Lists and Strings

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Processing a String

Lists and Strings are somewhat similar, since they are both a sequence of things. Strings are sequences of characters, but Lists can be a sequence of anything. The key idea is that both Strings and Lists are sequences, which means that you can iterate over them with a FOR loop. When you iterate over a list, you get each element. When you iterate over a string, you get each character.

word = "HELLO"

for character in word:
    print(character)

The string "Hello" is shown as the following boxes:

| H | E | L | L | O | |—|—|—|—|—|

Which results in the following printed output:

H
E
L
L
O

Using the split() method

Often, instead of processing a list character-by-character, we want to process it word-by-word, or by some other chunking of characters. To make this easy, strings have a method named split. Split is an awesome method because it is easy to use.

>>> "A multi-word string!".split()
['A', 'multi-word', 'string!']

>>> "ONE".split()
['ONE']

>>> "".split()
[]

For loop and Split

As you can see below, after we have split a string, it is easy to loop over each word. In this example, we separate each author to print them individually.

authors = "Alice Bob Carol"

for author in authors.split():
    print("By", author)

Splitting on Characters

If you do not pass anything to split, then it splits on any kind of whitespace - spaces, tabs, new lines. Sometimes, we want to split on other characters. You can pass a string as an argument to split on a different character.

>>> "Apple Pie,Yellow Cake,Plum Tart".split(',')
['Apple Pie', 'Yellow Cake', 'Plum Tart']

>>> "hokiebird@vt.edu".split("@")
['hokiebird', 'vt.edu']

>>> "Banana".split("nan")
['Ba', 'a']

String Iteration in Three Ways

Just to summarize briefly, there are three major ways to iterate over a string. Without the split method, you iterate by character. With a parameter in the split method, you iterate by splitting the string on the parameter. And without a parameter in the split method, you iterate by splitting the string on whitespace.

a_string = "1, 2, 3"

# By character
for a_character in a_string:
    print(a_character)
    
# On a split list
for an_item in a_string.split(","):
    print(an_item)

# On whitespace
for a_chunk in a_string.split():
    print(a_chunk)

Input/Split/Loop

Here is a useful pattern. You take a string separated by a specific character from the user, split the elements on that character, and process each component in turn. Notice how we can use this to process a string of numbers separated with commas by converting them using the “int” function. Study each statement of this pattern carefully, and make sure you understand it.

# Get user input
user_input = input("Type numbers separated by commas: ")

# Split into parts
user_values = user_input.split(',')

# Process each part independently
for value in user_values:
    value = int(value)
    print(value)