Loops with for

At the end of the lesson on lists a question was left open: how do you do something of its own with each element of a list, such as converting every temperature to Fahrenheit or writing every item of shopping on a line of its own? With what you know so far, you would need a line of its own for each element, and as soon as the list got longer you would have to change the program. A loop writes the step down only once and repeats it for each element, just as you peeled every onion in the net in the basic ideas, one after the other, until the net was empty.

For each element of a list

The for loop reads almost like a sentence: for each item in the shopping list, write it on a line of its own.

shopping = ["milk", "bread", "apples", "cheese"]

for item in shopping:
    print(f"- {item}")

The line starts with for, followed by a name, the word in, the list and a colon. Everything indented below it belongs to the loop and is called its body; as with your own functions, the indentation alone shows where it ends. Python runs the body once for each element, here four times, and item stands for the next element of the list each time.

The body may also have several lines. Here it converts every temperature to Fahrenheit, and the last line is not indented, so it no longer belongs to the loop and runs only afterwards, a single time:

temperatures = [12.5, 14.0, 9.5, 16.0, 13.5]

for celsius in temperatures:
    fahrenheit = celsius * 9 / 5 + 32
    print(f"{celsius} °C is {fahrenheit:.1f} °F")

print("All values converted.")

Indent the last line as a test and run the code again: the sentence then appears after every temperature, because it now belongs to the body.

The loop variable

The name between for and in is called the loop variable. It is a perfectly ordinary variable that is assigned the next element before every round, and you choose its name yourself. It is common to use the singular of what the list holds in the plural, such as for item in shopping, for celsius in temperatures or for name in names. Python would also be happy with for x in temperatures, but anyone reading the program understands celsius at once.

After the loop, the variable keeps the value from the last round:

names = ["Ada", "Alan", "Grace"]

for name in names:
    print(f"Hello {name}!")

print(f"Last greeted: {name}")

Adding up

sum() adds all the values of a list at once. You can write what happens inside it yourself with a loop, and you need this pattern whenever there is no ready-made function. Before the loop you create a variable that starts at 0, and in every round the current element is added to it:

prices = [2.49, 1.19, 3.99, 0.89]
total = 0

for price in prices:
    total += price
    print(f"+ {price:.2f} makes {total:.2f}")

print(f"Total: {total:.2f} euros")

Such a variable is called an accumulator, because it collects the result round by round. total += price is the short form of total = total + price from the lesson on calculating. It matters that total = 0 stands before the loop: in the body, the variable would be set back to 0 in every round, and at the end it would hold only the last price.

Counting with a condition

An accumulator can also count. Instead of adding the value, it goes up by one for every element that fits, and an if in the body decides which element fits, as you know it from Decisions with if, elif and else:

temperatures = [12.5, 14.0, 9.5, 16.0, 13.5]
warm_days = 0

for celsius in temperatures:
    if celsius > 13:
        warm_days += 1

print(f"{warm_days} of {len(temperatures)} days were warmer than 13 degrees.")

Mind the two levels of indentation: the if belongs to the loop, and warm_days += 1 belongs to the if and therefore runs only in the rounds in which the condition holds.

Building a new list

Often a new list should come out of a list, such as a list of all temperatures in Fahrenheit that the program can go on calculating with. The accumulator then does not start at 0 but as an empty list, and append() adds an element in every round:

temperatures = [12.5, 14.0, 9.5, 16.0, 13.5]
fahrenheit = []

for celsius in temperatures:
    fahrenheit.append(round(celsius * 9 / 5 + 32, 1))

print(fahrenheit)
print(f"At most {max(fahrenheit)} °F")

The new list is a perfectly ordinary value that the program can go on working with, here with max(), for instance. If the append() stands inside an if, the loop takes over only some of the elements; try writing only the values above 13 degrees into the new list.

Sequences of numbers with range()

Sometimes there is no list at all, only a number of rounds, for example when a program should print the seven times table. For this, range() creates a sequence of whole numbers for the loop to go through:

for number in range(1, 11):
    print(f"{number} × 7 = {number * 7}")

range(1, 11) starts at 1 and stops before 11, just as the index of a list never reaches its length. This feels unusual at first, but it has an advantage: how many numbers come out is simply the difference, here 11 − 1 = 10. With one, two or three arguments, range() gives these sequences:

print(list(range(5)))
print(list(range(2, 6)))
print(list(range(0, 20, 5)))
print(list(range(10, 0, -2)))

With one argument the sequence starts at 0 and ends before the number, so that range(5) gives exactly five numbers. With two arguments you set start and end, and a third is the step, which may also be negative in order to count backwards. list() turns the sequence into a list so that print() shows all the numbers at once; in a loop this is not needed.

The characters of a text

To a loop, a text is a sequence of characters, and for takes them one after the other:

word = "Avocado"

for letter in word:
    print(letter)

Together with an accumulator and an if, the loop can count the vowels of a word, for example. With text, in checks whether a character occurs in it, and lower() makes sure that the capital A at the start counts as well:

word = "Avocado"
vowels = 0

for letter in word.lower():
    if letter in "aeiou":
        vowels += 1

print(f"{word} has {vowels} vowels.")

Numbered with enumerate()

For output such as "Day 1", "Day 2" you need the number of each element as well. enumerate() gives both in every round, the number and the element, and the loop takes them into two variables:

temperatures = [12.5, 14.0, 9.5, 16.0, 13.5]

for day, celsius in enumerate(temperatures, start=1):
    print(f"Day {day}: {celsius} degrees")

Without start=1, counting begins at 0, like the index. The order of the two variables is fixed: first the number, then the element.

Takeaways

• for element in list: runs the indented body once for each element; the loop variable holds the current element and is best named after the singular of the list.
• An accumulator starts at 0 or as an empty list before the loop and collects the result in the body with += or append().
• An if in the body counts or takes over only the elements that meet a condition.
• range(end), range(start, end) and range(start, end, step) create whole numbers, and the end is never part of them.
• A loop goes through a text character by character, and enumerate(list, start=1) gives each element its number.