Lecture 18

More on booleans and iterables

MCS 260 Fall 2020
Emily Dumas

Reminders

  • Quiz 6 due today
  • Project 2 due Friday at 6:00pm central
  • 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]
    
    and another list
    
    ycoords = [5,5,-1,0,1]
    
    How would you make the list of corresponding pairs
    
            [ (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.

    zip() is most often used in loops

    
    cols = ["name", "quiz 1", "quiz 2"]
    vals = ["Anne Example", "82.5", "95.0"]
    for column,value in zip(cols,vals):
        print("Found value {} in column {}".format(value,column))
    

    Exercise: Given the list

    
    [ 4, 8, 15, 16, 23 42 ]
    
    How would you iterate over the adjacent pairs without using indices?

    
    >>> for a,b in adjacent_pairs( [ 4, 8, 15, 16, 23, 42 ] ):
    ...     print("Pair: {} and {}".format(a,b))
    ... 
    Pair: 4 and 8
    Pair: 8 and 15
    Pair: 15 and 16
    Pair: 16 and 23
    Pair: 23 and 42
    
    
    def adjacent_pairs(L):
        return zip(L,L[1:])
    

    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] )
    

    References

    Revision history

    • 2020-10-04 Correction about early return from all()
    • 2020-10-03 Initial publication