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SECTION 10 / 24
Unpack a list into its first element and the rest
![first([3,13,23,25]) is the element 3. rest gives the list [13,23,25].](/images/computational-thinking/week-5/first-rest.png)
REAL COURSE DATA · BUILD AND UNPACK LISTS
Unpack first, rest, init and last
Follow one occurrence at a time. Choose another input to test empty lists, repeated IDs and different parameters.
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Predict the next change, then step through the list. The original course records remain unchanged.
first(L) returns one element. rest(L) returns a list containing all occurrences after the first position. Neither operation means “remove every occurrence equal to the first value”. For [3,3,25], rest is [3,25]. The second 3 remains.
L = [3,13,23,25]
x = first(L) # x is 3; L is still the original list
R = rest(L) # R is [13,23,25]
L = rest(L) # NOW L refers to the shorter listKeep the difference between evaluating an operation and assigning its result. A local working variable can become shorter while the source data remains available. This lesson’s demonstrations construct result lists rather than mutating the original course dataset.
first([]) has no defined element result. Guard it with a non-empty check. The tutorial defines rest([]) as [] and rest([x]) as []. For a comparison with first(rest(L)), you need at least TWO elements in L, because the rest must also be non-empty.
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What are first([3]) and rest([3])?
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The element 3 and the empty list [], respectively. [3] is not the same value as 3.
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What is rest([3,3,25])?
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[3,25]. Drop one position, not all matching values.
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How many elements are needed before calling first(rest(L))?
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At least two in the original list. A one-element list has an empty rest.