1. Subroutines in Computer Programming
Functions
Subroutines in Computer Programming
Telerik Software Academy
Learning & Development Team

2. Table of Contents Using Functions Functions with Parameters*
Table of Contents
Using Functions
What is a Function? Why to Use Functions?
Declaring and Creating Functions
Calling Functions
Functions with Parameters
Passing Parameters
Returning Values
Best Practices
(c) 2007 National Academy for Software Development - All rights reserved. Unauthorized copying or re-distribution is strictly prohibited.*

3. What is a Function?
A Function is a kind of building block that solves a small problem
A piece of code that has a name and can be called from the other code
Can take parameters and return a value
Functions allow programmers to construct large programs from simple pieces
Functions are also known as methods, procedures, and subroutines

4. Why to Use Functions? More manageable programming*
Why to Use Functions?
More manageable programming
Split large problems into small pieces
Better organization of the program
Improve code readability
Improve code understandability
Avoiding repeating code
Improve code maintainability
Code reusability
Using existing Functions several times
(c) 2007 National Academy for Software Development - All rights reserved. Unauthorized copying or re-distribution is strictly prohibited.*

5. *
Defining Functions
(c) 2007 National Academy for Software Development - All rights reserved. Unauthorized copying or re-distribution is strictly prohibited.*

6. Defining and Creating Functions
return_type function_name( parameter list ) {
body of the function }
Each Function has a name
It is used to call the Function
Describes its purpose

7. Defining and Creating Functions (2)
int max(int num1, int num2) {
int result = min2;
if (num1 > num2) result = num1;
return result; }
Put return_type void when the Function does not return any result

8. Defining and Creating Functions (3)
int max(int num1, int num2) {
int result = min2;
if (num1 > num2) result = num1;
return result; }
Function body
Each Function has a body
It contains the programming code
Surrounded by { and }

9. *
Declaring Functions
(c) 2007 National Academy for Software Development - All rights reserved. Unauthorized copying or re-distribution is strictly prohibited.*

10. Declaring Functions Declaring function has these parts Example:
You can also use:
Declaring is used when you define a function in one file and call it in another
return_type function_name( parameter list );
int max(int num1, int num2);
int max(int, int);

11. Calling Functions

12. Calling Functions To call a Function, simply use:
The Function’s name
Parentheses (don’t forget them!)
A semicolon (;)
This will execute the code in the Function’s body
int a = 5;
Int b = 6;
int ret = max(a, b);

13. Calling Functions (2) A Function can be called from:
The Main() Function
Any other Function
Itself (process known as recursion)
int main () {
// ...
int ret = max(a, b); }

14. Declaring and Calling Functions*
Declaring and Calling Functions
Live Demo
(c) 2007 National Academy for Software Development - All rights reserved. Unauthorized copying or re-distribution is strictly prohibited.*

15. Functions with Parameters*
Functions with Parameters
Passing Parameters and Returning Values
(c) 2007 National Academy for Software Development - All rights reserved. Unauthorized copying or re-distribution is strictly prohibited.*

16. Function Parameters
To pass information to a Function, you can use parameters (also known as arguments)
You can pass zero or several input values
You can pass values of different types
Each parameter has name and type
Parameters are assigned to particular values when the Function is called
Parameters can change the Function behavior depending on the passed values

17. Defining and Using Function Parameters
Function’s behavior depends on
its parameters
Parameters can be of any type
Call by value - copies value
Call by pointer - copies memory address
Call by reference - copies reference
Call by pointer and by reference change the passed argument into the function scope

18. Defining and Using Function Parameters (2)
Functions can have as many parameters as needed:
The following syntax is not valid:
int max(int num1, int num2) {
int result = min2;
if (num1 > num2) result = num1;
return result; }
int max(int num1, num2)

19. Calling Functions with Parameters
To call a Function and pass values to its parameters:
Use the Function’s name, followed by a list of expressions for each parameter
Examples:
PrintSign(-5);
PrintSign(balance);
PrintSign(2+3);
PrintMax(100, 200);
PrintMax(oldQuantity * 1.5, quantity * 2);

20. Calling Functions with Parameters (2)
Expressions must be of the same type as Function’s parameters (or compatible)
If the Function requires a long expression, you can pass int instead
Use the same order like in Function declaration
For Functions with no parameters do not forget the parentheses

21. Using Functions With Parameters*
Using Functions With Parameters
Examples
(c) 2007 National Academy for Software Development - All rights reserved. Unauthorized copying or re-distribution is strictly prohibited.*

22. Months – Example
Display the period between two months in a user-friendly way
#include <iostream>
using namespace std;
void SayMonth(int month) {
string monthNames[] =
"January", "February", "March", "April",
"May", "June", "July", "August",
"September", "October", "November", "December" };
cout << monthNames[month-1]; }
(the example continues)

23. Months – Example (2) void SayPeriod(int start, int end) {
int period = end - start;
if (period < 0)
period += 12; }
cout << "There are " << period << " months between " << SayMonth(start) << " and " << SayMonth(end);

24. *
Months
Live Demo
(c) 2007 National Academy for Software Development - All rights reserved. Unauthorized copying or re-distribution is strictly prohibited.*

25. Printing Triangle – Example
Creating a program for printing triangles as shown below: 1
1 1 2
n=5  n=6 

26. Printing Triangle – Example
void printLine(int start, int end) {
for(int i = start; i <= end; i++)
cout << i << " "; }
cout << endl;
int main()
int n;
cin >> n;
for(int i = 1; i <= n; i++) printLine(1, i);
for(int i = n - 1; i >= 1; i--) printLine(1, i);

27. Printing Triangle
Live Demo

28. Optional Parameters
C++ supports default values assigned at their declaration:
The above Function can be called in several ways:
int sum(int a = 10, int b = 20) {
int result;
result = a + b;
return (result); }
sum(5, 10);
sum(15);
sum();

29. Optional Parameters
Live Demo

30. Returning Values From Functions*
Returning Values From Functions
(c) 2007 National Academy for Software Development - All rights reserved. Unauthorized copying or re-distribution is strictly prohibited.*

31. Returning Values From Functions
A Function can return a value to its caller
Returned value:
Can be assigned to a variable:
Can be used in expressions:
Can be passed to another Function:
int a = max(b, c);
int price = getPrice() * q * 1.20;
int age = max(getValue());

32. Defining Functions That Return a Value
Instead of void, specify the type of data to return
Functions can return any type of data (int, string, array, etc.)
void Functions do not return anything
The combination of Function's name and parameters is called Function signature
Use return keyword to return a result

33. The return Statement The return statement:
Immediately terminates Function’s execution
Returns specified expression to the caller
Example:
To terminate void Function, use just:
Return can be used several times in a Function body
return -1;
return;

34. Returning Values From Functions*
Returning Values From Functions
Examples
(c) 2007 National Academy for Software Development - All rights reserved. Unauthorized copying or re-distribution is strictly prohibited.*

35. Positive Numbers – Example
Check if all numbers in a sequence are positive:
bool arePositive(int numbers[]) {
for(int i = 0; i < sizeof(numbers); i++)
if(numbers[i] < 0)
return false; }
return true;

36. Positive Numbers Live Demo *
(c) 2007 National Academy for Software Development - All rights reserved. Unauthorized copying or re-distribution is strictly prohibited.*

37. Overloading Functions
Multiple Functions with the Same Name

38. Overloading Functions
What means "to overload a Function name"?
Use the same Function name for multiple Functions with different signature (parameters)
int print(int a) {
return a; }
int print(int a, int b)
return a + b;
int print(int a, int b, int c)
return a + b + c;

39. Functions – Best Practices
Each Function should perform a single, well-defined task
Function’s name should describe that task in a clear and non-ambiguous way
Good examples: calculatePrice, readName
Bad examples: f, g1, Process
In C# Functions should start with capital letter
Avoid Functions longer than one screen
Split them to several shorter Functions

40. *
Summary
Break large programs into simple Functions that solve small sub-problems
Functions consist of declaration and body
Functions are invoked by their name
Functions can accept parameters
Parameters take actual values when calling a Function
Functions can return a value or nothing
(c) 2007 National Academy for Software Development - All rights reserved. Unauthorized copying or re-distribution is strictly prohibited.*

41. Functions

42. *
Exercises
Write a Function that asks the user for his name and prints “Hello, <name>” (for example, “Hello, Peter!”). Write a program to test this Function.
Write a Function getMax() with two parameters that returns the bigger of two integers. Write a program that reads 3 integers from the console and prints the biggest of them using the Function getMax().
Write a Function that returns the last digit of given integer as an English word. Examples: 512  "two",  "four",  "nine".
(c) 2007 National Academy for Software Development - All rights reserved. Unauthorized copying or re-distribution is strictly prohibited.*

43. Exercises (2)
Write a Function that counts how many times given number appears in given array. Write a test program to check if the Function is working correctly.
Write a Function that checks if the element at given position in given array of integers is bigger than its two neighbors (when such exist).
Write a Function that returns the index of the first element in array that is bigger than its neighbors, or -1, if there’s no such element.
Use the Function from the previous exercise.

44. *
Exercises (3)
Write a Function that reverses the digits of given decimal number. Example: 256  652
Write a Function that adds two positive integer numbers represented as arrays of digits (each array element arr[i] contains a digit; the last digit is kept in arr[0]). Each of the numbers that will be added could have up to digits.
Write a Function that return the maximal element in a portion of array of integers starting at given index. Using it write another Function that sorts an array in ascending / descending order.
(c) 2007 National Academy for Software Development - All rights reserved. Unauthorized copying or re-distribution is strictly prohibited.*

45. Exercises (4)
Write a program to calculate n! for each n in the range [1..100]. Hint: Implement first a Function that multiplies a number represented as array of digits by given integer number.
Write a Function that adds two polynomials. Represent them as arrays of their coefficients as in the example below:
x2 + 5 = 1x2 + 0x + 5 
Extend the program to support also subtraction and multiplication of polynomials. 5 1

46. Exercises (5) Write a program that can solve these tasks:*
Exercises (5)
Write a program that can solve these tasks:
Reverses the digits of a number
Calculates the average of a sequence of integers
Solves a linear equation a * x + b = 0
Create appropriate Functions.
Provide a simple text-based menu for the user to choose which task to solve.
Validate the input data:
The decimal number should be non-negative
The sequence should not be empty
a should not be equal to 0
(c) 2007 National Academy for Software Development - All rights reserved. Unauthorized copying or re-distribution is strictly prohibited.*

47. *
Exercises (6)
Write Functions to calculate minimum, maximum, average, sum and product of given set of integer numbers. Use variable number of arguments.
* Modify your last program and try to make it work for any number type, not just integer (e.g. decimal, float, byte, etc.). Use generic Function (read in Internet about generic Functions in C#).
(c) 2007 National Academy for Software Development - All rights reserved. Unauthorized copying or re-distribution is strictly prohibited.*

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