HW6: Definite Loop Practice

The for x in y: loop behaves differently depending on what type of object y refers to. This exercise has you investigate what happens for a few different types of y.

The first step of this assignment is to create a directory called definite-loops.

The second step of this assignment is to create an empty Python file called definite-loops.py in that directory. Then open your Terminal program, navigate to the definite-loops directory, and you're ready to go.

This assignment is composed of a number of short code snippets. You should work through each section one by one, copying the snippets in that section into your definite-loops.py file. There are lots of snippets, and they print out lots of output. So each time you move onto a new section, you should replace the old code with the new code, so that you don't get distracted by the old output while you're working on the new section.

Again, this assignment is about “investigating” — don't just run the code snippets, mess around with them. Change values and print out types to get a deeper understanding of what's going on. It might be worthwhile to take notes as you go, writing down what you believe is happening.

Suppose y is a list. Then what kind of thing is x during each iteration of the loop?

print '====== looping over a list ======'
y = ['emu', 'auk', 'moonlight']
for x in y:
    print x
print

print '====== looping over another list ======'
y = [3, 1, 4]
for x in y:
    print x
print

print '====== looping over a mixed list ======'
y = [3, 'auks', 4, 'emus']
for x in y:
    print x
print

What if y is a string? When this loop runs, what kind of thing is x?

print '====== looping over a string ======'
y = 'emus and auks in the moonlight'
for x in y:
    print x
print

Suppose y is an open File object. What is x then? For this example, create a file in the same directory as your code,

print '====== looping over a file ======'
y = open('definite-loops.py', 'r')
for x in y:
    print x
print
# Remember you have to close a file after you're done with it!
y.close()

Suppose y is an integer. What is x then?

print '====== looping over an integer ======'
y = 15
for x in y:
    print x
print

You might not be able to loop over an integer (d'oh! I gave it away!), but you can loop over a “range” of integers.

print '====== looping over a range ======'
m = 7
for x in range(m):
    print x
print

What's the relationship between m and the last value that x takes? Where have we seen similar behavior before?

And why does looping over a range work? Let's check:

print range(7)
print type(range(7))

So what kind of object is range(7)? (That is, what kind of object does the expression range(7) evaluate to?)

Ranges are pretty flexible:

print '====== looping over other types of range ======'
for x in range(3,8):
    print x
print

for x in range(3, 27, 5):
    print x
print

What can you tell about the arguments to range()? Have we seen something similar before?

Another way of looping through a list (or string) is to use range() to loop over the valid indices of that list, and to access the list at that index.

print '====== looping through a list by index ======'
L = ['spam', 'eggs', 'sausage']
for i in range(len(L)):
    print L[i]
print

Why might you want to do this? Well, the index is just a number, and you can do other computations with it.

L = ['spam', 'eggs', 'sausage']
for i in range(len(L)-1):
    print L[i], L[i+1]
print

What just happened here? Why did we need to do range(len(L)-1)?

The “body” of a loop is a set of Python statements that you want to execute on each pass through the loop. In most of the examples above, the body has been a simple print statement. But the body can be any sequence of Python commands at all! It's very similar, in that way, to the body of a function.

total_length = 0
menu = ['spam', 'eggs', 'sausage']
for dish in menu:
    print dish
    total_length = total_length + len(dish)
    print "The total length computed so far is", total_length
print "The grand total of the lengths of the dishes is", total_length

Notice that we use indentation to indicate the body of a loop.

The body of a loop can be any Python code! This includes loops, in fact. Putting loops inside of other loops is a very common thing in programming. Before you run this code snippet, try to predict what it will print out. Then check and see if you were right.

print '====== nested loops, first example  ======'
for j in range(4):
    for k in range(3):
        print j, k
print

Nested loops can be pretty confusing, so study this one carefully. What exactly is going on?

Here's a trickier nested loop. Same as before: first try to predict what this snippet will print out, before you actually run it.

print '====== nested loops, trickier ======'
for j in range(4):
    for k in range(j):
        print j, k
print