If {...} Else {...}:Does the Line Break Between "}" and "Else" Really Matters

if {...} else {...} : Does the line break between } and else really matters?

Original question and answer

If we put in R console:

if (1 > 0) {
  cat("1\n");
  }
else {
  cat("0\n");
  }

why does it not work?

R is an interpreted language, so R code is parsed line by line. (Remark by @JohnColeman: This judgement is too broad. Any modern interpreter does some parsing, and an interpreted language like Python has no problem analogous to R's problem here. It is a design decision that the makers of R made, but it wasn't a decision that was forced on them in virtue of the fact that it is interpreted (though doubtless it made the interpreter somewhat easier to write).)

Since

if (1 > 0) {
  cat("1\n");
  }

makes a complete, legal statement, the parser will treat it as a complete code block. Then, the following

else {
  cat("0\n");
  }

will run into error, as it is seen as a new code block, while there is no control statement starting with else.

Therefore, we really should do:

if (1 > 0) {
  cat("1\n");
  } else {
  cat("0\n");
  }

so that the parser will have no difficulty in identifying them as a whole block.

In compiled language like C, there is no such issue. Because at compilation time, the compiler can "see" all lines of your code.

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Final update related to what's going on inside a function

There is really no magic here! The key is the use of {} to manually indicate a code block. We all know that in R,

{statement_1; statement_2; ...; statement_n;}

is treated as a single expression, whose value is statement_n.

Now, let's do:

{
  if (1 > 0) {
    cat("1\n");
    }
  else {
    cat("0\n");
    }
}

It works and prints 1.

Here, the outer {} is a hint to the parser that everything inside is a single expression, so parsing and interpreting should not terminate till reaching the final }. This is exactly what happens in a function, as a function body has {}.

Does the line break between } and else really matters?

Note that the if else need not be in a function for it to work. It just need to be in a continuous form or in a way to be expressed as a continuous block of code. One way is being in a function. The other is to write it in one line, or even ensure that there is no line break between the if block and the else block:

x <- 1
if(x != 1) print('haha') else print('hihi')
[1] "hihi"

More blocks of statements:

x <- 1
if(x != 1){
  print("haha")
} else if( x == 1){ # Note how else begins immediatley after }
  print("hihi")
}

[1] "hihi"

Note that you need to know when to put the line breaks whether in a function or outside of a function. Otherwise the code might fail or even give incorrect results.

Using subtraction:

 x <- 1
 x - 
 2
 [1] -1

 x <- 1
 x
 - 2
 [1] -2

You need to know when/where to have the line breaks. Its always safe to have else follow the closing brace } of the previous if statement. ie:

  if(...){
    ....
  } else if(...){
    ....
  } else {
   ...
  }

JSLint, else and Expected exactly one space between '}' and 'else' error

JSLint is based on Crockford's preferences (which I share in this case).

It's a matter of opinion which is "better".

(Although clearly his opinion is right ;)

Putting an if-elif-else statement on one line?

No, it's not possible (at least not with arbitrary statements), nor is it desirable. Fitting everything on one line would most likely violate PEP-8 where it is mandated that lines should not exceed 80 characters in length.

It's also against the Zen of Python: "Readability counts". (Type import this at the Python prompt to read the whole thing).

You can use a ternary expression in Python, but only for expressions, not for statements:

>>> a = "Hello" if foo() else "Goodbye"

Edit:

Your revised question now shows that the three statements are identical except for the value being assigned. In that case, a chained ternary operator does work, but I still think that it's less readable:

>>> i=100
>>> a = 1 if i<100 else 2 if i>100 else 0
>>> a
0
>>> i=101
>>> a = 1 if i<100 else 2 if i>100 else 0
>>> a
2
>>> i=99
>>> a = 1 if i<100 else 2 if i>100 else 0
>>> a
1

Replacements for switch statement in Python?

The original answer below was written in 2008. Since then, Python 3.10 (2021) introduced the match-case statement which provides a first-class implementation of a "switch" for Python. For example:

def f(x):
    match x:
        case 'a':
            return 1
        case 'b':
            return 2
        case _:
            return 0   # 0 is the default case if x is not found

The match-case statement is considerably more powerful than this simple example.

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You could use a dictionary:

def f(x):
    return {
        'a': 1,
        'b': 2,
    }[x]

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