Iteration, Bool, Selection

Booleans (values and expressions)

There are only two boolean values. They are True and False. They are called boolean literals. To work in Python must be capitalized. Type <class 'bool'>.

A boolean expression is an expression that evaluates to a boolean value.

3 > 4
>>> False
5 == 5
>>> True

Boolean operators

x != y               # x is not equal to y
x > y                # x is greater than y
x < y                # x is less than y
x >= y               # x is greater than or equal to y
x <= y               # x is less than or equal to y

The result of any expression evaluation can be returned by a function (using the return statement), functions can return boolean values aka. boolean functions.

It is common to give boolean functions names that sound like yes/no questions.

def isDivisible(x, y):
    return x % y == 0

Truthy values

Can be checked with bool()

bool(1) # True
bool(0)
bool([])
bool("")
bool(" ") # True

the is operation

a = []
b = []
a is b
>>> False

The is operator is rather useless as it checks memory address, but it is very useful for comparing to None.

a is None
>>> True # if of course a is None

[1] == [1]
>>> True
[1] is [1]
>>> False

the in operation

"py" in "python"
>>> True

5 in [3,4,5]
>>> True

The not keyword ca be used both ways:

if "a" not in "hello":
    pass

if not "a" in "hello":
    pass

Logical operators

# and
# or
# not
x > 0 and x < 10
n % 2 == 0 or n % 3 == 0 
not  x > y

number = 6
number == 5 or number == 6 or number == 7
# number == 5 or 6 or 7 will not work

Conditional execution

if statement

if BOOLEAN EXPRESSION:
    STATEMENTS_1        # executed if condition evaluates to True
else:
    STATEMENTS_2        # executed if condition evaluates to False

unary selection

the result of any expression evaluation can be returned by a function (using the return statement), functions can return boolean values

if x < 0:
    print("The negative number ",  x, " is not valid here.")

In unary selection the else clause is omitted and nothing happens if the statement is False.

else block cannot be used by itself.

conditionals can be nested

the indentation makes it clear what belongs where, and is thus very important

if x < y:
    print("x is less than y")
else:
    if x > y:
        print("x is greater than y")
    else:
        print("x and y must be equal")

chained conditionals

if x < y:
    print("x is less than y")
elif x > y:
    print("x is greater than y")
else:
    print("x and y must be equal")

• Only a single (and optional) final else statement is allowed and it must be the last branch in the statement

• Each condition is checked in order. If the first is false, the next is checked, and so on.

• Even if more than one condition is true, only the first true branch executes.

while statement

The while statement provides a much more general (than for loop) mechanism for iterating. The body of while will be repeated as long as the controlling boolean expression evaluates to True.

We can use the while loop to create any type of iteration we wish, including anything that we have previously done with a for loop.

def sumTo(aBound):
    """ Return the sum of 1+2+3 ... n """

    theSum  = 0
    aNumber = 1
    while aNumber <= aBound:
        theSum = theSum + aNumber 
        aNumber = aNumber + 1
    return theSum

The body of the loop should change the value of one or more variables so that eventually the condition becomes False and the loop terminates. Otherwise the loop will repeat forever. This is called an infinite loop.

• for loops create definite iteration because they loop through define number of items in a range

• while statement creates an indefinite iteration because we do not know how many times it may iterate.

escape character

The string '\t' represents a tab character. The backslash character in '\t' indicates the beginning of an escape sequence. Escape sequences are used to represent invisible characters like tabs and newlines. The sequence '\n' represents a newline.

break me out of jail (or not) keywords

break

You can stop a loop early by using the keyword break.

for letter in "abcdef":
    if letter == "c":
        break
    print(letter)

This loop will print "a" and "b" and then stop because of the break when letter is "c". break can only be used inside of a loop.

continue

for letter in "abcdef":
    if letter == "c":
        continue
    print(letter)

Nearly the same loop but this one will print "a", "b", "d", "e", and "f". It skips "c" because of the continue, which causes the loop to immediately move on to the next step in the loop.

Like break, continue can only be used inside of a loop.