310201 Fundamental Programming

310201Fundamental Programming Introduction to Arrays

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…

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

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...

Programs Number of marks in exam ==> 50 Student's mark ==> 30 Student's percentage: 60

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

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]

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

Arrays • to display the first three values in Marks, we can write (in pseudocode): writeMarks[0] writeMarks[1] writeMarks[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];

Arrays • we assign values to an array using the same notation: readMarks[0] (read value and assign as first in Marks) setMarks[5] =0(assign sixth value in Marks to 0) • no surprises here in C++: cin >> Marks[0]; Marks[5] = 0;

Arrays • the power of arrays comes from the ability to refer to values stored in an array using a variable ( display first mark ) setMarkNbr=0 write Marks[MarkNbr] ( display second mark ) setMarkNbr=1 write Marks[MarkNbr]

Activity • if Marks holds the values shown on the right, what output will the following design produce? setNbrMarks=3 setMarkNbr=0 whileMarkNbr < NbrMarks write”Mark number “, MarkNbr, ” = “ write Marks[MarkNbr] writeNewLine setMarkNbr=MarkNbr +1 ( end of loop over marks ) Marks [0] [1] [2] 17 29 8

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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 Marks 17 29 8 [1] [2] [0]

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?...

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

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++

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]

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]

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++) :

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!

Debugging • to debug looping logic effectively, you need to know how to set watches and to step through a loop

More on Initialising Arrays • if the array size is omitted when it is declared, the compiler canautomatically 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 Referencing Arrays • 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]

A Common Error Using Arrays • 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...

Activity • 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];

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Activity Feedback • 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];

A Common Error Using Arrays • C++ does notgive 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 • if you assign a value to an array elementthat 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

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

310201 Fundamental Programming