I am new to C++. I often see conditional statement like below:
if statement_0; else if statement_1;
Question:
Syntactically, shall I treat else if
as a single keyword? Or is it actually an nested if
statement within the outer else
like below?
if statement_0; else if statement_1;
The first elseif expression (if any) that evaluates to true would be executed. In PHP, you can also write 'else if' (in two words) and the behavior would be identical to the one of 'elseif' (in a single word).
Yes, they are two separate keywords—the Java language specification does not specify an else if keyword. It is actually, as the other posters here have said, an if statement contained inside of an else statement.
So as the default case handles all possibilities the idea behind else if is to split this whole rest into smaller pieces. The difference between else and else if is that else doesn't need a condition as it is the default for everything where as else if is still an if so it needs a condition.
Alternatively referred to as elsif, else if is a conditional statement performed after an if statement that, if true, performs a function.
They are not a single keyword if we go to the draft C++ standard section 2.12
Keywords table 4
lists both if
and else
separately and there is no else if
keyword. We can find a more accessible list of C++ keywords by going to cppreferences section on keywords.
The grammar in section 6.4
also makes this clear:
selection-statement: if ( condition ) statement if ( condition ) statement else statement
The if
in else if
is a statement following the else
term. The section also says:
[...]The substatement in a selection-statement (each substatement, in the else form of the if statement) implicitly defines a block scope (3.3). If the substatement in a selection-statement is a single statement and not a compound-statement, it is as if it was rewritten to be a compound-statement containing the original substatement.
and provides the following example:
if (x) int i; can be equivalently rewritten as if (x) { int i; }
So how is your slightly extended example parsed?
if statement_0; else if statement_1; else if statement_2 ;
will be parsed like this:
if { statement_0; } else { if { statement_1; } else { if { statement_2 ; } } }
Note
We can also determine that else if
can not be one keyword by realizing that keywords are identifiers and we can see from the grammar for an identifier in my answer to Can you start a class name with a numeric digit? that spaces are not allowed in identifiers and so therefore else if
can not be a single keyword but must be two separate keywords.
Syntactically, it's not a single keyword; keywords cannot contain white space. Logically, when writing lists of else if
, it's probably better if you see it as a single keyword, and write:
if ( c1 ) { // ... } else if ( c2 ) { // ... } else if ( c3 ) { // ... } else if ( c4 ) { // ... } // ...
The compiler literally sees this as:
if ( c1 ) { // ... } else { if ( c2 ) { // ... } else { if ( c3 ) { // ... } else { if ( c4 ) { // ... } // ... } } }
but both forms come out to the same thing, and the first is far more readable.
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