Parameter Passing (OCR A Level Computer Science)

Parameter Passing

What are Parameters?

• A parameter is a piece of data that is input into a code block such as a function or a procedure so that it can complete its task

• For example this could be a password that the user has entered which is passed into a code block as a parameter so it can check it meets password complexity rules

• When passing parameters into a code block, two standard methods are used:

  • Passing by value (also known as byVal) 

  • Passing by reference (also known as byRef) 

• The choice of which method to use depends on the specific requirements of the code block and how the programmer wants to handle data modifications inside and outside of this.

Parameter Passing by Value

• When a parameter is passed by value, a copy of the actual value is made and assigned to the function's parameter

• Any changes to the parameter within the function's scope do not affect the original value outside the function

• The function works with and changes its own local copy of the value

• This approach ensures that the original data remains unchanged, making it suitable when you want to protect the original data from being modified within the function

• When the function ends, the copy of the value is destroyed

• However, it can lead to performance overhead when working with large data structures since a copy is made

Pseudocode example

In this example pseudocode, a variable called number is assigned a random number between 1 and 10. This is then passed into the function printValue by value. 

number = random (1,10)
function printValue(number : byVal) // value is passed by value
  return("The random value is:", number )
endfunction
printValue(number)

Parameter Passing by Reference

• When a parameter is passed by reference, the function receives the memory address of the value rather than a copy of the value

• Any changes made to the parameter within the function directly affect the original value outside the function

• Both the function and the calling code share the same memory location

• This approach allows functions to directly modify the original data, making it efficient for working with large data structures

• However, it can also lead to unexpected side effects if not handled carefully, as changes made within the function affect other parts of the program

Pseudocode example

In this example pseudocode, a variable called number is assigned a random number between 1 and 10. This is then passed into the function printValue by reference. 

number = random (1,10)
function printValue(number : byRef) // value is passed by reference
  return("The random value is:", number )
endfunction
printValue(number) 

Comparison