[Courses] [python] Lesson 3: Fun with Strings and Lists

[Allison Moore]

Hello!  I'm new, but I come bearing homework!  Mmmmhomework

1.  I would use the len() command that returns the length of a list.  So in
this case:
>>> list = ['allison', 'paul', 'princess']
>>> len(list)
3

2. It counts back from the end.  So in the above example, list[-1] returns
'princess'

3. No idea

4.

list = ['one','two','three','four','five']
i = 0

while i < 5:
print list[i]
i = i+1

5.

vals = [ 0, 2, 4, 8, 16, 18, 17, 14, 9, 7, 4, 2, 1 ]

i = 0

while i < len(vals):
stars = vals[i]
print '*' * stars
i = i + 1



Thanks so much for putting this together Akkana - one of my friends referred
me to this list when he found out I was trying to learn python.  Great
timing for me!

Cheers,
Allison

On 2 July 2011 00:57, Akkana Peck <akkana at shallowsky.com> wrote:

> [Trying re-sending this -- sorry if you get two copies, but the
> first one doesn't seem to have shown up.]
>
> Today's lesson covers two important types of Python variables:
> strings and lists.
>
> ============== Strings ================
>
> You've already used simple strings like "Hello, world".
> But Python can do all sorts of useful things with strings. It's a
> great language for handling text (this lesson will only scratch the
> surface).
>
> You may remember from the discussion on earlier lessons that you can
> use + to "concatenate" (stick together) two strings. Like this:
>
> firstname = "Monty"
> lastname = "Python"
> fullname = firstname + " " + lastname
>
> You can also use += to build up a string by adding stuff on the end of it.
> fullname += lastname
> is just a shorthand way of saying
> fullname = fullname + lastname
>
> So you could build up that name like this:
> fullname = "Monty"
> fullname += " "
> fullname += "Python"
>
> You can get the length of that string (how many letters) with len(fullname)
> You'll see len again for other Python data types, so it's worth
> remembering it.
>
> ============== Splitting strings ===================
>
> What if you want to go the other way? What if you have a full name,
> like "Linus Torvalds", and you want to split it into firstname and
> lastname?
>
> Python strings have a handy way of splitting strings. It works like this:
>
> >>> fullname = "Linus Torvalds"
> >>> fullname.split(" ")
> ['Linus', 'Torvalds']
>
> As you see, the full name has been split at the space into two
> separate strings.
>
> The syntax, fullname.split(" "), might look a little odd. This is the
> first time in this course that we've encountered object-oriented code.
> "Object oriented" just means that Python objects, like strings, can
> have their own set of functions, like split(). If fullname is a string
> and you say fullname.split(" "), Python knows to call the function that
> splits strings. If fullname was some other type of object, like an
> integer number or a dictionary or a type you'd defined yourself, split()
> might mean something totally different, or might not exist at all.
>
> If that paragraph made no sense to you, don't worry about it. Just
> remember that if you have a string named s, you can call s.split().
>
> You can split on any set of characters, not just spaces. So if someone
> gave you a comma-separated list, maybe something that came out of a
> spreadsheet, like
>
> >>> veggies = "peas,green beans,spinach,corn,squash"
>
> you could say
>
> >>> veggies.split(',')
> ['peas', 'green beans', 'spinach', 'corn', 'squash']
>
> But about that ['Linus', 'Torvalds'] thing -- what is it really?
>
> ==================== Lists ========================
>
> ['Linus', 'Torvalds'] is what's called a list. You can have any number
> of items in a list, and you can reference them by number using square
> brackets, with 0 denoting the first item:
>
> >>> fullname = "Linus Torvalds"
> >>> names = fullname.split()
> >>> print "Hello,", names[0]
> Hello, Linus
>
> Lists can contain any type, and the items don't all have to be the
> same type. A list like this:
>
> mylist = [1, "eeny meeny miny moe", 3.14159]
>
> is perfectly valid. Lists can even contain other lists, but let's not
> worry about that just now.
>
> You can loop over the items in a list:
>
> for n in names :
>    print n
>
> ================ Slices =====================
>
> Okay, firstname and lastname are easy if there are only two names.
> What about people with multi-part names?
>
> >>> fullname = "Guido van Rossum"
> >>> names = fullname.split()
> >>> names
> ['Guido', 'van', 'Rossum']
>
> You can still use names[0] to get the first name. But the last name is
> no longer names[1] -- that would be "van". Python gives you a neat way
> of referring to several list items at once, called slices, where you
> can specify a start and end position with a colon in between. So if
> you knew Guido had a 2-part last name, you could say
>
> >>> names[1:3]
> ['van', 'Rossum']
>
> Or you might want to say "Give me everything except the first name".
> How would you do that? You can get the number of items in the list
> with len, the same way you get the length of a string. Then use it
> to take a slice (I'll space things out to make it more readable):
>
> >>> names[ 1 : len(names) ]
> ['van', 'Rossum']
>
> Getting everything to the end of the list, though, is such a common case
> that if you include a colon with nothing after it, Python assumes you
> want everything to the end of the list:
>
> >>> names[1:]
> ['van', 'Rossum']
>
> ============ Similaries between lists and strings ===========
>
> Many of the things you can do with lists, you can do with strings,
> and vice versa. You can take slices of a string:
>
> >>> fullname[0:5]
> 'Guido'
> >>> fullname[6:]
> 'van Rossum'
>
> and you can loop over a string's individual characters:
> for c in fullname :
>    if c == " " :
>        print "It's a space!"
>
> You can add to the end of a list; but instead of using += like you
> would for strings, use append: names.append("Jr.")
>
> ================== Homework ======================
>
> 1. How would you count the number of words in a single string?
>   Assume words are separated by spaces ... don't worry about
>   things like newlines, commas or hyphens.
>
> 2. What does an index of [-1], or another negative number mean in a
>   list or string? Take a guess, then try it and see if you were right.
>
> 3. Who is Guido van Rossum and why am I using him as an example?
>
> 4. Rewrite the exercise from lesson 2, the one where
>   you printed "one", "two", "three", "four", "five",
>   using a list instead of a series of if-elif.
>   (A few people already posted solutions that worked that way
>   in their lesson 2 answers. If you already did this for lesson 2, no
>   need to do it again. If you read other people's, it's still worth
>   writing it yourself now without going back and looking.)
>
> 5. This one's a little harder, but give it a try if you have time.
>   Plot a histogram graph from a list of numbers, with each number in
>   the list on its own line.
>
>   For instance, if you start with numbers like this:
> vals = [ 0, 2, 4, 8, 16, 18, 17, 14, 9, 7, 4, 2, 1 ]
>   you might plot something like this:
>
> **
> ****
> ********
> ****************
> ******************
> *****************
> **************
> *********
> *******
> ****
> **
> *
>    where the first line has no stars, the second has two, then 4, etc.
>    Hint: you'll need two loops, one inside the other.
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