09_strings.md

Strings

• Declaring strings
• string methods
• Functions with strings

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The string datatype

• Strings are simply a collection of characters
• However, in Python, a string is a scalar type and is immutable
• Strings are declared with single, double, or triple quotes.
  • Mostly interchangeable. Single and double quotes only work with strings that span one line.
  • Strings that span multiple lines need to be declared using triple quotes.

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Basic string Operators

• We can use the addition operator to concatenate strings.
  • 'My first string ' + 'My second string' => 'My first stringMy second string'
• The multiplication operator will repeat the string n times.
  • 'Repeating string' * 3 => 'Repeating stringRepeating stringRepeating string'

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Casting to and from a string

• Just about any data type can be cast to a string.
  • Use the string casting function: str()
• Not every string can be cast to another data type.
  • int('a') will not work
• String concatenation will only work between strings
  • Need to cast any numeric variables to a string before concatenating.

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Join a list to create a string

• Use the join string method to create a single string that contains all the elements of a list.
  • ''.join(list)
• All the elements of the list must be strings for the join method to work
  • You can use the map function to quickly cast all the elements of a list to strings
  • list(map(str, list))

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string Slicing

• Use slice indexing (similar to a list) to access characters or subsequences of characters
• string[index]
• string[start:end]
• string[start:end:skip]

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string membership

• Check if a string is a substring of another string using the in keyword.
  • 'str' in 'string'
  • Returns a boolean

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replace

• The replace method is used to replace every occurence of a specified substring in the original substring with another substring
  • Syntax: string.replace(old_substring, new_substring, 1)
• The third argument here will limit the amount of times the replace method will be applied to only once.

string = 'I love to look at the moon'

string = string.replace('o', 'puppy', 2)

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enumerate

• The enumerate function can be used on strings:

for idx, ch in enumerate(string):
    print(idx, ch)

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BREAKOUT (5 minutes)

Write a function that takes a list like the following list of column names and change any spaces in the column names to underscores. Return a modified list with the updated names.

column_names = ['gender', 'longest absence from school', 'is enrolled', 'enlist', 'unemployed', 'filed for bankruptcy', 'school', 'peace corps']

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BREAKOUT SOLUTION

column_names = ['gender', 'longest absence from school', 'is enrolled', 'enlist', 'unemployed', 'filed for bankruptcy', 'school', 'peace corps']

def add_underscores(feature_list):
    new_list = feature_list.copy()
    for idx, column in enumerate(new_list):
        if ' ' in column:
            new_list[idx] = column.replace(' ', '_')

    return new_list

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The list() Function

• A string can be cast to a list using the list function
  • list(string)
• The list() function will separate each character in the string into a new element in the returned list.

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The split() Method

• Quick aside: Strings are immutable so methods will not change the string in place like with lists.
• This method splits a string at each occurrence of a specified delimiter and will return a list with each split substring.
  • string.split(delimiter)
• The delimiter will not be included in the returned list

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lower(), upper(), swapcase(), and capitalize()

• The lower method will return a string where all the letters are lowercase
  • Syntax: string.lower()
• The upper method will return a string where all the letters are uppercase
  • Syntax: string.upper()
• The swapcase method will return a string where all the letter’s cases are swapped
  • Syntax: string.swapcase()
• The capitalize method will return a string where the first letter of the string will be capitalized and the rest will be lower case.
  • Syntax: string.capitalize()

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string Formatting

• The format method will dynamically insert the variables passed in as parameters into the placeholders in a string
  • Syntax: 'Inserting {} into this string'.format(variable_to_insert)
• f strings will dynamically insert variables into strings. The difference is that we just need to place the variable name in the curly braces.
  • Syntax: f'Inserting {variable_to_insert} into this string'

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BREAKOUT (8 minutes)

Write a function that takes in a string. Return two parallel lists: one that contains the unique characters in the string and another that has the number of times that character appears in the original string.

s = 'This is a string, we want you to count how many times each unique character appears in this string!'

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BREAKOUT SOLUTION Examples

s = 'This is a string, we want you to count how many times each unique character appears in this string!'

def get_letter_count(string):
    letters = []
    counts = []

    for let1 in string:
        index = 0
        if let1 not in letters:
            letters.append(let1)
            counts.append(0)
            index = len(letters) - 1
        else:
            for idx, let2 in enumerate(letters):
                if let1 == let2:
                    index = idx
                    break
        counts[index] += 1
    return letters, counts

letters, counts = get_letter_count(s)

for i in range(len(letters)):
    print(f'{letters[i]}: {counts[i]}')

or

s = 'This is a string, we want you to count how many times each unique character appears in this string!'

def get_letter_count(string):
    letters = []
    counts = []

    for let in string:
        if let not in letters:
            letters.append(let)
            counts.append(1)
        else:
            counts[letters.index(let)] += 1
    
    return letters, counts

letters, counts = get_letter_count(s)

for i in range(len(letters)):
    print(f'{letters[i]}: {counts[i]}')

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count()

• The count method is used to count every occurrence of a specified substring in the original string
  • Syntax: string.count('a')

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Applying count() to the letter counter

def get_letter_count(string):
    letters = []
    counts = []

    for let in string:
        if let not in letters:
            letters.append(let)
            counts.append(string.count(let))
        else:
            continue
    
    return letters, counts