2014-03-12-06-string-lists.md

layout	post
number	6
chili	2
categories	exercise
tags	strings lists index
light_text	turning strings into lists

{% include exercise_header.md number=page.number %}

Ask the user for a string and print out whether this string is a palindrome or not. (A palindrome is a string that reads the same forwards and backwards.)

Discussion

Concepts for this week:

• List indexing
• Strings are lists

List indexing

In Python (and most programming in general), you start counting lists from the number 0. The first element in a list is "number 0", the second is "number 1", etc.

As a result, when you want to get single elements out of a list, you can ask a list for that number element:

{% highlight pycon %}

a = [5, 10, 15, 20, 25] a[3] 20 a[0] 5 {% endhighlight %}

There is also a convenient way to get sublists between two indices:

{% highlight pycon %}

a = [5, 10, 15, 20, 25, 30, 35, 40] a[1:4] [10, 15, 20] a[6:] [35, 40] a[:-1] [5, 10, 15, 20, 25, 30, 35] {% endhighlight %}

The first number is the "start index" and the last number is the "end index."

You can also include a third number in the indexing, to count how often you should read from the list:

{% highlight pycon %}

a = [5, 10, 15, 20, 25, 30, 35, 40] a[1:5:2] [10, 20] a[3:0:-1] [15, 10, 5] {% endhighlight %}

To read the whole list, just use the variable name (in the above examples, a), or you can also use [:] at the end of the variable name (in the above examples, a[:]).

Strings are lists

Because strings are lists, you can do to strings everything that you do to lists. You can iterate through them:

{% highlight python %} string = "example" for c in string: print "one letter: " + c {% endhighlight %}

Will give the result:

{% highlight pycon %} one letter: e one letter: x one letter: a one letter: m one letter: p one letter: l one letter: e {% endhighlight %}

You can take sublists:

{% highlight pycon %}

string = "example" s = string[0:5] print s examp {% endhighlight %}

Now s has the string "examp" in it.

Moral of the story: a string is a list.

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