1. 310201: Fundamental Programming 310201 Fundamental Programming Introduction to Arrays

2. 310201: Fundamental Programming Arrays in this class we introduce the last substantial topic in the course the idea is quite simple – to begin, let’s go back to the beginning…

3. 310201: Fundamental Programming Variables write “Number of marks in exam ==> “ read NbrMarks write “Student’s mark ==> “ read StudentMark set Percentage to 100 * StudentMark / NbrMarks write “ Student’s percentage: “ write Percentage this design uses 3 variables: NbrMarks, StudentMark, Percentage

4. 310201: Fundamental Programming Variables write “Number of marks in exam ==> “ read NbrMarks write “Student’s mark ==> “ read StudentMark set Percentage to 100 * StudentMark / NbrMarks write “ Student’s percentage: “ write Percentage this design uses 3 variables: NbrMarks, StudentMark, Percentage

5. 310201: Fundamental Programming Variables write “Number of marks in exam ==> “ read NbrMarks write “Student’s mark ==> “ read StudentMark set Percentage to 100 * StudentMark / NbrMarks write “ Student’s percentage: “ write Percentage this design uses 3 variables: NbrMarks, StudentMark, Percentage

6. 310201: Fundamental Programming Variables each variable can hold a single value each variable is associated with a single memory location after running this program with the following inputs...

7. 310201: Fundamental Programming Programs Number of marks in exam ==> 50 Student's mark ==> 30 Student's percentage: 60

8. 310201: Fundamental Programming Variables after running this program with the previous inputs... the memory locations used by this program hold the following values... NbrMarks: 50 StudentMark: 30 Percentage: 60 each variable is associated with one memory location and can hold one value

9. 310201: Fundamental Programming Arrays an array is a variable that can hold more than one value An array is a series of elements of the same type placed in contiguous memory locations that can be individually referenced by adding an index to a unique identifier. an array called Marks might hold the collection of marks from an exam we refer to values in an array using square brackets and a position number of the value of interest: Marks[1] Marks[17] Marks[123]

10. 310201: Fundamental Programming Arrays in some languages, the first value in an array is at position 1 however, in C++, the first value in an array is at position 0 in pseudocode examples, we will adopt this 0-based convention Marks[0]  first value held in Marks Marks[1]  second value held in Marks Marks[2]  third value held in Marks

11. 310201: Fundamental Programming Arrays to display the first three values in Marks, we can write (in pseudocode): write Marks[0] write Marks[1] write Marks[2] we’ll look at arrays in C++ in detail later – to output array values we write: cout << Marks[0]; cout << Marks[1]; cout << Marks[2];

12. 310201: Fundamental Programming Arrays we assign values to an array using the same notation: read Marks[0] (read value and assign as first in Marks) set Marks[5] = 0 (assign sixth value in Marks to 0) no surprises here in C++: cin >> Marks[0]; Marks[5] = 0;

13. 310201: Fundamental Programming Arrays the power of arrays comes from the ability to refer to values stored in an array using a variable ( display first mark ) set MarkNbr = 0 write Marks[MarkNbr] ( display second mark ) set MarkNbr = 1 write Marks[MarkNbr]

14. 310201: Fundamental Programming Activity if Marks holds the values shown on the right, what output will the following design produce? set NbrMarks = 3 set MarkNbr = 0 while MarkNbr < NbrMarks write ”Mark number “, MarkNbr, ” = “ write Marks[MarkNbr] write NewLine set MarkNbr = MarkNbr + 1 ( end of loop over marks ) Marks [0] [1] [2] 17 29 8

15. 310201: Fundamental Programming Activity Break

16. 310201: Fundamental Programming Activity Feedback if Marks holds the values shown on the right, this design will produce the following output? Mark number 0 = 17 Mark number 1 = 29 Mark number 2 = 8 Marks [0] [1] [2] 17 29 8 17298 Marks [0] [1][2]

17. 310201: Fundamental Programming Array References below, a variable is used to refer to the values of interest in an array read Mark[StudentNbr] set Percentages[StudentNbr] to Percentage a useful convenience is that we can use an expression to refer to the values of interest in an array set Mark[StudentNbr] to Mark[StudentNbr + 1] why is this useful?...

18. 310201: Fundamental Programming Array References Because we can now do something like for (i=0; i<5; i++) { cout << “Mark for next student? “; cin >> mark[i] }

19. 310201: Fundamental Programming Implementing Arrays in C++ let’s have a look at the basics of using arrays in C++ - we’ll look at: –declaring arrays –initialising arrays we’ve already seen examples of how arrays are used in C++, more follow… in the next class we’ll learn some more details about using arrays in C++

20. 310201: Fundamental Programming Declaring Arrays in C++ no surprises in declaring arrays – below we declare an array to hold 10 integers: int Results[10]; as mentioned, our ten integers are held in positions 0 to 9: Results[0] Results[1] Results[2] : Results[9]

21. 310201: Fundamental Programming Declaring Arrays in C++ a constant is sometimes used to hold the number of values in an array const int NBR_RESULTS = 10; int Results[NBR_RESULTS]; a constant is useful when looping, and also makes it easy to change the array size: for (ResultNbr = 0; ResultNbr < NBR_RESULTS; ResultNbr++) cout << “Result[“ << ResultNbr << “]: “ Results[ResultNbr]

22. 310201: Fundamental Programming Declaring Arrays in C++ take care with loop conditions – one common form is: for (ResultNbr = 0; ResultNbr < NBR_RESULTS; ResultNbr++) : an alternative form is: for (ResultNbr = 0; ResultNbr <= NBR_RESULTS - 1; ResultNbr++) :

23. 310201: Fundamental Programming Initialising Arrays in C++ to initialise values in an array, enclose the initial values in braces: const NBR_RESULTS = 3; int Results[NBR_RESULTS] = {0,1,2}; you can initialise all values in an array of integers to zero using: int Results[NBR_RESULTS] = {0}; the default initial value for an array of integers is zero – if you do not provide enough initial values, others are set to zero!

24. 310201: Fundamental Programming Debugging to debug looping logic effectively, you need to know how to set watches and to step through a loop

25. 310201: Fundamental Programming if the array size is omitted when it is declared, the compiler can automatically determine the size from initial values int Primes[] = {1, 3, 5, 7, 11}; is equivalent to int Primes[5] = {1, 3, 5, 7, 11}; More on Initialising Arrays

26. 310201: Fundamental Programming we normally use an integer literal, or an integer variable, to refer to a value of interest in an array Results[0] Results[ResultNbr] also, an integer expression can be used Results[CurrentPos - 1] More on Referencing Arrays

27. 310201: Fundamental Programming a common error when using arrays is to refer to an element that does not exist that is: to use an index value that is outside the acceptable range... A Common Error Using Arrays

28. 310201: Fundamental Programming can you see how this code refers to an array element that does not exist? const NBR_RESULTS = 3; int Results[NBR_RESULTS] = {0}; for (int ResultNbr = 0; ResultNbr <= NBR_RESULTS; ResultNbr++) cout << Results[ResultNbr]; Activity

29. 310201: Fundamental Programming Activity Break

30. 310201: Fundamental Programming in the last trip through the loop below, the code refers to Results[3], which does not exist const NBR_RESULTS = 3; int Results[NBR_RESULTS] = {0}; for (int ResultNbr = 0; ResultNbr <= NBR_RESULTS; ResultNbr++) cout << Results[ResultNbr]; Activity Feedback

31. 310201: Fundamental Programming C++ does not give an error when using an index value that is out-of-range it is the programmer’s responsibility to ensure that array references are always within the acceptable range if, in an expression, you refer to an array element that does not exist: results are unpredictable – the program may give erroneous output, or go into an infinite loop A Common Error Using Arrays

32. 310201: Fundamental Programming if you assign a value to an array element that does not exists: with luck, you’ll get a memory violation error – so you know there’s a problem without luck, results are unpredictable: erroneous output produced infinite loops lock-up the program memory used by the operating system is overwritten causing a system crash A Common Error Using Arrays

33. 310201: Fundamental Programming Summary simple variables hold a single value arrays hold a collection of related values first value in a C++ array is at position 0 arrays can be defined for any of the basic data types – char, int, double, float