8.2 Selection Statements 351
statements, which serve as the bodies of then and else clauses. In Fortran 95,
Ada, Python, and Ruby, the then and else clauses are statement sequences,
rather than compound statements. The complete selection statement is termi-
nated in these languages with a reserved word.^1
Python uses indentation to specify compound statements. For example,
if x > y :
x = y
print "case 1"
All statements equally indented are included in the compound statement.^2
Notice that rather than then, a colon is used to introduce the then clause in
Python.
The variations in clause form have implications for the specification of the
meaning of nested selectors, as discussed in the next subsection.
8.2.1.4 Nesting Selectors
Recall that in Chapter 3, we discussed the problem of syntactic ambiguity of a
straightforward grammar for a two-way selector statement. That ambiguous
grammar was as follows:
<if_stmt> → if <logic_expr> then <stmt>
| if <logic_expr> then <stmt> else <stmt>
The issue was that when a selection statement is nested in the then clause of a
selection statement, it is not clear to which if an else clause should be associ-
ated. This problem is reflected in the semantics of selection statements. Con-
sider the following Java-like code:
if (sum == 0)
if (count == 0)
result = 0;
else
result = 1;
This statement can be interpreted in two different ways, depending on whether
the else clause is matched with the first then clause or the second. Notice that
the indentation seems to indicate that the else clause belongs with the first
then clause. However, with the exceptions of Python and F#, indentation has
no effect on semantics in contemporary languages and is therefore ignored by
their compilers.
- Actually, in Ada and Fortran it is two reserved words, end if (Ada) or End If (Fortran).
- The statement following the compound statement must have the same indentation as the if.
