Lecture 18

More on booleans and iterables

MCS 260 Fall 2020
David Dumas

Reminders

None

None is the only value of type NoneType. It represents the absence of a value, in cases where some value is needed.

E.g. None is the return value of a function that doesn't have a return statement.

>>> def f(x):
...     "Do nothing"
... 
>>> f(1) == None
True
    

bool()

The built-in function bool(x) converts a value x to a boolean, i.e. to either True or False.

How? A few values convert to False (are "falsy"):

• False
• None
• Zero in any numeric type (0, 0.0, 0j)
• Empty containers, i.e. (), [], "", {}, range(0)

Anything else converts to True, i.e. is "truthy" (unless you use an advanced technique to override this).

Automatic bool conversion

Python implicitly applies bool() to any value appearing where a boolean is expected, i.e. after if, elif, or while, or as operand of not, or, and.

>>> x = 5
>>> while x:  # not recommended; `while x!=0` is better.
...     print(x,end=" ")
...     x = x - 1
... 
5 4 3 2 1 >>>

    if not username:
        # Handle empty username
        print("The username must not be empty.")
        continue

Sequences and iterables

Reminder: Sequence is an ordered collection that can be accessed by integer index, e.g. tuple, list, string.

Sequences and iterables

Reminder: Iterable is a collection that can return items one by one upon request, e.g. range(), dict, dict_keys, ...

zip

You have a list

xcoords = [1,2,7,0,2]

ycoords = [5,5,-1,0,1]

        [ (1,5), (2,5), (7,-1), (0,0), (2,1) ]
    

?

Could use indexing and a for loop or comprehension, e.g.

>>> [ (xcoords[i], ycoords[i]) for i in range(len(xcoords)) ]
[(1, 5), (2, 5), (7, -1), (0, 0), (2, 1)]
    

But remember range(len()) usually means there is a better way?

zip(A,B,C,...) takes a bunch of iterables and returns tuples of values until one iterable is exhausted.

>>> zip(xcoords,ycoords)
<zip object at 0x7f51a3e36dc0>
>>> list(zip(xcoords,ycoords))
[(1, 5), (2, 5), (7, -1), (0, 0), (2, 1)]

Note zip() returns an iterable that we can convert to a list if needed.

Any & all

The functions any(L) and all(L) convert an iterable L into a single boolean.

any(L) returns True if at least one item from L is truthy. It returns as soon as it finds a truthy value. It is like a chain of or.

all(L) returns True if all items from L are truthy. It returns as soon as it finds a falsy value. It is like a chain of and.

Example: Check whether all characters in a string satisfy a condition.

left_keys = "qwertasdfgzxcvb"

def is_left_hand(word):
    "Can `word` be typed with only left hand on en-us keyboard?"
    return all( [c in left_keys for c in word] )

Example: Check whether a list of numbers contains at least one positive number.

def contains_a_positive(L):
    "Does `L` contain an element greater than zero?"
    return any( [x>0 for x in L] )