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SECTION 15 / 28
Unpack the four returned grade lists
![The returned outer order is [A-list,B-list,C-list,D-list]. Each extracted component remains a list of IDs.](/images/computational-thinking/week-6/unpack-grades.png)
REAL COURSE DATA · SORT, GROUP AND VERIFY
Unpack [A,B,C,D] grade lists
Follow the selected procedure. Change a field, tie rule or weekday and explain why the result changes.
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Predict the next change. Keep track of which list level, field and position convention the procedure uses.
G = SimpleGradeAssignment(SortedMarks)
A = first(G)
B = first(rest(G))
C = last(init(G))
D = last(G)The grading procedure returns one outer list containing four lists. Its outer length is 4 even though there are 30 IDs across the inner lists. Each inner list can have a different length or be empty. Evaluate nested expressions from the inside out.
first(G) is the complete A-grade list. first(first(G)) would be its first student ID, and is safe only if A is non-empty. last(init(G)) selects the third grade list; it does not automatically select its final student.
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What is the type of first(G)?
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A list of student IDs.
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What is length(G) for this contract?
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4, because the outer result has A, B, C and D components.
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Is first(first(G)) always defined?
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No. The A list might be empty even when G itself contains four lists.