Coding flow

Summary

Objectives

By the end of this session you should be able to:

• Explain what control flow is, and how flow can be manipulated using:
  • For and While Loops
  • If statements
• Demonstrate common comparisons that can be used in control flow statements

Key Points

• Control Flow comprises of the rules that tell a computer to do things conditionally and repetitively
• In code, programmers create:
  • Branching using if statements
  • Repetition using for and while loops
• We use comparisons to make decisions in Control Flow

Breakdown

Control Flow

Control flow is the order in which individual instructions in a program are executed, and it can be visualised as a flowchart. We can manipulate control flow to run instructions repeatedly (more than one time) and conditionally (if certain criteria are met). By default, most computer programs run instructions one line at a time, starting at the top.

Comparisons

Comparisons are statements that evaluate to either True or False, and rely on comparison operators. Comparison operators can be placed between two values to form a statement - the program then evaluates whether that statement is true or not. Common comparison operators include:

• ==, or "equal". 7 == 7 would evaluate to True, because both values are equal.
• !=, or "not equal". 7 != 7 would evaluate to False, because both values are not not equal.
• < and >, or "less than" and "more than". 7 > 7 would evaluate to False, because 7 is not more than 7.
• <= and >=, or "less than or equal to" and "more than or equal to". 7 >= 7 would evaluate to True, because 7 is more than or equal to 7.

Branching

An if statement allows us to use comparisons as conditional "gates"; if the comparison evaluates to True then the associated instructions are executed, if it evaluates to False then it does not. If statements can also have "else if" and "else" clauses, which give further branch options; these are optional. Consider this example:

doorbell = input("Who is it?")
if doorbell == "Peter":
  print("Welcome home!")
elif doorbell == "Paul":
  print("Paul, you've got the wrong door again. Your house is next door.")
else:
  print("Welcome, visitor!")

From this code we can see that Peter, who owns this house, has written a little script to welcome him home - and to welcome visitors, too. However, if Paul (his neighbour) rings the doorbell it will politely remind him that he has the wrong house.

Loops

Loops are a way to run a block of code many times. Two common loops are the while loop and the for loop.

While loops

While loops run the block of code while a condition is true. Consider this example, which will run infinitely (since True will always evaluate to True):

while True:
  print("This will run forever")

More commonly, while loops have comparison statements that eventually evaluate to false:

while bottles != 0:
  print(bottles + "green bottles, hanging on the wall")
  bottles = bottles - 1

For loops

For loops iterate through every item in a range, such as a list. This is possible in a while loop, but for loops are more suited to some tasks:

bottles = ["Ten", "Nine", "Eight", "Seven", "Six", "Five", "Four", "Three", "Two", "One"]
for bottle in bottles:
  print(bottle + "green bottles, hanging on the wall")