Editing Community Data Science Course (Spring 2015)/Day 2 Followup

In the lecture on Monday, we covered assigning to variables, addition, conditionals, and the special <code>.append()</code> function associated with lists. Sovling the [[Community Data Science Course (Spring 2015)/Day 2 Coding Challenges|Day 2 Coding Challenges]] with the [[Baby names]] project is going to require you use not only use these things but put them together.

I wanted to show you some concrete examples of this. These aren't answers but hopefully you'll see how they might be applied to make progress on some of the coding challenges.

Here's an example of a counter variable I can use to count the number of names in a list that should be familiar from lecture:

<syntaxhighlight lang="python">
 names = ["mako", "mika", "william", "bettamax"]
 
 counter = 0
 for name in names:
     counter = counter + 1

 print(counter)
</syntaxhighlight>

Sure enough, that code will print "4" which is the number of things in the list. 

The only thing might seem new here is the line with <code>counter</code> in it twice. All we're doing in that line is just adding 1 to the variable <code>counter</code> and then replacing the old value with the new, slightly bigger, version.

Of course, instead of that counter and <code>for</code> loop, we could have just used <code>len(names)</code> and it would have done the same thing! This is powerful because loops are much more flexible and combined with other things we've learned. For example, lets say I wanted to count the number of names that start with the letter "m". I could combine that counter above with an <code>if</code> statement that checks if the "0th" item in each name (i.e., the first letter) to see if it's "m" like this:

<syntaxhighlight lang="python">
 names = ["mako", "mika", "william", "bettamax"]
 
 counter = 0
 for name in names:
     if name[0] == "m":
         counter = counter + 1

 print(counter)
</syntaxhighlight>

Now, this program will print out "2" (the number of names that start with "m") instead of "4".

I can also use append to create a new list from within a for loop. So lets say I wanted to create a new list that contains only those "m" names. In this case, the code I'll write is very similar except I will replace the counter with an empty list (<code>[]</code>) and I'll append things to it every time there's a match. Here's an example:

<syntaxhighlight lang="python">
 names = ["mako", "mika", "william", "bettamax"]
 
 m_names = []
 for name in names:
     if name[0] == "m":
         m_names.append(name)

 print(m_names)
</syntaxhighlight>

Run these little programs in ipython (or save them to files and run them in your terminal) and try to use the concepts to make some progress on the [[Community Data Science Course (Spring 2015)/Day 2 Coding Challenges|Day 2 Coding Challenges]].