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.

Subsections

• The Simple Conditional Statement
• The Simple Conditional Expression
• The Case Statement
• The Case Expression
• Error Checking and Assertions

if boolexpr_1 then statements end if : ->

The standard conditional statement: the value of the Boolean expression is evaluated. If the result is `true', the first block of statements is executed, 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.

siglitnosplit{if} {if boolexpr_1 then statements_1 elif boolexpr_2 then statements_2} {else statements_3 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;

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

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

> 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

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

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. If no action is desired in the latter case, the construction may be abbreviated to the second form above.

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

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.

Print the values of the expressions and terminate execution of Magma. It is useful, for example, when an illegal value of an argument is passed to a function.

If the given boolean expression evaluates to true, print the values of the expressions and terminate execution of Magma. This is designed for checking that certain conditions must be met, etc.

If the given boolean expression evaluates to false, terminate execution of Magma, giving an appropriate message. If the Assertions flag is set to false (see SetAssertions), the check is not made and the statement has no effect.