Lecture 8

List methods and comprehensions

MCS 260 Fall 2021
David Dumas

Reminders

• Worksheet 3 solutions posted
• Homework 3 posted (due Tues 10am)
• Project 1 due Fri Sep 17 at 6pm CDT

Project 1 discussion

321, 174, 258, 385, 448, 480, 496, 612, 414, ...

Iterables

Recall a thing that can appear in a for loop in Python is called an iterable. So iterables include:

• Sequences (strings, lists, tuples*)
• $\texttt{range(...)}$, $\texttt{enumerate(...)}$
• Other built-in types we'll discuss soon (dict, set)

List methods

Lists in Python have many useful features we haven't talked about.

Any list, say $\texttt{L}$, comes with its own set of functions (called methods) that operate directly on the list.

L.append(x)    # Add x to the end of the list
L.insert(i,x)  # Insert x at position i
L.remove(x)    # Remove first instance of x in L
L.pop()        # Remove and return the last item of L
L.index(x)     # Find x in L, return its index

All except $\texttt{index()}$ change the list.

Example: Suppose $\texttt{L}$ is a list of strings representing integers, and we need to convert it to a list $\texttt{M}$ of ints.

A for loop can be used to do this:

L = ["42", "16", "15", "8", "4"]
M = []
for s in L:
    M.append( int(s) )
# now M == [42, 16, 15, 8, 4]

This pattern is very common: Iterate over a list doing something to each element, producing a new list.

This pattern is so common that Python has a more compact way of writing it. The code:

M = []
for s in L:
    M.append( int(s) )

Can instead be written:

M = [ int(s) for s in L ]

The expression $\texttt{[ ... for ... in ... ]}$ is called a list comprehension. It is a compact way of writing a common type of for loop.

Comprehension examples

The basic comprehension syntax is:

[ expression for varname in iterable ]

For example:

[ x**2 for x in range(5) ]
# Gives [0, 1, 4, 9, 16]

[ s[1:] for s in ["cat", "spot", "blot"] ]
# Gives ["at", "pot", "lot"]

[ float(s[:-1]) for s in ["6C", "12.5C", "25C"] ]
# Gives [6.0, 12.5, 25.0]

The variable name in a comprehension can be anything, it just needs to be used consistently.

These are all equivalent:

[ x**2 for x in range(5) ]
[ t**2 for t in range(5) ]
[ apple**2 for apple in range(5) ]

The name in a comprehension is not assigned to anything outside the comprehension:

>>> [ x**2 for x in range(5) ]
[0, 1, 4, 9, 16]
>>> x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'x' is not defined

Filtering

There is another common type of for loop, where elements are not just transformed but also filtered.

words = [ "alpha", "bridge", "assemble", "question" ]
a_words = []
for s in words:
    if s[0] == "a":
        a_words.append(s)
# Now a_words is [ "alpha", "assemble" ]

a_words = [ s for s in words if s[0]=="a" ]

[ expression for name in iterable if condition ]

Filtering examples

[ x+x**2 for x in range(5) if x!=2 ]

# range(5) gives      [0, 1, 2, 3, 4]
# !=2 gives           [0, 1, 3, 4]
# add to square gives [0+0, 1+1, 3+9, 4+16]
# Final result:
[0, 2, 12, 20]

namepairs = [ ["Mr.","Nabil Weber"],
              ["Ms.","Janet Leon"],
              ["Ms.","Mariana Wang"],
              ["Dr.","Lisa Young"] ]

What if we want a greeting (as salutation name) of the people with salutation "Ms."?

[ sal+" "+name for sal,name in namepairs if sal=="Ms." ]
# Gives ["Ms. Janet Leon","Ms. Mariana Wang"]

mss = []
for sal,name in namepairs:
    if sal=="Ms.":
        mss.append(sal+" "+name)

Convert every digit from the input string to an int, and make a list of these:

[ int(c) for c in input() if c in "0123456789" ]

If the keyboard input is $\texttt{I like 0 more than 157}$, then the above will evaluate to

[ 0, 1, 5, 7 ]