Fundamental Programming Fundamental Programming Introduction to Functions

Fundamental Programming Review Aims Of Course  develop an understanding of what programs do and how they do it  develop logical and procedural thinking  develop familiarity with the C++ language  develop skill in software development  provide an introduction to programming languages – some fundamental concepts  develop analysis and design skills  provide an introduction to structured design  encourage good s/w engineering practices

Fundamental Programming Structure Revisited  so far we’ve covered a minimal level of C++ to get you started with simple programs  also, we’ve talked about structured designs – designs broken into chunks - a main program and one or more sub-programs  in C++, sub-programs are called functions – all of our C++ programs so far have a single function – the main function; all C++ programs have a main function  even our Hello World program had a main function…

Fundamental Programming Hello World Program  in C++, the Hello World program is: #include using namespace std; void main (void) { cout << "Hello World!"; }  this program has just one function – the main function

Fundamental Programming Introduction to Functions  functions will be the focus of our work over the next two weeks, and in Assignment 2  our strategy is to get these ideas out early and use tutes and labs to build up your comfort level  so, if the topics covered this week are a bit slippery – DON’T PANIC!

Fundamental Programming Structured Programs  perhaps the place to begin our examination of functions is to use a very simple example  lets take a look at a version of the Hello World program that uses two functions…

Fundamental Programming Structured Hello World Program #include using namespace std; void DisplayHelloWorld(void); void main(void) { DisplayHelloWorld(); } void DisplayHelloWorld(void) { cout << "Hello World!"; }

Fundamental Programming Hello World Program  in this design, the main function simply calls the DisplayHelloWorld function void main(void) { DisplayHelloWorld(); }  the DisplayHelloWorld function sends a message to the display and returns to the calling function – the main function  like this…

Fundamental Programming #include using namespace std; void DisplayHelloWorld(void); void main(void) { DisplayHelloWorld(); } void DisplayHelloWorld(void) { cout << "Hello World!"; } program starts in main function

Fundamental Programming #include using namespace std; void DisplayHelloWorld(void); void main(void) { DisplayHelloWorld(); } void DisplayHelloWorld(void) { cout << "Hello World!"; } => program starts in main function

Fundamental Programming #include using namespace std; void DisplayHelloWorld(void); void main(void) { DisplayHelloWorld(); } void DisplayHelloWorld(void) { cout << "Hello World!"; } =>

Fundamental Programming #include using namespace std; void DisplayHelloWorld(void); void main(void) { DisplayHelloWorld(); } void DisplayHelloWorld(void) { cout << "Hello World!"; } main function calls DisplayHelloWorld function =>

Fundamental Programming #include using namespace std; void DisplayHelloWorld(void); void main(void) { DisplayHelloWorld(); } void DisplayHelloWorld(void) { cout << "Hello World!"; } control passes to function DisplayHelloWorld =>

Fundamental Programming #include void DisplayHelloWorld(void); void main(void) { DisplayHelloWorld(); } void DisplayHelloWorld(void) { cout << "Hello World!"; } =>

Fundamental Programming #include using namespace std; void DisplayHelloWorld(void); void main(void) { DisplayHelloWorld(); } void DisplayHelloWorld(void) { cout << "Hello World!"; } DisplayHelloWorld function sends Hello World! message to display =>

Fundamental Programming Example dialog Hello World!

Fundamental Programming #include using namespace std; void DisplayHelloWorld(void); void main(void) { DisplayHelloWorld(); } void DisplayHelloWorld(void) { cout << "Hello World!"; } no more to do in DisplayHelloWorld - return to main function =>

Fundamental Programming #include using namespace std; void DisplayHelloWorld(void); void main(void) { DisplayHelloWorld(); } void DisplayHelloWorld(void) { cout << "Hello World!"; } control returns to main function… =>

Fundamental Programming #include using namespace std; void DisplayHelloWorld(void); void main(void) { DisplayHelloWorld(); } void DisplayHelloWorld(void) { cout << "Hello World!"; } no more to do in main function - program ends =>

Fundamental Programming Declarations  we saw previously that, in C++, we had to declare a variable before we use it  the same is true for functions…

Fundamental Programming #include using namespace std; void DisplayHelloWorld(void); void main(void) { DisplayHelloWorld(); } void DisplayHelloWorld(void) { cout << "Hello World!"; } DisplayHelloWorld function is declared here DisplayHelloWorld function is called here DisplayHelloWorld function is defined here

Fundamental Programming #include using namespace std; void DisplayHelloWorld(void); void main(void) { DisplayHelloWorld(); } void DisplayHelloWorld(void) { cout << "Hello World!"; } notice semicolon at end of this line – this is the end of the declaration notice no semicolon at end of this line – this is the start of the definition Jargon : we call the declaration of a function a function prototype

Fundamental Programming What do we mean by void ?  we’re ready to start talking about void … void DisplayHelloWorld(void)  the second void indicates that the function has no input variables or output variables  lets’ look at a function with some input and output variables…  the structured design used previously has a sub-program with one input variable and one output variable…

Fundamental Programming Pseudocode Example main write “Number of marks in exam ==> “ read NbrMarks get valid student mark set Percentage to 100 * StudentMark / NbrMarks write “ Student’s percentage: “ write Percentage if Percentage < 50 then write “ (Fail)” else write “ (Pass)” sub-program get valid student mark

Fundamental Programming Pseudocode Example get valid student mark input: NbrMarks set StudentMark to -1 while StudentMark NbrMarks write “Student’s mark ==> “ read StudentMark if StudentMark NbrMarks then write “ERROR: Enter a value between 0 and ” write NbrMarks write NewLine output: StudentMark sub-program uses the value of input variable NbrMarks to perform it’s task sub-program assigns a value to output variable StudentMark – it is used later in the program

Fundamental Programming Input-Process-Output  sub-programs can have one or more inputs and can produce one or more outputs - recall the input-process-output model  example: get valid student mark input process output NbrMarks StudentMark

Fundamental Programming Structure Chart main get valid student mark NbrMarks StudentMark

Fundamental Programming Function Inputs and Outputs  in C++, the get valid student mark sub- program looks like this…

Fundamental Programming void GetValidStudentMark(int NbrMarks, float &StudentMark) { StudentMark = -1; while ((StudentMark NbrMarks)) { cout "; cin >> StudentMark; if ((StudentMark NbrMarks)) { cout << "ERROR: Enter a value between 0 and "; cout << NbrMarks; cout << endl; } notes: * no spaces in function name

Fundamental Programming void GetValidStudentMark(int NbrMarks, float &StudentMark) { StudentMark = -1; while ((StudentMark NbrMarks)) { cout "; cin >> StudentMark; if ((StudentMark NbrMarks)) { cout << "ERROR: Enter a value between 0 and "; cout << NbrMarks; cout << endl; } notes: * no spaces in function name * input and output variables instead of void

Fundamental Programming void GetValidStudentMark(int NbrMarks, float &StudentMark) { StudentMark = -1; while ((StudentMark NbrMarks)) { cout "; cin >> StudentMark; if ((StudentMark NbrMarks)) { cout << "ERROR: Enter a value between 0 and "; cout << NbrMarks; cout << endl; } notes: * no spaces in function name * input and output variables instead of void * data types declared in function header

Fundamental Programming void GetValidStudentMark(int NbrMarks, float &StudentMark) { StudentMark = -1; while ((StudentMark NbrMarks)) { cout "; cin >> StudentMark; if ((StudentMark NbrMarks)) { cout << "ERROR: Enter a value between 0 and "; cout << NbrMarks; cout << endl; } notes: * no spaces in function name * input and output variables instead of void * data types declared in function header * output variable name prefixed by & Jargon : input and output variables are called parameters

Fundamental Programming Function Inputs and Outputs  in C++, the rest of the program looks like this…

Fundamental Programming #include void GetValidStudentMark(int NbrMarks, float &StudentMark); void main (void) { int NbrMarks = 0; float StudentMark = 0, Percentage = 0; cout "; cin >> NbrMarks; GetValidStudentMark(NbrMarks,StudentMark); Percentage = 100 * StudentMark / NbrMarks; cout << " Student's percentage: "; cout << Percentage; if (Percentage < 50) { cout << " (Fail)"; } else { cout << " (Pass)"; } notes: * declaration – header plus ”;”

Fundamental Programming #include void GetValidStudentMark(int NbrMarks, float &StudentMark); void main (void) { int NbrMarks = 0; float StudentMark = 0, Percentage = 0; cout "; cin >> NbrMarks; GetValidStudentMark(NbrMarks,StudentMark); Percentage = 100 * StudentMark / NbrMarks; cout << " Student's percentage: "; cout << Percentage; if (Percentage < 50) { cout << " (Fail)"; } else { cout << " (Pass)"; } notes: * declaration – as defined plus ; * no data types in function call

Fundamental Programming #include void GetValidStudentMark(int NbrMarks, float &StudentMark); void main(void) { int NbrMarks = 0; float StudentMark = 0, Percentage = 0; cout "; cin >> NbrMarks; GetValidStudentMark(NbrMarks,StudentMark); Percentage = 100 * StudentMark / NbrMarks; cout << " Student's percentage: "; cout << Percentage; if (Percentage < 50) { cout << " (Fail)"; } else { cout << " (Pass)"; } notes: * declaration – as defined plus ; * no data types in function call * order of variables in call must match order in declaration

Fundamental Programming program declarations main() function implementation code #include directives function declarations (prototypes)    function definitions (implementation code)    Function1 Function2 Function1 Function2 Structure of a C++ Program

Fundamental Programming Naming Functions  same as variable names…  start with a letter - A to Z  can contain upper or lower case letters, or numerals - 0 to 9  good idea to limit names to 32 characters  must avoid C++ reserved words like int, float, void, etc  more later…

Fundamental Programming void parameters  we now know that a void in parentheses after the function name means that the function has no input of output variables  what about the first void ?  we’ll talk about it later… void GetValidStudentMark(int NbrMarks, float &StudentMark); void DisplayHelloWorld(void)

Fundamental Programming Activity  a program is needed to encrypt personal data  a prime number is needed for the encryption algorithm  to get started, we will write a program to get a prime number from the user  ask the user to enter a prime number  if number entered is not prime, ask user if they wish to use the nearest prime number  make a start on this program now…

Fundamental Programming Example dialog Enter a prime for encryption key ==> is nearest prime – use it? [Y/N] ==> n Enter a prime for encryption key ==> is nearest prime – use it? [Y/N] ==> n Enter a prime for encryption key ==> 23

Fundamental Programming Activity  use the following functions in your design: GetNearestPrime - input: Number (integer) output: NearestPrime (integer)  note: nearest prime to 7 is 7; if the number entered is equal to it’s nearest prime – it’s a prime number  here is a starting point for this program, finish it…

Fundamental Programming #include void GetNearestPrime(int Number, int &NearestPrime); void main(void) { char Done = ‘N’; int Number = 0; :  other variables? while (Done == ‘N’) { cout "; cin >> Number; : } Activity

Fundamental Programming Activity Break

Fundamental Programming #include void GetNearestPrime(int Number, int &NearestPrime); void main(void) { char Done = ‘N’; int Number = 0, Prime = 0; while (Done == ‘N’) { cout "; cin >> Number; GetNearestPrime(Number, Prime); if (Number = = Prime) Done = ‘Y’; else { cout << Prime; cout “; cin >> Done; } note: name of variable in function call can differ from name in function Activity Sample Solution

Fundamental Programming Functions and Variables  let’s learn more about the relationship between variables and functions …

Fundamental Programming Functions and Variables  GetValidStudentMark uses only 2 variables – NbrMarks and StudentMark (the parameters) void GetValidStudentMark(int NbrMarks, float &StudentMark) { StudentMark = -1; while ((StudentMark NbrMarks)) { cout "; cin >> StudentMark; if ((StudentMark NbrMarks)) { cout << "ERROR: Enter a value between 0 and "; cout << NbrMarks; cout << endl; }

Fundamental Programming Functions and Variables  however, most functions will use addition variables to perform their task  another design for GetValidStudentMark is…

Fundamental Programming void GetValidStudentMark(int NbrMarks, float &StudentMark) { int InputMark = -1; while ((InputMark NbrMarks)) { cout "; cin >> InputMark ; if ((InputMark NbrMarks)) { cout << "ERROR: Enter a value between 0 and "; cout << NbrMarks; cout << endl; } StudentMark = InputMark; } Jargon : InputMark is called a local variable – it is local to the function; it cannot be used in main, or any other function

Fundamental Programming Local Variables  most functions use local variables  later we will learn about global variables – and why we avoid using them  as the name suggests, local variables are only available (or visible) locally - they cannot be used from outside the function  unauthorised use of a function’s local variables will result in a compilation error…

Fundamental Programming #include void SubFunction(void); void main(void) { int LocalToMain; SubFunction(); LocalToSub = 1; } void SubFunction(void) { int LocalToSub; LocalToMain = 1; } local variables declared in main and SubFunction compilation errors here…

Fundamental Programming Local Variables  unless authorised, a function’s local variables cannot be used by other functions  global variables do not provide this protection – they are visible everywhere  use of global variables is discouraged as they can make programs more difficult to debug…

Fundamental Programming Local vs Global Variables  programmers debug programs by tracing the value of variables as a program runs…  a variable assigned a value at the start of a function – a function that does not authorise sub-functions to use it - may be assumed to still hold that value at the end of the function  if it’s a global variable, this may not be the case – a sub-function may have changed it

Fundamental Programming void SomeFunction(void) { : GlobalVariable = InitialValue; : < functions called here, but no authority given to change GlobalVariable > : OutputValue = ; : } programmer may assume GlobalVariable still holds initial value here… but, it’s value may have been changed by one of the functions called here

Fundamental Programming void SomeFunction(void) { : GlobalVariable = InitialValue; : < functions called here, but no authority given to change GlobalVariable > : OutputValue = ; : } functions do not need authority to change global variables - dangerous! - can make programs harder to debug - do not use global variables after assignment 2

Fundamental Programming Local vs Global Variables  we will learn how global variables are declared later…  for now, lets’ explore this idea of authorising other functions to use local variables  how do you do it?  why would you want to?  how does it work?

Fundamental Programming Exposing Local Variables  the first two questions are easy to answer  how do you do it?  why would you want to?  we’ve already seen an example of how you do it…

Fundamental Programming #include void GetValidStudentMark(int NbrMarks, float &StudentMark); void main (void) { int NbrMarks = 0; float StudentMark = 0, Percentage = 0; cout "; cin >> NbrMarks; GetValidStudentMark(NbrMarks,StudentMark); Percentage = 100 * StudentMark / NbrMarks; cout << " Student's percentage: "; cout << Percentage; if (Percentage < 50) { cout << " (Fail)"; } else { cout << " (Pass)"; } notes: StudentMark is a local variable

Fundamental Programming #include void GetValidStudentMark(int NbrMarks, float &StudentMark); void main (void) { int NbrMarks = 0; float StudentMark = 0, Percentage = 0; cout "; cin >> NbrMarks; GetValidStudentMark(NbrMarks,StudentMark); Percentage = 100 * StudentMark / NbrMarks; cout << " Student's percentage: "; cout << Percentage; if (Percentage < 50) { cout << " (Fail)"; } else { cout << " (Pass)"; } notes: function GetValidStudentMark is called to obtain a valid student mark

Fundamental Programming #include void GetValidStudentMark(int NbrMarks, float &StudentMark); void main (void) { int NbrMarks = 0; float StudentMark = 0, Percentage = 0; cout "; cin >> NbrMarks; GetValidStudentMark(NbrMarks,StudentMark); Percentage = 100 * StudentMark / NbrMarks; cout << " Student's percentage: "; cout << Percentage; if (Percentage < 50) { cout << " (Fail)"; } else { cout << " (Pass)"; } notes: this mark is assigned to variable StudentMark – this is an output value produced by GetValidStudentMark

Fundamental Programming #include void GetValidStudentMark(int NbrMarks, float &StudentMark); void main (void) { int NbrMarks = 0; float StudentMark = 0, Percentage = 0; cout "; cin >> NbrMarks; GetValidStudentMark(NbrMarks,StudentMark); Percentage = 100 * StudentMark / NbrMarks; cout << " Student's percentage: "; cout << Percentage; if (Percentage < 50) { cout << " (Fail)"; } else { cout << " (Pass)"; } notes: so, main’s local variable called StudentMark is assigned a value by function GetValidStudentMark

Fundamental Programming #include void GetValidStudentMark(int NbrMarks, float &StudentMark); void main (void) { int NbrMarks = 0; float StudentMark = 0, Percentage = 0; cout "; cin >> NbrMarks; GetValidStudentMark(NbrMarks,StudentMark); Percentage = 100 * StudentMark / NbrMarks; cout << " Student's percentage: "; cout << Percentage; if (Percentage < 50) { cout << " (Fail)"; } else { cout << " (Pass)"; } conclusion: authority to change a local variable is granted whenever we call a function with an output variable

Fundamental Programming Exposing Local Variables  so, answers to our first two questions are:  how do you do it?  answer: call a sub-function with an output variable  why would you want to?  answer: to receive output values produced by sub-functions

Fundamental Programming Memory Management  an answer to our third question takes a bit more explaining - how does it work?  to answer this question, we need to explain how output variables work  later, we will explore the meaning of void in: void GetNearestPrime(int Number, int &NearestPrime); void GetYorNReply(char &Reply);

Fundamental Programming Functions - The Jargon #include void SubFunction(int LocalVariable); void main(void) { int LocalVariable = 1; SubFunction(LocalVariable); cout << "LocalVariable after call is: "; cout << LocalVariable; } void SubFunction(int LocalVariable) { LocalVariable = 2; } function declaration or prototype function parameter function call function header Prototype is a copy of Header with a semicolon