MCS 260 Fall 2020
David Dumas
Last time we discussed for loops, which run a block of code for each element of a sequence or certain other "container" types.
The term for a thing that can appear in a for loop in Python is an iterable. So iterables include:
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 in $\texttt{[ ]}$ is called a list comprehension. A comprehension is a compact way of writing a common type of for loop.
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
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 ]
[ 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 = [ ("Frances","Beal"),
("David", "Bowie"),
("Justin","Roberts"),
("David", "Cameron") ]
What if we want the full names (as first last) of the people with first name David.
[ first+" "+last for first,last in namepairs if first=="David" ]
# Gives [ "David Bowie", "David Cameron" ]
That comprehension,
[ first+" "+last for first,last in namepairs if first=="David" ]
davids = []
for first,last in namepairs:
if first=="David":
davids.append(first + " " + last)
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 $\texttt{i16 n+0 20B}$, then the above will evaluate to
[ 1, 6, 0, 2, 0 ]
Use when their brevity improves readability, i.e. when a for loop spreads a simple idea out over multiple lines.