In a programming language, parameters are variables that are used to pass information into or out of a method (or a function or procedure). There are 2 main ways to pass parameters: by value and by reference.
Pass by value
When a parameter is passed by value, the value of the parameter is copied into the method. If the value of the parameter is changed in the method, the change will not be reflected in the value of the original parameter.
For istance, the following pseudocode passes an integer by value to a method:
proc foo(by val number)
number = 10
endproc
number = 5
foo(number)
print(number) # the output is 5
In the above pseudocode, the foo() method is called with the value 5 as an argument. The value 5 is copied into the number variable inside the method. The number variable is then set to the value 10. However, the change in the value of number is not reflected in the value of the original number. When the foo() method terminates, the value of number is still 5.
Pass by reference
When a parameter is passed by reference, the reference to the parameter is copied into the method. If the value of the parameter is changed in the method, the change will be reflected in the value of the original parameter.
For example, the following pseudocode passes an object by reference to a method:
proc foo(by ref person)
person.name = "Mark"
endproc
person = Person("Luke")
foo(person)
print(person.name) # Prints Mark
In the above pseudocode, the foo() method is called with the object person as an argument. The reference to the object person is copied into the person variable inside the method. The person variable is then used to change the name of the object. The change in the name of the object is reflected in the original object. When the foo() method terminates, the name of the object is still Mark.
The type of parameter passing to use depends on the type of parameter and the operation that needs to be performed. If you need to change the value of the parameter in the method, you need to use pass by reference. If you do not need to change the value of the parameter in the method, you can use pass by value.