Class #7 – Strings & Objects

The thing about strings

Activity

How is a string different from the other data types we’ve seen (int, float, numpy.float32, bool)?

  • I can print individual characters of a string, by indexing the string:

    >>> a='My little string'
    >>> print(a[0])
        M
    
    >>> print(a[1])
    y
    
  • A string is just a… string… of characters

  • In a compound object, indexing allows us to pick out individual components.

  • Note that in Python, the first index is 0, not 1!
    • Whether to start at zero or one is an arbitrary decision made by programming language designers.

    • Is the “first floor” of a building the same as le “premier étage”? Natural language problem, too!

    • Python inherited the 0-based convention from C (this actually makes sense if/when you learn about memory pointers)

    • MATLAB inherited 1-based indexing from Fortran

    • This is a small gotcha for folks switching from one language to another; very easy to fix, but perplexing if you don’t know to look for it!

Activity

Write a single line command to print the first 4 characters of some string a.

How about the 2nd to 7th characters?

How about the last three characters?

Hint: what does a negative index do?

  • Bonus if you’re reading ahead: what does print(a[0:4]) do?

  • This is called slicing the string.

More loops

  • I can get the length of string like this:
    >>> len(a)
    15
    
  • Let’s apply that…

Activity

Write a function vert_print that takes a string as an argument and then uses a while loop to print each character on its own line. (See sample output below)

>>> vert_print(a)
M
y
_
l
i
t
t
l
e
_
s
t
r
i
n
g
  • The while loop certainly worked OK there, but it was a bit awkward.

  • while loops are meant to continue until some logical condition is met.
    • Which this technically is, buttttt…

  • Maybe there is another kind of loop that says “Do the indented code block once for each item in a compound object” rather than “Do the code block until an arbitrary condition is met”.

  • Such a thing exists: the for loop:

    for char in a:
        print(char)
    
  • for each character in the string a, we run the indented code block.

  • You don’t have to use the for loop. A while loop can do exactly the same thing.

  • The for loop is just cleaner here (and less typing!).

  • But there is nothing wrong with the while! There are just a lot of ways to solve a problem.

  • You can actually prove that there are a (countably) infinite number of programs to do any given task.
    • Some are just more efficient, and easier to read, than others.

Mutability

  • So… if we can access an individual character in a string with an index…

  • … you might be feeling tempted to try to set an individual character with an index, too.

  • Let’s try::
    >>> a[7]='X'
    Traceback (most recent call last):
        File "<stdin>", line 1, in <module>
    TypeError: 'str' object does not support item assignment
    
  • Variables of some types (like int) are mutable… that is: they can be changed.

  • Based on the above… do you think strings are mutable?

  • You can’t change a string. You have to make a new one based on the old one.
    >>> new_a = a[:7] + 'X' + a[8:]
    >>> print(new_a)
    My littXe string
    

in

Activity

Write a function char_is_in(char,string) that returns True if the character char appears in the string string.

  • HINT: what does the in operator do in Python?

  • You can do the above exercise the hard way, with loops, or you can look up in.

Tricky Activity

What’s wrong with this?:

def char_is_in(char, string):
    count = 0
    while count < len(string):
        if string[count] == char:
            return True
        else:
            return False
        count = count + 1
  • Try: char_is_in(‘t’, ‘test’)

  • Try: char_is_in(‘z’, ‘test’)

  • Try: char_is_in(‘e’, ‘test’)

Activity

Write a function where_is(char,string) that returns the index of the first occurrence of char in string.

String Trivia

  • '' or "" will work for the quotes needed for strings
    • But you can put '' inside "" s

    >>> a = "Hello, 'world'"
    >>> print(a)
    Hello, 'world'
    
  • We can concatenate strings
    >>> a = 'FuN' + ' ' + 'tImEs'
    >>> print(a)
    FuN tImEs
    
  • We can make a string repeat
    >>> a = 'FuN' * 3
    >>> print(a)
    FuNFuNFuN
    
  • We can convert an int to a str
    >>> print(type(1))
    <class 'int'>
    
    >>> print(type(str(1)))
    <class 'str'>
    
  • The string '' is a string, but it’s empty
    • This is a weirdly important detail actually

    >>> a = ''
    >>> print(len(a))
    0
    
    >>> print(type(a))
    <class 'str'>
    
  • We have some special characters that we have no buttons for.
    • ‘\n’

    • ‘\t’

    • ‘\\’

    • There are a bunch

    >>> a = 'hello\nWorld\tFUN\\!'
    >>> print(a)
    hello
    World   FUN\!       # A weird string
    
  • ASCII Table
    • Every character is a number

    _images/ascii.gif
    >>> wut = ord('a')  # get the num of 'a'
    >>> print(wut)
    97
    
    >>> wut = chr(65)   # convert num to char
    >>> print(wut)
    A
    

Formatting output (the ol’ trusty way)

%.2f (percent dot two eff)

  • f is for float

  • Right side of . is for decimal places

    >>> a = 1.235
    >>> print('Format to 2 decimal places: %.2f' %a) # it will round too!
    Format to 2 decimal places: 1.24
    
    >>> b = 4.39999
    >>> print('a: %.2f b: %.4f' %(a, b))        # need parentheses if more than one value to be inserted
    a: 1.24 b: 4.4000
    
  • Left side of . is for specifying total string length

    >>> a = 1.311
    >>> print('3 of the 5 chars: %5.1f' %(a))
    3 of the 5 chars:   1.3     # len('1.3') = 3
    
    >>> print('4 of the 5 chars: %5.2f' %(a))
    4 of the 5 chars:  1.31
    
    >>> print('5 of the 5 chars: %5.3f' %(a))
    5 of the 5 chars: 1.311
    
  • Left justify

    >>> a = 1.311
    >>> print('%-10.2f neato' % a)
    1.31       neato
    
    >>> print("%-10s%10.2f" %('Total:', a))
    Total:          1.31
    
  • Many old programming languages do it this way
    • And there are a billion other options too

  • There are new ways to format your strings in Python though
    • .format()
      • Probably the best way to do it in Python these days

    • f-strings

    • Check them out if you care

Objects

Warning

Some of the following is not actually true for Python, but will be the case for many of the commonly used programming languages.

Also, we will be going into more detail on Objects later in the class.

  • We have seen primitive types
    • Int

    • Float

    • Booleans

  • There are other types:
    • Strings (actually kinda’ a primitive type in Python, but let’s ignore this …)

    • Numpy things

  • These are objects!

  • We can even make our own objects
    • stay tuned

  • These objects act a little differently inside the computer
    • For good reason too, but stay tuned.

Methods

  • We’ve seen built in functions
    • print('this is a function')

  • We’ve written our own functions
    • char_is_in('a','bleh')

Activity

In Python (or whatever IDE):
  1. Make a string

  2. Assign it to a variable (if using Colab, hit run too)

  3. Type the name of the variable

  4. Press dot (period)

  5. Wait… (or space or press ctrl-space (depends on IDE))

_images/methods2.png _images/methods.png

Activity

  1. Try writing a_string.upper() and printing it out.

  2. Try some other methods

  • Methods are very very similar to functions

  • But we’re telling a specific object to do something

  • Long story short:
    • Sometimes we do things with functions

    • Sometimes we do things with methods

BUT WAIT…

  • Why do we have to do it like this a_string.upper()

  • As opposed to like this: upper(a_string)

Answer

  1. Because…

  2. upper(a_string) is not actually defined

    • unless we define it ourselves

  3. These methods were written by someone, and they wrote them to work a certain way

    • Not necessarily the best way, or a way you like

  4. There’s also a good bookkeeping argument too

    • Put all the string methods with the strings

  5. But really… because

How are you supposed to keep track of what’s what?

  • Don’t worry, you’ll get it with practice

  • Do note though, the key is practice

Heavy lifting with strings

For next class