1 Topic 2- Functions Chapter 7 in the shrinkwrap Part 1 Announcements  Lab #3 - Functions Topic 2 - Ch 7 - Functions 1 of 162 Divide and Conquer  We break big complex problems into smaller comprehensible problems ● This is how we build big programs  WHY? ● Smaller problems are easier to manage and code ● Smaller code segments are easier to code & test (debug) ● If we can break it into small independent tasks then we can have many programmers each working on their own task Topic 2 - Ch 7 - Functions 2 of 16 Divide & Conquer  Break up large problems into smaller manageable problems Topic 2 - Ch 7 - Functions 3 Hard problem Easy subproblem Easy subproblem Hard subproblem Easy subproblem Easy subproblem of 163 Divide & Conquer Example Calculate a grade point average. Problem statement  We need an interactive program (user will input data) that calculates a students’ GPA given a series of letter grades. Input/Output description ● Input  Letter grades ● Output  Grade point average Algorithm development (set of steps, decomposition outline) 1. Read the letter grade 2. Convert it to a numerical grade 3. Calculate the grade point average 4. Output the grade point average Topic 2 - Ch 7 - Functions 4 Each of these can be thought of as separate tasks! of 16 A divide and conquer block diagram of our problem How can we further break this down? Divide & Conquer Example (2) Topic 2 - Ch 7 - Functions 5 Calculate the Grade Point Avg INPUT letterGrade OUTPUT gradePointAvg CALCULATE gradePointAvg of 164 How do we do this in C++?  Each separate task is called a component, module, subprogram ● Each of these are logical groupings of code  In C++ we call these functions  Each function has its own task ● It is a small program that we can use over and over again by calling it  We have been discussing functions all along ● e.g. toupper, ceil, fabs  Functions should accomplish one task ● This way we can reuse it ● Make it general Topic 2 - Ch 7 - Functions 6 These are provided. We want to create our own functions of 16 Example  Programs often have different tasks ● Input, validate input, get the grade pt value of a letter grade, calculate, gpa, etc..  We can split these up into different functions ● Each function processes its own task ● We call upon it as necessary Topic 2 - Ch 7 - Functions 7 Coding is similar – just have to separate them out into new functions and call them! of 165 The Main function int main ()  Up until now we have used the function main  All C++ programs must have this function  The operating system runs the program until main returns 0 Topic 2 - Ch 7 - Functions 8 of 16 Function basics  All Functions must have a return datatype ● This represents the datatype of the value it is returning ● For main we use int ● Some functions return some value ● Others do not  They still have to have a datatype Example ● we can have a function that calculates the square root of a number.  will return a float ● We can have a function that outputs a header (such as your name, date, class, etc.)  doesn’t return anything but needs a datatype none-the-less  Once we create these functions we can call them over and over again and don’t have to rewrite them every time we want to use them 9 of 166 Function Process For every function we need to.. 1 - Declare the function 2 - Define the function 3 - Use the function Topic 2 - Ch 7 - Functions 10 of 16 Using a Function theSquareRoot = sqrt(num1); Argument  sometimes we need to send data to the function, we use arguments to do this. Argument_List  a list of arguments that follows the function call Variable  if our function returns a value we need a place to store it. NOTE: This function returns a value so we need to either assign that value to a variable or cout it. e.g. cout << sqrt(num1); Topic 2 - Ch 7 - Functions 11 Function Name Argument Variable The parens are required of 167 How do we define a function?  Somewhere in statements you need to have a return statement.  The function terminates when the return statement is executed  The return statement returns the value of the code. Example int AddTwoInts(int num1, int num2) { return num1 + num2; } Topic 2 - Ch 7 - Functions 12 Syntax returnType functionName ( type parameterName...) { statements; return value; } NOTE: We have to let the compiler know what the types are of our arguments (incoming data) of 16 Declaring a function We can declare them Somewhere in our main source file (main.cpp for example)  before int main () We have to tell the compiler about the function We do this using a function prototype Example int AddTwoInts(int num1, int num2); Topic 2 - Ch 7 - Functions 13 Syntax returnType functionName (type parameterName...); Note: this is exactly like the first line of our function definition except the ; of 168 Why use a Prototype? We need the prototype to be first because the compiler reads from top to bottom 1. We can put it in the same file as our int main()  Before the int main() 2. We can put it in a separate header file ◘ We will cover this later 3. We can define the function before the int main() { } (rather then declaring the prototype) and then we won’t need the prototype ◘ This will require that our functions appear in a particular order making our code hard to maintain Topic 2 - Ch 7 - Functions 14 For now we will do this NEVER use this option 1 of 16 Calling our function Now we just need to call our function #include <iostream> using namespace std; int main () { int n1 int n2; int sum; cout << "Enter the first value to be summed: "; cin >> n1; cout << "Enter the second: "; cin >> n2; sum = AddTwoInts(n1, n2); cout << "\nThe sum is: " << sum << endl; return 0; } 15 NOTE: we could have put AddTwoInts here of 16 Prototype goes here9 Putting it all together #include <iostream> using namespace std; int AddTwoInts(int num1, int num2); int main () { int n1, n2, sum; cout << "Enter the first value: "; cin >> n1; cout << "Enter the second: "; cin >> n2; sum = AddTwoInts(n1, n2); cout << "\nThe sum is: " << sum << endl; return 0; } int AddTwoInts(int num1, int num2) { return num1 + num2; } 16 Lab #3  Modify this code of 16 Prototype Calling Function function call Function Definition } How do the values get into the