Choosing the Pattern From the Words of the Problem

Six mechanisms have been derived, each from a piece of structure in an input.

What has not been said is how to pick one in the three minutes before the

writing starts, from a statement nobody has labelled, with nothing to recognise

and nothing to recall. That decision is the whole skill, and it is almost never

taught, because it is the part that looks like judgement.

It is not judgement. It is a short list of questions asked about the input in a

fixed order, where the first one that matches decides and nothing after it is

read. Two of those questions rule the whole thing out, one of them is pure

arithmetic on the stated bound, and the rest are one structure each.

Nearly every statement is two halves. A sentence or two describing what you are

given, and a sentence saying what to produce. The instinct is to read the

second half first, and the second half is almost useless for choosing, because

the verbs in it are shared by every pattern here. Find appears in all of them.

So do count, minimise, and how many. A verb tells you what to accumulate inside

the loop and nothing about which loop to write.

The deciding facts are all in the first half. Does the input arrive in some

order. Does the question concern neighbours, or any items at all. Is only part

of the answer wanted. What bound is given on the size. Four facts, each a

phrase long, and between them they pick the method.

A quick way to feel this is to take two statements with the same verb. Find two

items summing to a target in a sorted list, and find two items summing to a

target in a list in arbitrary order. The verb is identical and the methods are

different patterns from different lessons. Now take two with different verbs

and the same input sentence. The longest run of neighbours whose total is under

a limit, and the count of runs of neighbours whose total is under a limit. The

verbs differ and both are a sliding window with a different line inside it.