Formatting Strings

Jed Rembold & Fred Agbo

February 7, 2025

Announcements

Problem set 3 is due next week Monday at 10 pm
I am working on returning PS 2. You should get it hopefully, by next week
Our class on Friday February 16 will be at Eaton 311
Remember the only way to master coding is to keep practicing
Link to Polling https://www.polleverywhere.com/agbofred203

Review!

Suppose you have the string x = "consternation" and you’d like to just extract and print the word "nation". Which expression below will not give you the string "nation"?

x[7:len(x)]
x[7:]
x[-6:len(x)]
x[-6:-1]

Transforming Methods

Method	Description
`string.lower()`	Returns a copy of `string` with all letters converted to lowercase
`string.upper()`	Returns a copy of `string` with all letters converted to uppercase
`string.capitalize()`	Returns a copy of `string` with the first character capitalized and the rest lowercase
`string.strip()`	Returns a copy of `string` with whitespace and non-printing characters removed from both ends
`string.replace(old, new)`	Returns a copy of `string` with all instances of `old` replaced by `new`

Classifying Character Methods

Method	Description
`char.isalpha()`	Returns `True` if `char` is a letter
`char.isdigit()`	Returns `True` if `char` is a digit
`char.isalnum()`	Returns `True` if `char` is letter or a digit
`char.islower()`	Returns `True` if `char` is a lowercase letter
`char.isupper()`	Returns `True` if `char` is an uppercase letter
`char.isspace()`	Returns `True` if `char` is a whitespace character (space, tab, or newline)
`char.isidentifier()`	Returns `True` if `char` is a legal Python identifier

Igpay Atinlay

Suppose we wanted to write a script that converted English to Pig Latin
Rules of Pig Latin:
- If the word begins with a consonant, move everything up to the first vowel to the end and append on “ay” at the end fleet ⟶ eetflay
- If the word starts with a vowel, just append “way” to the end orange ⟶ orangeway
- If the word has no vowels, do nothing
Our decomposition:
- Find first vowel
- Convert a single word

Indingfay Owelsvay

def find_first_vowel_index(word):
    """
    Find the first vowel in a word and return its index,
    or return None if no vowels found.
    """
    for i in range(len(word)):
        index = "aeiou".find(word[i].lower())
        if index != -1:
            return i
    return None

Onvertcay Oneway Ordway

def word_2_pig_latin(word):
    """
    Convert a single word with no special characters from
    English to Pig Latin.
    """
    vowel = find_first_vowel_index(word)
    if vowel is None:
        return word
    elif vowel == 0:
        return word + "way"
    else:
        return word[vowel:] + word[:vowel] + "ay"

Definitely not G-Strings

Constructing text or a sentence by interleaving strings and other objects comes up a lot in communicating code results to a user
We’ve already seen that, for any version past 3.6, the nicest and easiest way to do this in Python is with f-strings:
```
A = 10
print(f"The value of A is: {A}!")
```
You can define an f-string anytime you would normally define a string, just be aware that the substitution happens with the values of variable at that point
```
A = 10
s = f"The value of A is {A}"
A = 12
print(s)
```

Formatting Strings

F-strings allow up to easily substitute in values, but what if we want those values to have a particular format?
- Rounded to the nearest two decimal places, for example
One option would be to handle all this before the substitution manually
Python gives a more streamlined method, using a format spec
A format spec will be given inside the {} placeholder
- Comes after the variable/value itself
- There is a colon between the variable/value and the format spec
```
A = 10.234
print(f"The value of A is {A:0.2f}")
```

Shaping your format

A Format spec is a special string that determine the desired format
Lots we can do, but we rarely need to do it all at once
The basic building blocks (square brackets just to group):
[[fill]align][sign][width][,][.precision][type]

Type
- How you want the object represented as a string
- “d” - base-10 integer
- “f” - float or decimal
- “e” - scientific notation
- More on next slide

Precision
- How many digits to display after a decimal point
- Details can vary a little by conversion type

Grouping?
- A comma here indicates that numbers should be grouped in sets of 3 and separated with a comma

Width
- The minimum number of characters you want the formatted value to have
- If not otherwise specified, pads the value with space characters

Sign?
- If the sign of the number should be specified
- A + sign ensures all numbers will have either a + or - sign in front
- A space just adds a space before positive numbers (negatives would have a - sign in front)

Fill and Align
- How you want the text aligned if it is shorter than the minium width
  - Can be <, >, or ^ for left, right, or center justified
- Any fill characters you want to fill the empty space come before
  - Default is space

Output Conversion Types

Code	Description
`b`	Inserts an integer using its binary representation
`d`	Inserts an integer using its decimal representation
`e` or `E`	Inserts a number using scientific notation
`f` or `F`	Inserts a number using a decimal point format
`g` or `G`	Choose `e` or `f` to get the best fit
`o`	Inserts an integer using its octal representation
`s`	Inserts a string value
`x` or `X`	Inserts an integer using its hexadecimal representation

Uppercase conversion types use capital characters where possible in output

Learning English

When working with sequences of characters, it is often useful or desirable to determine if they form actual valid English words
This class provides for you a new library, through the file english.py
The english library provides two objects you can import into your programs:
- The constant ENGLISH_WORDS, which is a list of all the valid words in the English dictionary
- The function is_english_word(), which accepts a single string as an argument and returns True if the string represents a valid English word.
This library will be particularly useful for Wordle!

Example: How many 2 letter words?

Before we start writing code, let’s pause. Give a physical English dictionary, how could you go about figuring out the number of two letter words?

from english import ENGLISH_WORDS

count = 0
for word in ENGLISH_WORDS:
    if len(word) == 2:
        count += 1
print(count)

from english import is_english_word

count = 0
alphabet = 'abcdefghijklmnopqrstuvwxyz'
for letter1 in alphabet:
    for letter2 in alphabet:
        word = letter1 + letter2
        if is_english_word(word):
            count += 1
print(count)