Lists and 2D lists

Working through a list

Most list tasks are one loop with a variable that remembers something: a total, a count, or the best value so far.

function highest(values)
    best = values[0]
    for i = 1 to values.length - 1
        if values[i] > best then
            best = values[i]
        endif
    next i
    return best
endfunction
print(highest([-4, -9, -2, -7]))

SUBROUTINE highest(values)
    best ← values[0]
    FOR i ← 1 TO LEN(values) - 1
        IF values[i] > best THEN
            best ← values[i]
        ENDIF
    ENDFOR
    RETURN best
ENDSUBROUTINE
OUTPUT highest([-4, -9, -2, -7])

def highest(values):
    best = values[0]
    for i in range(1, len(values)):
        if values[i] > best:
            best = values[i]
    return best
print(highest([-4, -9, -2, -7]))

This prints -2. Starting from the first item, not from 0, matters: if every value is negative, 0 is not in the list at all.

Some tasks build a new list. Start with an empty list, result = [], and add to the end of it with result.append(x). So a loop that appends each score over 50 from [42, 77, 51] gives [77, 51].

To find a mean (average), add up the values and divide by how many there are: the mean of 4, 9 and 5 is 18 / 3 = 6.0. In Python / always gives a real number, while // gives the whole-number part.

A linear search checks each item in turn and returns its index as soon as it matches. Only after the whole list has been checked does it return -1 for "not found".

1. Decide what to remember: a total, a count, a best value or an index.
2. Set it up before the loop.
3. Loop through every index (or every item).
4. Update it inside the loop.
5. Return it after the loop, unless you have found what you were searching for.