Editing Community Data Science Course (Spring 2016)/Day 1 Tutorial (section)

==Strings==

[[File:Letter.png|100px]]

So far we've seen two data types: '''integers''' and '''floats'''. Another useful data type is a '''string''', which is just what Python calls a bunch of characters (like numbers, letters, whitespace, and punctuation) put together. Strings are indicated by being surrounded by quotes:

 "Hello"
 "Python, I'm your #1 fan!"

Like with the math data types above, we can use the <code>type</code> function to check the type of strings:

 type("Hello")
 type(1)
 type("1")

===String Concatenation===

You can smoosh strings together (called "concatenation") using the '+' sign:

 "Hello" + "World"

 name = "Jessica"
 "Hello " + name

How about concatenating different data types?

 "Hello" + 1

Hey now! The output from the previous example was really different and interesting; let's break down exactly what happened:

<code>>>> "Hello" + 1</code><br />
<code>Traceback (most recent call last):</code><br />
<code>  File "<stdin>", line 1, in <module></code><br />
<code>TypeError: cannot concatenate 'str' and 'int' objects</code>

Python is giving us a '''traceback'''. A traceback is details on what was happening when Python encountered an Exception or Error -- something it doesn't know how to handle.

There are many kinds of Python errors, with descriptive names to help us humans understand what went wrong. In this case we are getting a <code>TypeError</code>: we tried to do some operation on a data type that isn't supported for that data type.

Python gives us a helpful error message as part of the TypeError:

<code>"cannot concatenate 'str' and 'int' objects"</code>

We saw above the we can concatenate strings:

 "Hello" + "World"

works just fine.

However,

 "Hello" + 1

produces a <code>TypeError</code>. We are telling Python to concatenate a string and an integer, and that's not something Python understands how to do.

We can convert an integer into a string ourselves, using the <code>str</code> function:

 "Hello" + str(1)

Like the <code>type</code> function from before, the <code>str</code> function takes 1 argument. In the above example it took the integer 1. <code>str</code> takes a Python object as input and produces a string version of that input as output.

===String length===

There's another useful function that works on strings called <code>len</code>. <code>len</code> returns the length of a string as an integer:

 len("Hello")
 len("")
 fish = "humuhumunukunukuapua'a"
 name_length = len(fish)
 fish + " is a Hawaiian fish whose name is " + str(name_length) + " characters long."

===Quotes===

We've been using double quotes around our strings, but you can use either double or single quotes:

 'Hello'
 "Hello"

Like with spacing above, use whichever quotes make the most sense for you, but be consistent.

You do have to be careful about using quotes inside of strings:

 'I'm a happy camper'

This gives us another '''traceback''', for a new kind of error, a <code>SyntaxError</code>. When Python looks at that expression, it sees the string 'I' and then

<code>m a happy camper'</code>

which it doesn't understand -- it's not 'valid' Python. Those letters aren't variables (we haven't assigned them to anything), and that trailing quote isn't balanced. So it raises a <code>SyntaxError</code>.

We can use double quotes to avoid this problem:

 "I'm a happy camper"

One fun thing about strings in Python is that you can multiply them:

 "A" * 40
 "ABC" * 12
 h = "Happy"
 b = "Birthday"
 (h + b) * 10