Counting and Filtering

The second pattern counts: how many elements of a list meet a condition? It looks like adding up, but instead of the value, you add 1, and only when an if says so:

temperatures = [18, 24, 27, 15, 31, 22, 29]
hot_days = 0
for temperature in temperatures:
    if temperature >= 25:
        hot_days += 1
print("Hot days:", hot_days)

Hot days: 3

The counter hot_days starts at 0, before the loop. The loop looks at every temperature, and the if decides whether to count it. hot_days += 1 runs only for 27, 31 and 29, so at the end, the counter is 3. Notice that hot_days += 1 is indented twice: it belongs to the if, which is inside the loop. Again, you print the result after the loop.

Sometimes you want to know not only how many, but also which ones. Then start with an empty list and add each matching element to it with .append():

temperatures = [18, 24, 27, 15, 31, 22, 29]
hot = []
for temperature in temperatures:
    if temperature >= 25:
        hot.append(temperature)
print("Hot temperatures:", hot)
print("Hot days:", len(hot))

Hot temperatures: [27, 31, 29]
Hot days: 3

This way, you build a new list from the elements you want, and the original list stays as it was. And len() tells you how many elements the new list has.