Function std.algorithm.comparison.predSwitch
Returns one of a collection of expressions based on the value of the switch expression.
choicesswitchExpressionpred(switchExpression
Both the test and the return expressions are lazily evaluated.
Prototype
auto predSwitch(alias pred, T, R...)( T switchExpression, R choices );
Parameters
| Name | Description |
|---|---|
| switchExpression | The first argument for the predicate. |
| choices | Pairs of test expressions and return expressions. The test
expressions will be the second argument for the predicate, and the return
expression will be returned if the predicate yields true with |
Returns
The return expression associated with the first test expression that made the predicate yield true, or the default return expression if no test expression matched.
Throws
If there is no default return expression and the predicate does not
yield true with any test expression - SwitchError is thrown. SwitchError is also thrown if a void return expression was executed without
throwing anything.
Example
string res = 2.predSwitch!"a < b"(
1, "less than 1",
5, "less than 5",
10, "less than 10",
"greater or equal to 10");
assert(res == "less than 5");
//The arguments are lazy, which allows us to use predSwitch to create
//recursive functions:
int factorial(int n)
{
return n.predSwitch!"a <= b"(
-1, {throw new Exception("Can not calculate n! for n < 0");}(),
0, 1, // 0! = 1
n * factorial(n - 1) // n! = n * (n - 1)! for n >= 0
);
}
assert(factorial(3) == 6);
//Void return expressions are allowed if they always throw:
import std.exception : assertThrown;
assertThrown!Exception(factorial(-9));