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Control Flow in Python

elif Statement

The Python elif statement allows for continued checks to be performed after an initial if statement. An elif statement differs from the else statement because another expression is provided to be checked, just as with the initial if statement.

If the expression is True, the indented code following the elif is executed. If the expression evaluates to False, the code can continue to an optional else statement. Multiple elif statements can be used following an initial if to perform a series of checks. Once an elif expression evaluates to True, no further elif statements are executed.

# elif Statement


pet_type = "fish"


if pet_type == "dog":
  print("You have a dog.")
elif pet_type == "cat":
  print("You have a cat.")
elif pet_type == "fish":
  # this is performed
  print("You have a fish")
else:
  print("Not sure!")

if Statement

The Python if statement is used to determine the execution of code based on the evaluation of a Boolean expression.

  • If the if statement expression evaluates to True, then the indented code following the statement is executed.
  • If the expression evaluates to False then the indented code following the if statement is skipped and the program executes the next line of code which is indented at the same level as the if statement.
# if Statement


test_value = 100


if test_value > 1:
  # Expression evaluates to True
  print("This code is executed!")


if test_value > 1000:
  # Expression evaluates to False
  print("This code is NOT executed!")


print("Program continues at this point.")

else Statement

The Python else statement provides alternate code to execute if the expression in an if statement evaluates to False.

The indented code for the if statement is executed if the expression evaluates to True. The indented code immediately following the else is executed only if the expression evaluates to False. To mark the end of the else block, the code must be unindented to the same level as the starting if line.

# else Statement


test_value = 50


if test_value < 1:
  print("Value is < 1")
else:
  print("Value is >= 1")


test_string = "VALID"


if test_string == "NOT_VALID":
  print("String equals NOT_VALID")
else:
  print("String equals something else!")

Python Loops with range().

In Python, a for loop can be used to perform an action a specific number of times in a row.

The range() function can be used to create a list that can be used to specify the number of iterations in a for loop. 

# Print the numbers 0, 1, 2:
for i in range(3):
  print(i)


# Print "WARNING" 3 times:
for i in range(3):
  print("WARNING")

Infinite Loop

An infinite loop is a loop that never terminates. Infinite loops result when the conditions of the loop prevent it from terminating. This could be due to a typo in the conditional statement within the loop or incorrect logic. To interrupt a Python program that is running forever, press the Ctrl and C keys together on your keyboard.

Python while Loops

In Python, a while loop will repeatedly execute a code block as long as a condition evaluates to True.

The condition of a while loop is always checked first before the block of code runs. If the condition is not met initially, then the code block will never run.

# This loop will only run 1 time
hungry = True
while hungry:
  print("Time to eat!")
  hungry = False


# This loop will run 5 times
i = 1
while i < 6:
  print(i)
  i = i + 1

Python Nested Loops

In Python, loops can be nested inside other loops. Nested loops can be used to access items of lists which are inside other lists. The item selected from the outer loop can be used as the list for the inner loop to iterate over.

groups = [["Jobs", "Gates"], ["Newton", "Euclid"], ["Einstein", "Feynman"]]


# This outer loop will iterate over each list in the groups list
for group in groups:
  # This inner loop will go through each name in each list
  for name in group:
    print(name)

break Keyword

In a loop, the break keyword escapes the loop, regardless of the iteration number. Once break executes, the program will continue to execute after the loop.

In this example, the output would be:

  • 0
  • 254
  • 2
  • Negative number detected!
numbers = [0, 254, 2, -1, 3]


for num in numbers:
  if (num < 0):
    print("Negative number detected!")
    break
  print(num)
  
# 0
# 254
# 2
# Negative number detected!

The Python continue Keyword

In Python, the continue keyword is used inside a loop to skip the remaining code inside the loop code block and begin the next loop iteration.

big_number_list = [1, 2, -1, 4, -5, 5, 2, -9]


# Print only positive numbers:
for i in big_number_list:
  if i < 0:
    continue
  print(i)

or Operator

The Python or operator combines two Boolean expressions and evaluates to True if at least one of the expressions returns True. Otherwise, if both expressions are False, then the entire expression evaluates to False.

True or True      # Evaluates to True
True or False     # Evaluates to True
False or False    # Evaluates to False
1 < 2 or 3 < 1    # Evaluates to True
3 < 1 or 1 > 6    # Evaluates to False
1 == 1 or 1 < 2   # Evaluates to True

Comparison Operators

In Python, relational operators compare two values or expressions. The most common ones are:

  • < less than
  • > greater than
  • <= less than or equal to
  • >= greater than or equal too

If the relation is sound, then the entire expression will evaluate to True. If not, the expression evaluates to False.

a = 2
b = 3
a < b  # evaluates to True
a > b  # evaluates to False
a >= b # evaluates to False
a <= b # evaluates to True
a <= a # evaluates to True

and Operator

The Python and operator performs a Boolean comparison between two Boolean values, variables, or expressions. If both sides of the operator evaluate to True then the and operator returns True. If either side (or both sides) evaluates to False, then the and operator returns False. A non-Boolean value (or variable that stores a value) will always evaluate to True when used with the and operator.

True and True     # Evaluates to True
True and False    # Evaluates to False
False and False   # Evaluates to False
1 == 1 and 1 < 2  # Evaluates to True
1 < 2 and 3 < 1   # Evaluates to False
"Yes" and 100     # Evaluates to True

Equal Operator ==

The equal operator, ==, is used to compare two values, variables or expressions to determine if they are the same.

If the values being compared are the same, the operator returns True, otherwise it returns False.

The operator takes the data type into account when making the comparison, so a string value of "2" is not considered the same as a numeric value of 2.

# Equal operator


if 'Yes' == 'Yes':
  # evaluates to True
  print('They are equal')


if (2 > 1) == (5 < 10):
  # evaluates to True
  print('Both expressions give the same result')


c = '2'
d = 2


if c == d:
  print('They are equal')
else:
  print('They are not equal')

Not Equals Operator !=

The Python not equals operator, !=, is used to compare two values, variables or expressions to determine if they are NOT the same. If they are NOT the same, the operator returns True. If they are the same, then it returns False.

The operator takes the data type into account when making the comparison so a value of 10 would NOT be equal to the string value "10" and the operator would return True. If expressions are used, then they are evaluated to a value of True or False before the comparison is made by the operator. 

# Not Equals Operator


if "Yes" != "No":
  # evaluates to True
  print("They are NOT equal")


val1 = 10
val2 = 20


if val1 != val2:
  print("They are NOT equal")


if (10 > 1) != (10 > 1000):
  # True != False
  print("They are NOT equal")

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