Strings 1

Calvin (Deutschbein)

W4Wed: 18 Sep

Announcements

By next MONDAY 11:59 PM: "Problem Set 3: Strings" assignment.
- It is about strings
- Start now.
- We want to get ahead of the clock on problem sets since there's a project soon.
Sections today and tomorrow. Also on strings.

Today

Strings
String arithmetic
- Addition / Multiplication
- Comparison

Strings

Remember how we could do this: >>> x = "hello world" >>> print(x) hello world
This is snippets of text in Python are called 'strings' >>> type(x) <class 'str'>

Strings

Strings rule, they are basically what I use Python for the most. >>> for student in cs_majors: ... if 'Sophomore' in student: ... x += 1
For example, I can count how many sophomores have declared a CS major.
- If you want to declare a CS major, ckdeutschbein@willamette.edu

Strings

Python is pretty good at numbers. >>> 1234 * 5678 7006652
But could be better >>> 9.9 ** 20 + 1000 == 9.9 ** 20 True

Strings

I do not know how Python could be better with strings.
I copy-pasted the entirety of Pride and Prejudice into Python >>> letters = 0 >>> for letter in book: ... letters += 1 ... >>> letters 1496244

Strings

I copy-pasted the entirety of Pride and Prejudice into Python >>> es = 0 >>> for letter in book: ... if 'e' == letter: ... es += 1 ... >>> es 147006
There are 147006 letter 'e' characters in Pride and Prejudice.
That seems... hard to count otherwise.
This sees use in e.g. State Dept analysis, digital humanities, etc.

Strings 1

Four ways to make a variable that is a string.
- 1. Use single quotes. name = 'calvin'

Strings 2

Four ways to make a variable that is a string.
- 1. Use single quotes. name = 'calvin'
- 2. Use double quotes, which allows counting single quotes. name = "L'Ouverture"

Strings 3

Four ways to make a variable that is a string.
- 1. Use single quotes. name = 'calvin'
- 2. Use double quotes, which allows counting single quotes. name = "L'Ouverture"
- 3. Use three double quotes, which allows spanning multiple lines book = """It is a truth universally acknowledged, that a single man in possession of a good fortune must be in want of a wife. However little known the feelings or views of such a man may be on his first entering a neighbourhood, this truth is so well fixed in the mind"""

Strings 4

Four ways to make a variable that is a string.
- 1. Use single quotes. name = 'calvin'
- 2. Use double quotes, which allows counting single quotes. name = "L'Ouverture"
- 3. Use three double quotes, which allows spanning multiple lines book = """It is a truth universally acknowledged, that a single man in possession of a good fortune must be in want of a wife. However little known the feelings or views of such a man may be on his first entering a neighbourhood, this truth is so well fixed in the mind"""
- 4. Use the str() function, which makes a non-string into a string. number = str(105 * 4)

Strings 4 Again

Emphasis
4. Use the str() function, which makes a non-string into a string. number = str(105 * 4)
Be advised:
- The string "10" and the value 10 (no quotes) are not equal. >>> str(10) == 10 False
- Always check with type()! >>> x = 10 >>> type(x) <class 'int'> >>> x = str(x) >>> type(x) <class 'str'>
- Use type()!

Today

✓: Strings
String arithmetic
- Addition / Multiplication
- Comparison

String +

For some reason, Python let's us add strings together.
- You can decide if you like this or not. It's a bit unusual.
>>> print('CS' + '151') # there won't be spaces... CS151
In general, Python makes sure you don't add strings to ints. >>> print('CS' + 151) # won't work Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: can only concatenate str (not "int") to str
I am confident you can read and understand this error code if you see it.
Read the the last line.

String +

If we intend to add a string and an integer, we just have to say so. >>> number = 151 >>> prefix = 'CS' >>> prefix + str(number) 'CS151'
The most common case for me: adding spaces. >>> number = 151 >>> prefix = 'CS' >>> prefix + ' ' + str(number) 'CS151'
NOTE: I pretty much always forget a plus '+' on one side of the space.
Subtraction does not work
- What would it even do?

String *

I do a lot better reading things when they align vertically, like I did here with 'number' and 'prefix':
- That is, the single-equals assignment '=' is vertical aligned, one over another
>>> number = 151 >>> prefix = 'CS'

String *

I also teach classes called:
- CS 151
- IDS 101
- DATA 599
This is hard on my eyes - it isn't easy to see, for example, DATA 599 is a graduate class (500-level)
I solve this by... adding spaces!
- This is called 'right padding' is one of the most used techniques in text processing!

`len()`

Python includes a handy built-in function called len() to help us here.
- It is handy in the way print() and type() are - keep it in mind!
We can actually use it on more than just strings... >>> len(range(0,50,7)) # 0, 7, 14, 21... 28, 35, 42, 49 8

`len()`

Python includes a handy built-in function called len() to help us here.
It works very well on strings. >>> letters 1496244 >>> len(book) 1496244
Ranges and strings have other commonalities - more latter.

right pad

On like my course websites, etc., I right pad my course names: def right_pad(prefix, number): if len(prefix) == 2: # CS return prefix + " " + number if len(prefix) == 3: # IDS return prefix + " " + number if len(prefix) == 4: # DATA return prefix + " " + number if len(prefix) == 5: # PHEAL return prefix + number
This is what is known as bad code (I would make too many typos)
We are allowed to make bad code as long as we want to make it better!

right pad

def n_spaces(n):
    s = ""
    for i in range(n):
        s = s + " "
    return s

def right_pad(prefix, number):
    if len(prefix) == 2: # CS
        return prefix + n_spaces(3) + number
    if len(prefix) == 3: # IDS
        return prefix + n_spaces(2) + number
    if len(prefix) == 4: # DATA
        return prefix + n_spaces(1) + number
    if len(prefix) == 5: # PHEAL
        return prefix + n_spaces(0) + number

right pad

def n_spaces(n):
    s = ""
    for i in range(n):
        s = s + " "
    return s

def right_pad(prefix, number):
    # wait a minute...
    num_spaces = 5 - len(prefix) # the longest is 5
    return prefix + n_spaces(num_spaces) + number

right pad

By the way, we have a name for adding things 'n' times.
It's multiplication. def n_spaces(n): return " " * n def right_pad(prefix, number): # wait a minute... num_spaces = 5 - len(prefix) # the longest is 5 return prefix + n_spaces(num_spaces) + number
For some reason, multiplying strings is allowed.
This is a good way to be cute, among other things. >> print("hi" * 3) # mfw i see bae hihihi

Today

✓: Strings
String arithmetic
- ✓: Addition / Multiplication
- Comparison

String comparison

We can use comparison operators on strings.
They just do alphabetical, and test it if it seems non-obvious.
Vocab word: This is called "lexicographical" ordering (term might be on midterm).
I think
- "a" is before "b", and
- "1" is before "2", so
- "a" < "b" since "1" < "2"

String comparison

Just test it.
- Alphabetical ('a' before 'b')
- Capitals before lower
- Numbers before letters
- Shorter before longer
Can't compare strings and ints.
Don't do what I always do and confuse strings with variable names. >>> a = 'something' >>> b = 'another thing' >>> a < b False
Use descriptive variable names.

>>> 1 < 2 # baseline
True
>>> 'a' < 'b'
True
>>> 'a' < 'A'
False
>>> 'a' < '1'
False
>>> 'A' < '1'
False
>>> 'a' < 'aa'
True
>>> 'Oregon' < 'Washington'
True
>>>

Today

✓: Strings
✓: String arithmetic
- ✓: Addition / Multiplication
- ✓: Comparison

Announcements

By MONDAY 11:59 PM: "Problem Set 2: Checkers" assignment.
- One Karel problem (probably) using "if" and "while"
- One Python problem using arithmetic and (probably) "for" def divisors(n:int) -> int: # this is useful count = 0 for x in range(2,n): if n % x == 0: print(x) count = count + 1 return count
- One writing problem.

Announcements

By MONDAY 11:59 PM: "Problem Set 3: Strings" assignment.
- It is about strings
- Start now. def n_spaces_before(n:int,s:str) -> NoneType: # print -> None print(n * ' ', s) # test - that is 'n times space plus s'
- We want to get ahead of the clock on problem sets since there's a project soon.
Sections today and tomorrow. Also on strings.