Tutorial: implement length, first and last

Procedure length(L):
    Count = 0
    foreach x in L:
        Count = Count + 1
    return Count

Procedure first(L):
    # Precondition: L is non-empty
    foreach x in L:
        return x

Procedure last(L):
    # Precondition: L is non-empty
    foreach x in L:
        E = x
    return E

length accepts a list and returns its occurrence count. The empty list returns 0 because no increment occurs. first returns during the very first loop execution, which terminates the function. last overwrites E on every visit, so after the final visit E holds the final element.

first([]) has no first occurrence; last([]) never assigns E. The tutorial calls both results undefined. State a non-empty precondition or add an explicit error/unavailable result. Do not invent a return value of 0 or [] for an undefined element. Different implementations may share the same interface but use different amounts of work.