Re: [Tutor] Sequence Comparison

[Magnus Lycka]

Michael Janssen wrote:
> python compares each element from left to right until it founds a pair
> of element, where the condition (<) is true:
> 1 < 1 ---> python find's: not true therefore:
> 2 < 2 ---> not true, next try
> 3 < 4 ---> true. Overall condition is true

Wrong. Python only goes to the next element in a sequence
as long as all the previous element pairs compared equal.

If > is true for a pair, False is returned at once.

No element is less than any element, so it looks like
this...roughly.

(lhs = left hand side, you guess what rhs is)

def less_than(lhs, rhs):
    # Is lhs a sequence? (i.e. it has a length)
    if hasattr(lhs, '__len__'):
        # Are they both sequences?
        if hasattr(rhs, '__len__'):
            # Yes they are!
            # Either empty?
            if len(rhs) == 0:
                # lhs is either equal or greater
                return False
            elif len(lhs) == 0:
                # Nothing is less than something
                return True
            else:
                # Both are non-empty
                # Compare first elements
                if lhs[0] < rhs[0]:
                    return True
                elif lhs[0] == rhs[0]:
                    return less_than(lhs[1:], rhs[1:])
                else:
                    return False
        else:
            # Only lhs is a sequence. Make the arbitrary decision
            # that a sequence is always greater than a non-sequence
            return False
    else:
        # lhs is no sequence. Is rhs a sequence?
        if hasattr(rhs, '__len__'):
            # As we just said, sequence is greater...
            return True
        else:
            # Neither is a sequence, assume that we understand < here.
            return lhs < rhs

In real life, there are other non-obvious non-sequences, such as classes,
modules, files and mappings etc.

-- 
Magnus Lycka, Thinkware AB
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