Original
Case statements
The case
statement provides a readable alternative to complex nested if
conditionals. A case statement has the form
case selectorExpression
of
caseList1: statement1;
...
caseListn: statementn;
end
where selectorExpression is any
expression of an ordinal type (string types are invalid) and each caseList
is one of the following:
· A numeral, declared constant, or other expression that the compiler
can evaluate without executing your program. It must be of an ordinal type
compatible with selectorExpression. Thus 7, True, 4 + 5 * 3, 'A', and
Integer('A') can all be used as caseLists, but variables and most
function calls cannot. (A few built-in functions like Hi and Lo
can occur in a caseList. See Constant
expressions.)
· A subrange having the form First..Last, where First
and Last both satisfy the criterion above and First is less than
or equal to Last.
· A list having the form item1, ..., itemn, where each item
satisfies one of the criteria above.
Each
value represented by a caseList must be unique in the case
statement; subranges and lists cannot overlap. A case statement can have
a final else clause:
case selectorExpression
of
caseList1: statement1;
...
caseListn: statementn;
else
statements;
end
where statements is a
semicolon-delimited sequence of statements. When a case statement is
executed, at most one of statement1 ... statementn is executed. Whichever
caseList has a value equal to that of selectorExpression
determines the statement to be used. If none of the caseLists has
the same value as selectorExpression, then the statements in the else
clause (if there is one) are executed.
The case
statement
case
I of
1..5: Caption := 'Low';
6..9: Caption := 'High';
0, 10..99: Caption := 'Out of range';
else
Caption := '';
end;
is equivalent to the nested conditional
if
I in [1..5] then
Caption := 'Low'
else if I in
[6..10] then
Caption := 'High'
else if (I =
0) or (I in [10..99]) then
Caption := 'Out
of range'
else
Caption := '';
Other
examples of case statements:
case
MyColor of
Red: X := 1;
Green: X := 2;
Blue: X := 3;
Yellow, Orange, Black: X := 0;
end;
case
Selection of
Done: Form1.Close;
Compute: CalculateTotal(UnitCost,
Quantity);
else
Beep;
end;
See also
Structured statements: Overview
译文
Case语句
case语句为复杂的嵌套if语句提供了更加容易阅读的选择。case语句具有如下形式：
case selectorExpression
of
caseList1: statement1;
...
caseListn: statementn;
end
这里的选择表达式selectorExpression是任意序数类型的表达式（串类型是无效的），并且每个情况列表caseList是下列之一：
· 数字、声明的常量，或者编译器不运行程序就能求值的表达式。并且必需与选择表达式selectorExpression类型兼容。因此，7、True、4 + 5 * 3、'A'、Integer('A')等都能作为情况列表caseList使用，而变量和大多数函数调用不能。（个别内建函数，如Hi、Lo等可以出现在情况列表caseList中，见常量表达式。）
· 形如First..Last的子界，这里的First和Last都必需满足上面的准则，并且First小于或等于Last。
· 形如item1, ..., itemn的列表，这里的每个item也必需满足上面的准则。
在case语句中，每个由情况列表caseList表示的值都必需是唯一的；子界和列表不能重叠。case语句的末尾还可以含有一个else子句，形如：
case selectorExpression
of
caseList1: statement1;
...
caseListn: statementn;
else
statements;
end
这里的statements是分号隔开的语句序列。当执行case语句时，statement1...statementn中的语句最多只有一个被执行。无论哪一个情况列表caseList中含有与选择表达式selectorExpression相等的值，其相应的语句statement都会被执行。如果所有的情况列表中都不含选择表达式的值，那么else子句中的语句statements将被执行（如果有该语句）。
case语句
case
I of
1..5: Caption := 'Low';
6..9: Caption := 'High';
0, 10..99: Caption := 'Out of range';
else
Caption := '';
end;
等价于
if
I in [1..5] then
Caption := 'Low'
else if I in
[6..10] then
Caption := 'High'
else if (I =
0) or (I in [10..99]) then
Caption := 'Out
of range'
else
Caption := '';
下面是其他case语句的例子：
case
MyColor of
Red: X := 1;
Green: X := 2;
Blue: X := 3;
Yellow, Orange, Black: X := 0;
end;
case
Selection of
Done: Form1.Close;
Compute: CalculateTotal(UnitCost,
Quantity);
else
Beep;
end;
相关主题
编者注
if语句和case语句相同之处在于，最多只执行一个条件中的语句，因此有时候可以对嵌套if语句和case相互替换，但二者之间还有一些明显的差别。
差别一：嵌套if语句不受数据类型的约束，而case语句只能针对有序型，因此下面的嵌套if语句就无法用case语句替换：
if
S = 'Book' then
...
else
if S = 'Pen' then
...
else
if S := 'Ruler' then
...
else
...
差别二：嵌套if语句可以使用多种类型的表达式，而case语句的情况列表只能使用同一种序数类型，因此下面的嵌套if语句也无法用case语句替换：
if
N = 1 then
...
else
if S = 'Hello' then
…
else
if B = False then
…
else
...
差别三：嵌套if语句可以出现条件重叠，而case语句不允许，因此下面的嵌套if语句是合法的（尽管程序逻辑显然欠考虑），而case语句是不合法的：
if N = 1
then
...
else if
N in [2..6] then
...
else if
N in [3..8] then
...
else
...
case N of
1: ...
2..6: ...
3..8: ...
else
...
end;