Programming Techniques Flashcards
Define recursion
When a subroutine calls itself from within its own subroutine
State the characteristics of a recursive subroutine
- Contains a stopping condition - base case
- For any input value other than the stopping condition the subroutine should call itself
- The stopping condition should be reachable within a finite number of times
Why does a recursive subroutine need to have a stopping condition?
Otherwise it might call itself indefinitely resulting in a stack overflow
State an advantage of recursion
For certain problems they can be represented in fewer lines of code
- less prone to errors
What happens each time a function calls itself?
A new stack frame is created within the call stack
State a disadvantage of recursion
- Not very memory efficient: it uses stacks which takes up space in memory
- danger of stack overflow
- difficult to trace
What is stored in a call stack?
Parameters, local variables and return addresses
State examples of where recursion is useful
Tree Traversal algorithms
Fibonacci sequence
Why is a call stack needed?
To allow the subroutine to return to the point where it paused its execution
Why must a recursive subroutine reach its stopping condition within a finite number of times?
Otherwise it may call itself indefinitely, resulting in stack overflow
State the characteristics of a local variable
- Declared inside a subroutine
- Only accessible by that subroutine
- Created when the subroutine is called
- Destroyed when the subroutine ends
State the characteristics of a global variable
- Declared outside of any subroutine (at the top of the program)
- Accessible throughout the program
- Create when the program starts
- Destroyed when the program ends
When can two variables share the same name?
When they have different scopes
State a disadvantage of global variables
- Make programs hard to maintain, test and debug (considered poor programming practice)
- can be accidentally overwritten and edited
- require more memory
State an advantage of local variables
- Data cannot be accidentally altered
- ensures subroutines are self-contained
- no danger of variables being affected by code outside of the subroutine
Define modularity
The idea of breaking down a large program or problem into smaller chunks
What is the aim of modularity?
To design a program with modules that each carry out a single specific task
Define a procedure
A block of code that takes in one or more or no parameters and performs a set task
Define a function
A block of code that takes in one or more or no parameters, performs a set task and returns a value
What is included in the interface of a subroutine?
- The name of the subroutine
- The list of parameters it requires when called
- The order of those parameters
- The data type for those parameters
What is passing by value?
A copy of the parameter value is passed to the subroutine
- held in a separate memory location (only available to the subroutine)
- the original value is unaffected
What is passing by reference?
A pointer that contains the memory address of the original variable is created
- any change to the value will affect the value of the original variable
When a subroutine ‘unwinds’ , what does this mean?
It is the process of popping information off the call stack
What is a stack overflow?
When the call stack runs out of memory
Define scope
The section of code in which the variable is available/accessible
State a disadvantage of local variables
- do not enable data sharing
- limited scope
- destroyed once the subroutine ends
State an advantage of global variables
- enable data sharing
- useful for values that need to be used by multiple parts of the program
If a local variable exists in a subroutine with a global variable of the same name, what will happen?
The local variable will take precedence
