8.8. Iteration with Strings

One of the most common uses of a for loop is to carry out a task on each item in a collection. When using a loop with a collection in this way, we say that the loop iterates over the collection.

8.8.1. Iterate by Index or Characters

In the Loops chapter, we learned about iteration. The following examples illustrate two ways we can iterate over a string.

Example

One way to iterate over a string is to use the index values for each character. The following program prints each of the characters of the string 'LaunchCode' on a separate line.

Run the code to see the output, then complete the Try It suggestions.

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name = 'LaunchCode'

for index in range(len(name)):
   print(name[index])

repl.it

Since len(name) is 10, the loop executes once for each of the values 0 to 9, assigning them in turn to the loop variable index.

The loop body, print(name[index]), prints one of the characters from name (name[0] through name[9] for 'LaunchCode').

Try It!

  1. Replace the string 'LaunchCode' with your name. Note how using len(name) as the argument inside range makes the loop run the proper number of times, regardless of the string.

  2. Use range(start, stop, step) to print every third character from name.

  3. Loop through the string using negative index values. Run the program with range(-1, -len(range), -1). How does the output change?

  4. range(-1, -len(range), -1) does not quite get all of the letters from name. Modify the start, stop, step values as needed to print all of the characters.

Since a string is simply a sequence of characters, Python gives us a way to automatically iterate over those characters.

Example

Run the following code to see the output, then complete the Try It suggestions.

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fruit = 'Bananas'
fruit_copy = ''

for char in fruit:
   fruit_copy += char
   print(char, fruit_copy)

repl.it

The loop variable char is automatically reassigned each character in the string stored by fruit. This approach is called iteration by item.

We also see an example of the accumulator pattern. The program reassigns a new, longer string to fruit_copy each iteration.

Note that iteration by item moves through a collection from left to right. If we want to move from right to left, we must use the index values.

Try It!

  1. Replace fruit_copy += char with fruit_copy = char.upper() + fruit_copy. What happens to the output?

  2. Replace print(char, fruit_copy) with print(char, fruit.count(char)). Run the program to see the output.

  3. Use a template literal and format() to print a more readable message each iteration. Something like, 'Bananas' contains 3 'a' character(s). The output should change each time the loop repeats.

8.8.2. Check Your Understanding

Question

Given the following code, which character is printed during the fifth iteration?

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text = 'Hello, World!'

for index in range(len(text)):
   print(text[index])

  1. 'l'

  2. 'o'

  3. ','

  4. ' '

Question

Given the following code, which character is printed during the fifth iteration?

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text = 'Hello, World!'

for char in text:
   print(char)

  1. 'l'

  2. 'o'

  3. ','

  4. ' '

Question

Given the string my_dream_car = 'Tesla', which of the following will loop backwards through all of the characters in the string? Select ALL options that work.

  1. for index in range(len(my_dream_car)):

  2. for index in range(-1, -len(my_dream_car), -1):

  3. for index in range(len(my_dream_car)-1, -1, -1):

  4. for index in range(-1, -len(my_dream_car)-1, -1):

  5. for char in my_dream_car:

  6. for char in -my_dream_car: