Part 3.1Subprograms in Python

Subprograms

Python, like many scripting syntaxes, has just one subprogram declaration format.

Often called a procedure, a function, or a subroutine, a subprogram is a division of code that is separated from the rest. A subprogram is a way of organising code to perform one particular function. For instance, (in an abstract example), a subprogram could be used to turn on a light bulb or to turn off a plug socket. When it is needed, it is run (called). The part that gives the subprogram a name and parameters is called the signature. Below is a simple diagram that explains the subprogram structure:

Defining a subprogram

Below is an example subprogram:

def main():
  print ("Hello world")
    

Subprogram can however, return a value. If a subprogram returns a value, the value that it gives back can be used elsewhere.

Returning a value

A subprogram that returns, or gives back, a value is known as a function, since it's actions mimic that of a mathematical function:

A ⇒ B

In this example, A is an input the arrow is the process and B is the output. For every input there is exactly one output.

A function returning a value looks like the following sample:

def main():
  return "Hello world"
    

With this, assigning the function call to a variable will give the variable the value "Hello world".

Calling a subprogram

A subprogram can be called through a very simple syntax. Arguments are also given to subprogram calls within the brackets - these line up with parameters specified in the subprogram signature.

def main():
  print ("Hello world")

main()
    

The previous sample will call the subprogram named main. It will also provide no arguments to the call.

A subprogram with parameters can be called by inserting values in the positions of those parameters with arguments:

def add(n1, n2):
  print (n1 + n2)

add(4, 7)
    

Recursion

Recursion is a big part of learning to program efficently and it is basically a subprogram calling itself over and over again until a termination condition (base case) is met. Recursion can be used to multiply two numbers by simply using addition.

def multiply(n1, n2, total):
  if n2 == 0:
    return n1
  else:
    total = multiply(n1, n2 - 1, total)

  return total + n1

print(multiply(4, 3, 0))
    

Here is what happens in this program:

• multiply(4, 3, 0) calls multiply (4, 2, 0)
• multiply(4, 2, 0) calls multiply (4, 1, 0)
• multiply(4, 1, 0) calls multiply (4, 0, 0)
• multiply (4, 0, 0) then returns 0 + 4 (4) to multiply(4, 1, 0)
• multiply (4, 1, 0) then returns 4 + 4 (8) to multiply(4, 2, 0)
• multiply (4, 2, 0) then returns 8 + 4 (12) to multiply(4, 3, 0)

Recursion is something that to understand, one must first understand recursion. For more information on recursion, look at recursion.