Structure of a Loop

• Loop structures can be likened to a conditional statement
  • they run on a True or False set of values
  • the loop will continuously loop until the condition is True
  • the loop will terminate when the condition is False
• Loops can run for a desired length of time
  • or until a user-defined flag terminates it
• Loops are great for re-using code
  • limiting the number of statements that are required
  • re-uses the same conditional arguments for testing instead of hundreds

while Loop Statements (2)

• Initially x is 0 and the loop will increment x for each iteration
• This is repeated until x < 10

x = 0
while x < 10:
    print(x, end=" ")
    x += 1

while Loop with a Break Statement (1)

• break statements can be used to stop the loop if a condition is evaluated to True

x = 0
while x < 10:
    print(x, end=" ")
    if x == 5:
        break
    x += 1

while Loop with a Break Statement (2)

• Infinite loops can be constructed by using a True value after the while keyword
• Will continue incrementing x until it reaches a certain value
  • in this instance x must be equal to 10
• If there is no condition to check in the loop it will continue incrementing

x = 0
while True:
    print(x, end=" ")
    if x == 20:
        break
    x += 1

while Loop with Continue Statement

• continue statements can stop the current iteration and continue onto the next

x = 0
while x < 10:
    x += 1
    if x == 5:
        continue
    print(x, end=" ")

while Loop with an Else Statement

• else statements can be used to executee a block of code when a condition has been met
  • that is the condition is no longer True

x = 0
while x < 10:
    print(x, end=" ")
    x += 1
else:
    print("\n\nx is no longer less than 10")

Loop Structure (1)

• The structure of a for loop consists of the following:
  1. Initialisation of a counter
  2. Test the counter variable:
     1. less than: start < stop
     2. greater than: start > stop
  3. Update the counter variable

for variable in range(x, y, step):
    ...

Loop Structure (2)

• for loops can also be iterated forwards by using a positive step value

for x in range(1, 10, 1):
    print(x, end=" ")

Loop Structure (2)

• for loops can also be iterated backwards by using a negative step value

for x in range(10, 1, -1):
    print(x, end=" ")

Nested Loops

• These are loops that are located inside the body of another loop
  • consist of an inner (inside) and outer (outside) loops

for i in range(1, 4, 1):
    print(f"Iteration {i}: ")
    for j in range(1, 4, 1):
        print(i * j, end=" ")
    print("\n\n\n")

• The inner loop will go through all the repetitions for each repetition of the outside loop
  • the inner loop repetitions will complete sooner than the outside
• Nested loops are necessary for when a task performs a repetitive operation, and that task itself needs to be repeated

Looping Through Objects (1)

• for loops are great for looping through various objects:
  • i.e. strings, lists, tuples or dictionaries

Strings

• Strings are iterable, as they consist of a sequence of characters

title = "4061CEM"
for letter in title:
    print(letter, end=" ")

Looping Through Objects (2)

Lists

• Lists are also iterable, as such they can be looped through

module = [4061, "Programming and Algorithms", "Ian Cornelius"]
for item in module:
    print("\n\n", item)

Looping Through Objects (3)

Dictionaries (i)

• Dictionaries are also iterable, but can be looped through in a variety of ways

Items of a Dictionary

• Returns the key and value of each item in a dictionary

module = {"code": 4061,
          "title": "Programming and Algorithms",
          "leader": "Ian Cornelius"}
for key, value in module.items():
    print("\n\n", key, "=", value)

Looping Through Objects (4)

Dictionaries (ii)

By A Key of the Dictionary

• Returns the key of a dictionary, which can be used to access the item in a dictionary

module = {"code": 4061,
          "title": "Programming and Algorithms",
          "leader": "Ian Cornelius"}
for key in module.keys():
    print("\n\n", key, "=", module[key])

Looping Through Objects (5)

Dictionaries (iii)

Values of a Dictionary

• Returns all values in the dictionary, but not the key associated to it

module = {"code": 4061,
          "title": "Programming and Algorithms",
          "leader": "Ian Cornelius"}
for v in module.values():
    print("\n\n", v)

Looping Through Objects (6)

Dictionaries and Lists

module = {"code": 4061,
          "title": "Programming and Algorithms",
          "leader": "Ian Cornelius",
          "team": ["Terry Richards", "Daniel Goldsmith"]}
for key in module.keys():
    if type(module[key]) != list:
        print("\n\n", key, ":",  module[key])
    else:
        print("\n", key, ":", end=" ")
        for i in range(len(module[key])):
            print('\t', module[key][i], end=" ")

Infinite Loops

• Loops must have a way of terminating, otherwise the loop will continue to repeat until it is manually interrupted
• Infinite loops can occur when you forget to write code inside the loop to terminate the loop
  • i.e. there is no condition for a boolean expression to evaluate to False
• You must remember to include some condition to break out of a loop

x = 1
while x == 1:
    y = input("Enter a number: ")
    print(f"The number you entered is {y}!")

x = 1
while x == 1:
    y = input("Enter a number (Type 'q' to quit): ")
    if y == 'q':
        break
    print(f"The number you entered is {y}!")