Documentation

Conditional Statements and Expressions

The conditional statement has the usual form if ... then ... else ... end if;. It has several variants. Within the statement, a special prompt will appear, indicating that the statement has yet to be closed. Conditional statements may be nested.

The conditional expression, select ... else, is used for in-line conditionals.

The Simple Conditional Statement

if Boolean expression then
statements₁
else
statements₂
end if;

if Boolean expression then
statements
end if;

The standard conditional statement: the value of the Boolean expression is evaluated. If the result is true then the first block of statements is executed, while if the result is false the second block of statements is executed. If no action is desired in the latter case, the construction may be abbreviated to the second form above.

if Boolean expression₁ then
      statements₁
elif Boolean expression₂ then
      statements₂
else
      statements₃
end if;

Since nested conditions occur frequently, elif provides a convenient abbreviation for else if, which also restricts the `level':
if boolexpr then
   statments1
elif boolxpr2 then
   statments2
else
   statments3
end if;
is equivalent to
if boolxpr1 then
    statments1
else
    if boolxpr2 then
       statments2
    else
       statments3
    end if;
end if;

Example `State_if (H1E10)`

> m := Random(2, 10000);
> if IsPrime(m) then
>     m, "is prime";
> else
>     Factorization(m);
> end if;
[ <23, 1>, <37, 1> ]

The Simple Conditional Expression

Example `State_InLineConditional (H1E11)`

Using the select ... else construction, we wish to assign the sign of y to the variable s.

> y := 11;
> s := (y gt 0) select 1 else -1;
> s;
1

This is not quite right (when y = 0), but fortunately we can nest select ... else constructions:

> y := -3;
> s := (y gt 0) select 1 else (y eq 0 select 0 else -1);
> s;
-1
> y := 0;
> s := (y gt 0) select 1 else (y eq 0 select 0 else -1);
> s;
0

The select ... else construction is particularly important in building sets and sequences, because it enables in-line if constructions. Here is a sequence containing the first 100 entries of the Fibonacci sequence:

>  f := [ i gt 2 select Self(i-1)+Self(i-2) else 1 : i in [1..100] ];

The Case Statement

case expr : when expr_i : statements end case : ->

The expression following case is evaluated. The statements following the first expression whose value equals this value are executed, and then the case statement has finished. If none of the values of the expressions equal the value of the case expression, then the statements following else are executed, in the first form. If no action is desired in the latter case, the construction may be abbreviated to the second form above.

Example `State_case (H1E12)`

> x := 73;
> case Sign(x):
>    when 1:
>       x, "is positive";
>    when 0:
>       x, "is zero";
>    when -1:
>       x, "is negative";
> end case;
73 is positive

The Case Expression

case< expr | expr_{left, 1} : expr_((right), 1), ..., expr_((left), n) : expr_((right), n), default : expr_def> : ->

This is the expression form of case. The expr is evaluated to the value v. Then each of the left-hand expressions expr_((left), i) is evaluated until one is found whose value equals v; if this happens the value of the corresponding right-hand expression expr_((right), i) is returned. If no left-hand expression with value v is found the value of the default expression expr_def is returned.
The default case cannot be omitted, and must come last.

V2.28, 13 July 2023

Contents

if Boolean expression then statementsend if;

if Boolean expression then
statements
end if;