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Minimum, Total and Average

A report often needs several facts from the same list. You can update more than one accumulator during one loop.

Track a minimum and a total

For a non-empty list, start the minimum with the first real value. Starting at 0 would be wrong when every measurement is positive: none could be smaller than zero.

distances = [12, 7, 10]
shortest = distances[0]
total = 0

for distance in distances:
    if distance < shortest:
        shortest = distance
    total = total + distance

print(shortest)
print(total)

The minimum changes only when a smaller value appears. The total changes on every turn.

Calculate a mean

The arithmetic mean is the total divided by the number of measurements. In the preceding example, the total is 29 and there are 3 measurements.

mean = 29 / 3
print(mean)
print(round(mean))

Division gives approximately 9.6667; round(mean) gives the integer 10. With one argument, round() rounds to a whole number. Exact halfway cases choose the even integer: round(10.5) is 10, and round(11.5) is 12.

Calculate the mean after the loop, once the total includes every measurement. An empty list would give a divisor of zero, so this lesson's examples require a non-empty list.

Trace the two values

For the measurements [18, 12, 20], start lowest at 18 and total at zero. Write both values after each measurement is processed. Explain why the final measurement changes the total but not the lowest value.

Check your trace: the pairs are (18, 18), (12, 30) and (12, 50), written as (lowest, total).

Practise this in the free Python course, where every exercise checks your code instantly.