Chapter 56 - Subroutines and recursion
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What is a subroutine?
» A name block of code which performs a specific task within a program
What are the 2 types of subroutines?
» Functions » Procedures
What is the difference between a function and a procedure?
» The function returns a value but the procedure doesn’t » if there are values already in the subroutine, you use a procedure
How can you call the subroutine from the main program and output it to the user?
» Simply write the subroutine name
What does the first line of a subroutine specify?
» The interface for that subroutine
What is modularity?
» Concept of breaking a large program or problem down into smaller chuncks
What is the benefit of modularity?
» Each module carries out a single specific task
What does a procedure do?
» Performs a set task and can take in a zero, 1 or 2 inputs
What happens when a parameter is passed by a value?
» The value created for the subroutine being called is a copy of the original » Held in a separate memory location once passed » It is now a local variable
What happens if you change the parameter inside the subroutine when you pass a value?
» It will not affect its value outside the subroutine
What does it mean if a parameter is passed by a reference?
» The address of the parameter is passed to the subroutine » A pointer that contains the memory address of the original variable is created
What happens if you change the value in the parameter when passing by a reference?
» A change to this value within the called subroutine will also affect the value outside the subroutine » It becomes a global variable
What does it mean if you say a scope of a variable?
» Refers to where a variable is visible or accessible from inside the main program
What is a global variable?
» A variable which is defined in the main program, outside any functions » Has a global scope
What are some disadvantages of using a global variable?
» Needs to be preserved in memory all the time, this means memory space is kept throughout the running of the program » Makes it less memory efficient
Why do we avoid using global variables?
» Increases complexity of the program - as they can be used by many difference parts of the program » Make them harder to keep track of its value » If you want to re-use code, you have to track and remember the value of the global variable
Why do we used global variables?
» To use a variable which can be accessed from any parts of a program » When a value needs to stay the same throughout the program
What is a local variable?
» A variable which is defined in the procedure or function, and can be used only inside that function
What are the advantages of using a local variable?
» Local variables in difference subroutines can have the same identifier » Changing the value of a local variable in a subroutine, will not affect its value outside the subroutine
What are the disadvantages of using a local variable?
» Can overwrite global variables with the same name » Only exists during the execution of the program
What the principle of encapsulation?
» The subroutine written according to this principle can be tested independently, and used in many different programs without the programmer needing to know what variable it uses
What are the benefits of programming with subroutines?
» Relatively easy to understand and debug » Can be tested independently » It can be re-used » Can use a modular approach
When is a subroutine recursive?
» If its defined in terms of tiself
What is recursion?
» When a subroutine calls itself from within its own subroutine