SearchBirthday: inputs, return values and side effects

A procedure lets the caller delegate a well-defined task. SearchBirthday(Target, OtherCards) returns true if at least one card in OtherCards has the target day and month; otherwise it returns false. For an individual student search, OtherCards must exclude that student. The caller can pass only later unprocessed cards to avoid repeated pairs.

Procedure SearchBirthday(Target, OtherCards):
    for each card Y in OtherCards:
        if BirthdayKey(Y) == Target:
            restore any required order and markers
            return true
    restore any required order and markers
    return false
End Procedure

Returning false inside the loop after the first non-match is a bug: a later card may match. Returning true after a genuine match is valid for existence. Early return must still honour the restoration part of the contract.

Three possible contracts #

In the lecture, sideways cards mark students already processed. Losing that orientation makes the caller forget which outer cards are finished. A correct Boolean alone is not enough. The caller may rely on guarantees written in the contract, but must not assume stronger guarantees that were never promised.

The target birthday is a parameter. A marked deck can also carry a parameter implicitly: the mark tells the helper which position or card matters. Written code is usually clearer when it names the target, eligible records and preservation rules explicitly.