# Flow control

Martin McBride, 2018-04-08
In section Python language Conditions, if statements, loops.

## Conditions

True                # boolean True (capitalised)
False               # boolean False (capitalised)

x > 3               # True if x is greater than 3, else False
x < 3               # True if x is less than 3, else False
x == 3              # True if x is equal to 3, else False
x >= 3              # True if x is greater than or equal to 3,
# else False
x <= 3              # True if x is less than or equal to 3,
# else False
x != 3              # True if x is not equal to 3, else False

k = [2, 4, 6, 8]
x in k              # True if value x is present in
# sequence k, else False
x not in k          # True if value x is not present in
# sequence k, else False

a is b              # True is a and b are the same object
# equivalent to id(a)==id(b)
a is not            # True is a and b are not the same object


## Compound conditions

not x > 3           # True if x is NOT greater than 3
# equivalent to x <= 3
x < 3 and y < 2     # True if x is less than 3 AND y is less than 2
x < 3 or y < 2      # True if x is less than 3 OR y is less than 2

5 <= z < 10         # True if z is greater than or equal to 5 AND
# z is less than 10


## Falsey and truthy values

0                   # 0 or 0.0 count as False
# Any non-zero number counts as True

[]                  # Any empty collection counts as False
# Any non-empty collection counts as True
# Collections include lists, tuples, dicts, sets

None                # The value None counts as False


## If statements

if x > 10:
print('big')    # Only executed if x is greater than 10

if x > 10:
print('big')    # Only executed if x is greater than 10
else:
print('small')  # Executed if x is less than or equal to 10

if x > 10:
print('big')    # Only executed if x is greater than 10
elif x > 5:
print('mid')    # Only executed if x is greater than 5 but
# less than or equal to 10
else:
print('small')  # Executed if x is less than or equal to 5


## While loops

a = 0
while a < 3:
print(a)        # prints 0 1 2
a += 1

while True:
print('.')      # prints . . . forever
# useful with a break statement


## For loops

for i in range(5):      # from 0 up to but not including 5
print(i)            # prints:
#        0
#        1
#        2
#        3
#        4

for i in range(2, 5):   # from 2 up to but not including 5
print(i)            # prints:
#        2
#        3
#        4

for i in range(1, 9, 2):    # from  up to but not including 9
print(i)                # in steps of 2
# prints:
#        1
#        3
#        5
#        7

k = [20, 50, 30 10]
for x in k:                 # loops over each element in k
print(x)                # prints:
#        20
#        30
#        50
#        10

for i, x in enumerate(k):   # loops over each element in k
print(i, x)             # prints:
#        0 20
#        1 30
#        2 50
#        3 10

m = ['ab', 'cd', 'ef', 'gh']
n = [10, 20, 30, 40]
for a, b in zip(m, n):      # loops over each element in m, n
print(i, x)             # prints:
#        ab 10
#        cd 20
#        ef 30
#        gh 40


If you found this article useful, you might be interested in the book Functional Programming in Python, or other books, by the same author.

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