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Copyright © 2002 Pearson Education, Inc. Slide 1.

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1 Copyright © 2002 Pearson Education, Inc. Slide 1

2 Copyright © 2002 Pearson Education, Inc. Slide 2 Chapter 5 Created by Frederick H. Colclough, Colorado Technical University Arrays

3 Copyright © 2002 Pearson Education, Inc. Slide 3 Learning Objectives  Introduction to Arrays  Declaring and referencing arrays  For-loops and arrays  Arrays in memory  Arrays in Functions  Arrays as function arguments, return values  Programming with Arrays  Partially Filled Arrays, searching, sorting  Multidimensional Arrays

4 Copyright © 2002 Pearson Education, Inc. Slide 4 Introduction to Arrays  Array definition:  A collection of data of same type  First ‘aggregate’ data type  Means ‘grouping’  int, float, double, char are simple data types  Used for lists of like items  Test scores, temperatures, names, etc.  Avoids declaring multiple simple variables  Can manipulate ‘list’ as one entity

5 Copyright © 2002 Pearson Education, Inc. Slide 5 Declaring Arrays  Declare the array  allocates memory int score[5];  Declares array of 5 integers named ‘score’  Similar to declaring five variables: int score[0], score[1], score[2], score[3], score[4]  Individual parts called many things:  Indexed or subscripted variables  ‘Elements’ of the array  Value in brackets called index or subscript  Numbered from 0 to size - 1

6 Copyright © 2002 Pearson Education, Inc. Slide 6 Accessing Arrays  Access using index/subscript  cout << score[3];  Note two uses of brackets:  In declaration, specifies SIZE of array  Anywhere else, specifies a subscript  Size, subscript need not be literal  int score[MAX_SCORES];  score[n+1] = 99;  If n is 2, identical to: score[3]

7 Copyright © 2002 Pearson Education, Inc. Slide 7 Array Usage  Powerful storage mechanism  Can issue command like:  “Do this to i th indexed variable” where i is computed by program  “Display all elements of array score”  “Fill elements of array score from user input”  “Find highest value in array score”  “Find lowest value in array score”

8 Copyright © 2002 Pearson Education, Inc. Slide 8 Array Program Example Display 5.1, page 174

9 Copyright © 2002 Pearson Education, Inc. Slide 9 for-loops with Arrays  Natural counting loop  Naturally works well ‘counting thru’ elements of an array  Example: for (idx = 0; idx<5; idx++) { cout << score[idx] << “off by “ << max – score[idx] << endl; }  Loop control variable (idx) counts from 0 – 5

10 Copyright © 2002 Pearson Education, Inc. Slide 10 Major Array Pitfall  Array indexes always start with zero!  Zero is ‘first’ number to computer scientists  C++ will ‘let’ you go beyond range  Unpredictable results  Compiler will not detect these errors!  Up to programmer to ‘stay in range’

11 Copyright © 2002 Pearson Education, Inc. Slide 11 Major Array Pitfall Example  Indexes range from 0 to (array_size – 1)  Example: double temperature[24]; // 24 is array size // Declares array of 24 double values called temperature  They are indexed as: temperature[0], temperature[1] … temperature[23]  Common mistake: temperature[24] = 5;  Index 24 is ‘out of range’!  No warning, possibly disastrous results

12 Copyright © 2002 Pearson Education, Inc. Slide 12 Memory override example with out-of- range index value #include int main() { int m = 99, n[4] = {11, 22, 33, 44}, i; for (i = 0; i < 4; i++) cout << "n[" << i << "] =" << n[i] << '\t' << "&n[" << i << "] =" << &n[i] << endl; cout << "m = " << m << "\t\t" << "&m = " << &m << endl; n[4] = 55; cout << "n[4] was not accounted for in the array declaration” << “ it overwrites content of variable m" << endl; cout << "n[4] = " << n[4] << '\t' << "&n[4] = " << &n[4] << endl; cout << "m = " << m << "\t\t" << "&m = " << &m << endl; return 0; }

13 Copyright © 2002 Pearson Education, Inc. Slide 13 Memory override example (The hexadecimal value 0x0012FF6C is the actual physical address of n[0] of a run on Microsoft Visual C++ 6.0 where each integer variable is given 4 bytes in RAM)

14 Copyright © 2002 Pearson Education, Inc. Slide 14 Defined Constant as Array Size  Always use defined/named constant for array size  Example: const int NUMBER_OF_STUDENTS = 5; int score[NUMBER_OF_STUDENTS];  Improves readability  Improves versatility  Improves maintainability

15 Copyright © 2002 Pearson Education, Inc. Slide 15 Uses of Defined Constant  Use everywhere size of array is needed  In for-loop for traversal: for (idx = 0; idx < NUMBER_OF_STUDENTS; idx++) { // Manipulate array }  In calculations involving size: lastIndex = (NUMBER_OF_STUDENTS – 1);  When passing array to functions (later)  If size changes  requires only ONE change in program!

16 Copyright © 2002 Pearson Education, Inc. Slide 16 Arrays in Memory  Recall simple variables:  Allocated memory in an ‘address’  Array declarations allocate memory for entire array  Sequentially-allocated  Means addresses allocated ‘back-to-back’  Allows indexing calculations  Simple ‘addition’ from array beginning (index 0)

17 Copyright © 2002 Pearson Education, Inc. Slide 17 An Array in Memory Display 5.2, page 178

18 Copyright © 2002 Pearson Education, Inc. Slide 18 Initializing Arrays  As simple variables can be initialized at declaration: int price = 0;// 0 is initial value  Arrays can as well: int children[3] = {2, 12, 1};  Equivalent to following: int children[3]; children[0] = 2; children[1] = 12; children[2] = 1;

19 Copyright © 2002 Pearson Education, Inc. Slide 19 Auto-Initializing Arrays  If fewer values than size supplied:  Fills from beginning  Fills ‘rest’ with zero of array base type  If array-size is left out  Declares array with size required based on number of initialization values  Example: int b[] = {5, 12, 11};  Allocates array b to size 3

20 Copyright © 2002 Pearson Education, Inc. Slide 20 Arrays in Functions  As arguments to functions  Indexed variables  An individual ‘element’ of an array can be functin parameter  Entire arrays  All array elements can be passed as ‘one entity’  As return value from function  Can be done  chapter 10

21 Copyright © 2002 Pearson Education, Inc. Slide 21 Indexed Variables as Arguments  Indexed variable handled same as simple variable of array base type  Given this function declaration: void myFunction(double par1);  And these declarations: int i; double n, a[10];  Can make these function calls: myFunction(i);// i is converted to double myFunction(a[3]);// a[3] is double myFunction(n);// n is double

22 Copyright © 2002 Pearson Education, Inc. Slide 22 Subtlety of Indexing  Consider: myFunction(a[i]);  Value of i is determined first  It determines which indexed variable is sent myFunction(a[i*5]);  Perfectly legal, from compiler’s view  Programmer responsible for staying ‘in-bounds’ of array

23 Copyright © 2002 Pearson Education, Inc. Slide 23 Entire Arrays as Arguments  Formal parameter can be entire array  Argument then passed in function call is array name  Called ‘array parameter’  Send size of array as well  Typically done as second parameter  Simple int type formal parameter

24 Copyright © 2002 Pearson Education, Inc. Slide 24 Entire Array as Argument Example Display 5.3, page 183

25 Copyright © 2002 Pearson Education, Inc. Slide 25 Entire Array as Argument Example  Given previous example:  In some main() function definition, consider this calls: int score[5], numberOfScores = 5; fillup(score, numberOfScores);  1 st argument is entire array  2 nd argument is integer value  Note no brackets in array argument!

26 Copyright © 2002 Pearson Education, Inc. Slide 26 Array as Argument: How?  What’s really passed?  Think of array as 3 ‘pieces’  Address of first indexed variable (arrName[0])  Array base type  Size of array  Only 1 st piece is passed!  Just the beginning address of array  Very similar to ‘pass-by-reference’

27 Copyright © 2002 Pearson Education, Inc. Slide 27 Array Parameters  May seem strange  No brackets in array argument  Must send size separately  One nice property:  Can use SAME function to fill any size array!  Exemplifies ‘re-use’ properties of functions  Example: int score[5], time[10]; fillUp(score, 5); fillUp(time, 10);

28 Copyright © 2002 Pearson Education, Inc. Slide 28 The const Parameter Modifier  Recall: array parameter actually passes address of 1 st element  Similar to pass-by-reference  Function can then modify array!  Often desirable, sometimes not!  Protect array contents from modification  Use ‘const’ modifier before array parameter  Called ‘constant array parameter’  Tells compiler to ‘not allow’ modifications

29 Copyright © 2002 Pearson Education, Inc. Slide 29 Functions that Return an Array  Functions cannot return arrays same way simple types are returned  Requires use of a ‘pointer’  Will be discussed in chapter 10…

30 Copyright © 2002 Pearson Education, Inc. Slide 30 Programming with Arrays  Plenty of uses  Partially-filled arrays  Must be declared some ‘max size’  Sorting  Searching

31 Copyright © 2002 Pearson Education, Inc. Slide 31 Partially-filled Arrays  Difficult to know exact array size needed  Must declare to be largest possible size  Must then keep ‘track’ of valid data in array  Additional ‘tracking’ variable needed  int numberUsed;  Tracks current number of elements in array

32 Copyright © 2002 Pearson Education, Inc. Slide 32 Partially-filled Arrays Example Display 5.5, page 195

33 Copyright © 2002 Pearson Education, Inc. Slide 33 Global Constants vs. Parameters  Constants typically made ‘global’  Declared above main()  Functions then have scope to array size constant  No need to send as parameter then?  Technically yes  Why should we anyway?  Function definition might be in separate file  Function might be used by other programs!

34 Copyright © 2002 Pearson Education, Inc. Slide 34 Searching an Array  Very typical use of arrays  Display 5.6 next slide

35 Copyright © 2002 Pearson Education, Inc. Slide 35 Searching an Array Cont’d

36 Copyright © 2002 Pearson Education, Inc. Slide 36 Searching an Array Cont’d

37 Copyright © 2002 Pearson Education, Inc. Slide 37 Searching an Array Cont’d

38 Copyright © 2002 Pearson Education, Inc. Slide 38 Searching an Array Cont’d

39 Copyright © 2002 Pearson Education, Inc. Slide 39 Sorting an Array  Selection Sort Algorithm  E.W. Hu, CS240 S2003

40 Copyright © 2002 Pearson Education, Inc. Slide 40 Sorting an Array Example Display 5.8, page 201

41 Copyright © 2002 Pearson Education, Inc. Slide 41 Sorting an Array Example Cont’d

42 Copyright © 2002 Pearson Education, Inc. Slide 42 Sorting an Array Example Cont’d

43 Copyright © 2002 Pearson Education, Inc. Slide 43 Sorting an Array Example Cont’d Display 5.8, page 201

44 Copyright © 2002 Pearson Education, Inc. Slide 44 Multidimensional Arrays  Arrays with more than one index  char page[30][100];  Two indexes: An ‘array of arrays’  Visualize as: page[0][0], page[0][1], …, page[0][99] page[1][0], page[1][1], …, page[1][99] … page[29][0], page[29][1], …, page[29][99]  C++ allows any number of indexes  Typically no more than two

45 Copyright © 2002 Pearson Education, Inc. Slide 45 Multidimensional Array Parameters  Similar to one-dimensional array  1 st dimension size not given  Provided as second parameter  2 nd dimension size IS given  Example: void DisplayPage(const char p[][100], int sizeDimension1) { for (int index1=0; index1<sizeDimension1; index1++) { for (int index2=0; index2 < 100; index2++) cout << p[index1][index2]; cout << endl; } }

46 Copyright © 2002 Pearson Education, Inc. Slide 46 A 3 x 3 magic square int square[3][3] = {{8,1, 6}, {3, 5, 7}, {4, 9, 2}}  E.W. Hu, CS240 S2003

47 Copyright © 2002 Pearson Education, Inc. Slide 47 3x3 magic square example #include bool magic(int s[ ][3]); void displayArray(int [ ][3]); int main() { int s[3][3] = {{8, 1, 6}, {3, 5, 7}, {4, 9, 2}}; bool ans; displayArray(s); ans = magic(s); if (ans) cout << "\nThe square is magic!" << endl; else cout << "\nThe square is not magic!" << endl; return 0; }

48 Copyright © 2002 Pearson Education, Inc. Slide 48 3x3 magic square example continued void displayArray(int s[ ][3]) { int i, j; for (i = 0; i < 3; i++){ for (j = 0; j < 3; j++) cout << s[i][j] << '\t'; cout << endl; }

49 Copyright © 2002 Pearson Education, Inc. Slide 49 3x3 magic square example continued bool magic(int s[ ][3]) { int i, j, fdsum = 0, bdsum = 0, rsum, csum; // fdsum, bdsum are sums of forward & backward diagonals // rsum, csum are individual row and column sums for (i = 0; i<3; i++){ fdsum += s[i][i]; bdsum += s[i][3 - i - 1]; } if (fdsum != bdsum) return false; for (i = 0; i < 3; i++){ rsum = 0; csum = 0; for (j = 0; j < 3; j++){ rsum += s[i][j]; csum += s[j][i]; } if(rsum != fdsum || csum != fdsum) return false; else return true; }

50 Copyright © 2002 Pearson Education, Inc. Slide 50 Sort a list of names alphabetically #include #include // defines the strcmp and strcpy functions that are used in the program int fillArray(char name[ ][10]); void displayArray(char name[ ][10], int n); void sortArray(char name[ ][10], int n); void swap(char *s1, char *s2); int main() { char name[20][10];// assuming up to 20 names and each name contains up to 9 characters int n; n = fillArray(name);// pass the (constant) address of the 2-D array; same as &name[0][0] cout << "Name list before sort:\n"; displayArray(name, n); sortArray(name, n); cout << "Name list after sort:\n"; displayArray(name, n); return 0; }

51 Copyright © 2002 Pearson Education, Inc. Slide 51 Sort a list of names alphabetically continued int fillArray(char name[ ][10])// '10' is necessary { int n = 0, i = 0; char c = 'Y';// assume at least one name while(c == 'y' || c == 'Y') { cout << "Enter the name: "; cin >> name[i];// name[i] is the (constant) address of the ith row! n++; i++; cout << "More names to enter? press (Y/N) to continue/terminate: "; cin >> c; } return n; }

52 Copyright © 2002 Pearson Education, Inc. Slide 52 Sort a list of names alphabetically continued void displayArray(char name[ ][10], int n) { for (int i = 0; i < n; i++) cout << name[i] << '\t' << endl; // name[i] is the (constant) address of the ith row! return; }

53 Copyright © 2002 Pearson Education, Inc. Slide 53 Sort a list of names alphabetically continued void swap(char *s1, char *s2) { char hold[ ]; // char *hold; does not work strcpy(hold, s1); // It results in run time error: strcpy(s1, s2); // "the memory could not be written" strcpy(s2, hold); return; }

54 Copyright © 2002 Pearson Education, Inc. Slide 54 Sort a list of names alphabetically continued // bubble sort void sortArray(char name[ ][10], int n) { int pass, i, nc; // nc --- number of comparisons in each pass for (pass = 1; pass <= n - 1; pass++){ nc = n - pass - 1; // reduce # comparisons as pass # increases for (i = 0; i <= nc; i++) if (strcmp(name[i], name[i + 1]) > 0) swap(name[i], name[i + 1]); }

55 Copyright © 2002 Pearson Education, Inc. Slide 55 Row-wise mapping a 3x4 2-dimension array to 1-dimension physical memory space  E.W. Hu, CS240 S2003

56 Copyright © 2002 Pearson Education, Inc. Slide 56 Row-wise mapping a 4x3 2-dimension array to 1-dimension physical memory space  E.W. Hu, CS240 S2003

57 Copyright © 2002 Pearson Education, Inc. Slide 57 Summary 1  Array is collection of ‘same type’ data  Indexed variables of array used just like any other simple variables  for-loop ‘natural’ way to traverse arrays  Programmer responsible for staying ‘in bounds’ of array  Array parameter is ‘new’ kind  Similar to call-by-reference

58 Copyright © 2002 Pearson Education, Inc. Slide 58 Summary 2  Array elements stored sequentially  ‘Contiguous’ portion of memory  Only address of 1 st element is passed to functions  Partially-filled arrays  more tracking  Constant array parameters  Prevent modification of array contents  Multidimensional arrays  Create ‘array of arrays’

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