SECTION 14 / 31

A dictionary value can be another dictionary

Create the outer birthday dictionary, create an empty inner dictionary for a new date, then add an ID → True entry.

birthdays = {}
duplicates = {}
foreach X in scores {
    dob = normalizedDayMonth(X.DateOfBirth)
    id = X.CardNo
    if (isKey(birthdays,dob)) {
        duplicates[dob] = True
    }
    else {
        birthdays[dob] = {}
    }
    birthdays[dob][id] = True
}

birthdays[15 Mar] is now {7:True,21:True}. To find IDs, use keys(birthdays[15 Mar]). To ask whether ID 21 is in that date group, test isKey(birthdays[15 Mar],21). First check the outer date key before reading its inner dictionary.

The inner {} must be created only for a new date. Reinitializing it on every occurrence erases earlier IDs. With unique student records, the list and inner-dictionary versions identify the same students. If an ID is accidentally processed twice, a list retains the duplicate occurrence but an ID-keyed dictionary overwrites that ID’s True flag.

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What is the type of birthdays[15 Mar]?

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A dictionary whose keys are student IDs and values are True flags.

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What goes wrong if birthdays[dob]={} is executed for every record?

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It clears the existing group and loses earlier students at that date.

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Does an inner dictionary preserve repeated copies of the same ID?

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No. Each ID key has one current value. This differs from appending repeatedly to a list.

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