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

Question

I write my if {...} else {...} statement in R in the following way as I find it more readable.

Testifelse = function(number1)
{
     if(number1>1 & number1<=5)
     {
          number1 =1
     }
     else if ((number1>5 & number1 < 10))
     {
          number1 =2
     }
     else
     {
          number1 =3
     }

     return(number1)
}

According to ?Control:

... In particular, you should not have a newline between } and else to avoid a syntax error in entering a if ... else construct at the keyboard or via source ...

the function above will cause syntax error, but actually it works! What's going on here?

Thanks for your help.

Answer 1

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.

---

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 {}.