7. Strings

7.1. The thing about strings

Activity

How is a string different from the other data types we’ve seen (int, float, 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.

    • 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

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.

7.2. 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.

  • 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.

7.3. 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

7.4. 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.

7.5. 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

    img/ascii.gif 
    >>> wut = ord('a')  # get the num of 'a'
>>> print(wut)
97

    >>> wut = chr(65)   # convert num to char
>>> print(wut)
A

7.6. Formatting output

%.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

7.7. For next class