Making decisions (If, Select)

What you’ll build

A shipping calculator: given a parcel’s weight and where it’s going, it works out the price and how long delivery takes. Its decisions use If, which tests conditions in turn, and Select, which compares one value against several cases.

If

If runs a block when its condition is True. ElseIf tests another condition when the ones before it were False, and Else runs when none was True. Only the first branch whose condition holds runs.

Import std.core

Function Price(grams: Int): Int
    If grams <= 0
        Return 0
    ElseIf grams <= 500
        Return 450
    ElseIf grams <= 2000
        Return 900
    Else
        Return 900 + (grams - 2000) / 1000 * 300
    End If
End Function

For grams In [250, 1500, 2000, 5000]
    Print("{grams} g: {Price(grams)} cents")
End For

250 g: 450 cents
1500 g: 900 cents
2000 g: 900 cents
5000 g: 1800 cents

One line

When the code to run is a single statement, If ... Then ... fits on one line, with an optional Else. This is the only form of If that uses Then.

Import std.core

Local grams = 2600
Local fragile = True

If fragile Then Print("handle with care")
If grams > 2000 Then Print("heavy parcel") Else Print("standard parcel")

handle with care
heavy parcel

Select

Select compares one value with each Case in turn and runs the first that matches. A case can list several values, or a range with To, and Default runs when no case matches. Only one case runs; there’s no falling through to the next.

Import std.core

Function DeliveryDays(zone: Int): String
    Select zone
        Case 1
            Return "1 day"
        Case 2, 3
            Return "2 days"
        Case 4 To 7
            Return "3 to 5 days"
        Default
            Return "up to 2 weeks"
    End Select
End Function

For zone In [1, 3, 5, 9]
    Print("zone {zone}: {DeliveryDays(zone)}")
End For

zone 1: 1 day
zone 3: 2 days
zone 5: 3 to 5 days
zone 9: up to 2 weeks

Each Case value must be known when the program is compiled, and two cases can’t match the same value: that’s an error, so the order of the cases never hides one.

Strings and characters

Select also works on Strings, Chars, and Bools. Here a parcel’s destination is a country code, a String, and its service is a one-letter code, a Char: E for express.

Import std.core

Function Zone(country: String): Int
    Select country
        Case "FR"
            Return 1
        Case "DE", "BE", "NL"
            Return 2
        Case "US", "CA"
            Return 5
        Default
            Return 9
    End Select
End Function

Function Service(code: Char): String
    Select code
        Case 'E'
            Return "express"
        Case 'A' To 'D'
            Return "standard"
        Default
            Return "economy"
    End Select
End Function

Print(Zone("BE"), Zone("CA"), Zone("JP"))
Print(Service('E'), Service('B'), Service('X'))

2 5 9
express standard economy

A Select over an Enum must cover every member or have a Default, so adding a member later shows each place that has to handle it. Enums shows this, and the kind of Enum that always needs a Default.

What you learned

• If, ElseIf, and Else run the first branch whose condition is True.
• If condition Then statement is the one-line form, with an optional Else.
• Select runs the first Case that matches its value, with no falling through.
• A Case can list values or a range with To, and Default takes the rest.
• Select works on integers, Strings, Chars, Bools, and Enums.