If the same piece of code needs to be used several times we can use a loop – but only if those times are all together. If you need to run the same bit of code again later, you can use a procedure.

In this example I want to print the squares of the numbers from 1 to 20: #This is a procedure called output #It will print a given number and its square def output(value): print (value,"squared =",value*value) #This is the start of the main program number = 1 while number < 21: output(number) number = number + 1

Here the top two lines are a procedure called output. def output(value): print (value,"squared =",value*value) number = 1 while number < 21: output(number) number = number + 1 The procedure expects to be given one variable (which it calls value) and will happily do its thing when told to. The main program is underneath, and the line that says output(number) will run the output procedure and give it the variable number to work with.

A parameter is the variable (or variables) that get passed to the procedure. You can require more than one parameter: Try the following; Comment on each line of code def output(number1,number2): print (number1,"+",number2,"=",number1+number2) for counter in range(20): number = counter**2 output(counter,number) You can pass as many parameters as you like to a procedure as long as you set it up correctly.

Once you have defined a procedure, you can call it in a number of ways: Try The following and comment on each line; def output(grade,score=50,feedback="Well done!"): print ("You scored",score,"which is a grade",grade,feedback) output("C") output("A",87) output("E",12,"Rubbish!") You can pass as many (or few) of the parameters as you like - as long as they’re in the right order. This means you have to plan the procedure quite carefully.

So far our procedures have just printed an answer to the console. Sometimes a procedure will have to send some data back. return This is called a return. Procedures that return something are called functions. In Python that isn’t a huge distinction, but in some programming languages it is very important.

Try the following and add comments on each line; def calculate(number): newnumber = number / return (newnumber) for counter in range(5,101,5): answer = calculate(counter) print (counter,"% = ",answer) The line return(newnumber) passes the value of the newnumber variable back to the main program. The line y = calculate(x) shows that the main program expects a value to be returned and has somewhere to store it.

What happens if you change the line: newnumber = number/100.0 to newnumber = number/100.0 Try it

You are going to write a program that will give the user the option to perform different string manipulation tasks. To start, the options should be: 1.Find the length of the string 2.Print only the first letter of the string 3.Convert it to upper case 4.Print each character of the string on a separate line 5.Reverse the string Before starting the code, we need to plan. How could you decompose this problem? Write a solution in pseudocode.

There are some procedures already built in to Python (print, for example). Modules (sometimes known as libraries in other languages) are collections of extra procedures and functions that are pre- written. Try this program first. number = 49 answer = sqrt(49) print answer It won’t work, but I’m trying to prove a point (sqrt is short for square root):

Now try the below import math number = 49 answer = math.sqrt(49) print answer Comments on each line explaining what the code is doing

Open your program Swap seat with a partner Look through your partner program Think of three things you can do to the program to make it stop working Change or edit your partner code so that the whole program does not work