Decisions

A direction value may be negative, zero or positive. One decision classifies its sign; another chooses an action for a particular integer value.

Boolean chains

if tests a Boolean expression. Further tests use the single keyword elseif, and an optional else handles the remaining path.

if direction > 0
    observed = 1
elseif direction < 0
    observed = 2
else
    observed = 3
end

Conditions are tested in order until one is true. Only that body runs. There is one end for the complete chain.

elseif is a clause, not the two words else if. A nested conditional in an else body is still available, but it begins on the following logical line and has its own end. Use nesting when the inner decision belongs inside one outer path; use elseif when the tests are peers in one chain.

Integer selection

select evaluates one integer expression once, then compares it with constant case values:

select direction
case -1
    observed = observed + 10
case 0
    observed = observed + 20
else
    observed = observed + 30
end

Several comma-separated values may share a case body. The first matching case runs and then control continues after end; cases never fall through into one another. else is optional and handles a value that matched no case.

Choose select when the question is equality against a short list of integer constants. Choose if for ranges, Boolean combinations or conditions that are not all comparisons with one value.

The companion classifies -1, then selects its matching case. It leaves observed equal to 12.

var direction as i8 = -1
var observed as u16 = 0

sub main()
    if direction > 0
        observed = 1
    elseif direction < 0
        observed = 2
    else
        observed = 3
    end

    select direction
    case -1
        observed = observed + 10
    case 0
        observed = observed + 20
    else
        observed = observed + 30
    end
end

Summary

• if, elseif and else form one flat Boolean chain.
• A nested if is an ordinary statement with its own end.
• select evaluates one integer selector once.
• Case values are compile-time integer constants and do not fall through.

See conditional and selection control. The checked companion is 04-decisions.nu.