error is obvious here, in real life this code a couple years down the road could potentially become more complex, lengthy, and more difficult to read such that it's easier to make such a mistake.

Answer 10

The alternatives are pretty close semantically, so it doesn't matter that much. As suggested in some answers, putting the result in a string variable and return at the end might better represent what you are doing in the method.

Another alternative is to use the conditional operand, which gives you a less verbose code:

protected string GetNumberDescription(int value) {
  return
    value >= veryBigNumber ? veryBigNumberDescription :
    value >= bigNumber ? bigNumberDescription :
    value >= smallNumber ? smallNumberDescription :
    String.Empty;
}

Answer 11

With the return statements, it really doesn't matter. I totally disagree with those who say the second is in any way worse, though. What makes it worse?

In response to your second question: you are quite right. In fact, not only are the two ways of formatting if/else if effectively the same; they actually are literally the same. C# does not enforce a particular formatting style, so these are all identical:

// This...
if (A())
    return "Congrats!";
else if (B())
    return "Not bad!";
else if (C())
    return "Next time...";
else
    return "Uh-oh.";

// ...is the same as this...
if (A())
    return "Congrats!";
else
    if (B())
        return "Not bad!";
    else
        if (C())
            return "Next time...";
        else
            return "Uh-oh.";

// ...which is the same as this...
if (A()) return "Congrats!"; else
                       if               (B())
             return "Not bad!";


else
{ if
(C())


     return


"Next time...";else{
return "Oh-oh.";
}}

Answer 12

protected string GetNumberDescription(int value) 
{ 
    return new NumberDescriptor(value).toString();
} 

All you need is to hide your logic. Don't take this too serious, but...i would do it that way. I know, your question is not anserwed here, but there are already enough if/then/else examples in this topic.

Answer 13

I would say to use the first if the function is thought of as returning one of four possibilities, all of which are considered "successful". I would use the latter style if some returns denoted "failures". For example (toy example--throwing might be better for the error cases):

string get_suit_name(int suit)
{
  string suit_names[4] = {"clubs", "diamonds", "hearts", "spades"};

  if (0 > suit) /* Avoid LT sign */
    return "Suit is improperly negative!";
  if (suit >= 4)
    return "Suit is too big!";
  return suit_names[suit];
}

Answer 14

Well from my point of view it would be these priorities from highest to lowest.

Functionality / achieving the end goal Reduction of cost in any scope/level Readability

^^^^ note that many will dispute this on my list and call readability above reduction of cost, it really depends on your work envi. and the end goal but I find this the most common. But guess coming from a rookie that doesn't mean much huh :D

After that you it's pretty much peanuts. Same goes for ? : operator. If you are convinced it's more readable, go for it. Function wise it doesn't really matter. But if(){}else{} is significantly different from if(){}else if(){} So pick what's the optimum code (Highest wanted results minus unexpected results - 0.5* readability score ) pretty much what I'd go on.

Now seeing as the code does the same for you, a new attribute comes into play, will this code be edited at some later stage? And if so what would be best to go for then? Don't forget the future of the code, nor the comments, that a free bonus to pick along with it. You can even do both if you can't figure it out for the time and just comment the other but that does leave sloppy walls of text to be removed later on cleanup/release.

Answer 15

For the example provided, you don't need to worry about "if" statements if you're willing to sacrifice a ton of readability and possibly a little speed:

string [] descriptions = {"", smallNumberDescription, bigNumberDescription, veryBigNumberDescription};

protected string GetNumberDescription(int value)
{
    int index = Convert.ToInt32(value >= smallNumber) + Convert.ToInt32(value >= bigNumber)
                + Convert.ToInt32(value >= veryBigNumber);
    return descriptions[index];
}

Answer 16

In terms of performance, they may be the same depending on the compiler/interpreter. In my opinion the second one is better....much easier to debug.

Answer 17

You want to use "else if" because that only will fire if the first "if" is not accepted. In your case the 3 if statements are okay, simply because if a latter "if" is correct, then the previous "if"s cannot be.

In a case where all of your if statements could be correct, you'd want to use else if.

For Example:

if (8<10) {
return 10;
}
if (8<9) {
return 9;
}

That's a very simplistic example, but you get the idea. You use "else if" when you want only one "if" statement to fire.