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Chapter 8: Arrays

Chapter 8: Arrays. Starting Out with C++ Early Objects Seventh Edition by Tony Gaddis, Judy Walters, and Godfrey Muganda. Revised 2015 Edward L. Jones, Ph.D., FAMU. Topics. 8.1 Arrays Hold Multiple Values 8.2 Accessing Array Elements 8.3 Inputting and Displaying Array Contents

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Chapter 8: Arrays

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  1. Chapter 8: Arrays Starting Out with C++ Early Objects Seventh Edition by Tony Gaddis, Judy Walters, and Godfrey Muganda Revised 2015 Edward L. Jones, Ph.D., FAMU

  2. Topics 8.1 Arrays Hold Multiple Values 8.2 Accessing Array Elements 8.3 Inputting and Displaying Array Contents 8.4 Array Initialization 8.5 Processing Array Contents 8.6 Using Parallel Arrays

  3. Topics (continued) 8.7 The typedef Statement 8.8 Arrays as Function Arguments 8.9 Two-Dimensional Arrays 8.10 Arrays with Three or More Dimensions 8.12 Arrays of Class Objects

  4. 8.1 Arrays Hold Multiple Values • Array: variable that can store multiple values of the same type • Values are stored in adjacent memory locations • Declared using [] operator const int CAPACITY = 5; int T[CAPACITY]; … or int T[5];

  5. Array Storage in Memory This definition allocates the following memory: int T[5];

  6. Array Terminology In the definition int T[5]; • int is the data type of the array elements • T is the name of the array • 5, in [5], is the size declarator. It shows the maximum number of elements in the array (i.e., array capacity). • The allocated size of an array is the number of bytes required to store the array (number of elements) * (bytes needed for each element)

  7. Array Terminology Examples // Assume int uses 4 bytes and double uses 8 bytes const int CAPACITY = 5, DSIZE = 10; int T[CAPACITY];// holds 5 ints, array // occupies 20 bytes double volumes[DSIZE];// holds 10 doubles // occupies 80 bytes

  8. 0 1 2 3 4 subscripts 8.2 Accessing Array Elements Each array element is a variable whose “name” contains a subscript that uniquely identifies the element. Name of 3rd element array T is T[2], ”T sub-two” Subscripts start at 0

  9. T 0 1 2 3 4 Accessing Array Elements Array elements are regular variables of the base data type. T[0] = 79; cout << T[0]; cin >> T[1]; T[4] = T[0] + T[1]; cout << T; // illegal due to ??

  10. 8.3 Inputting and Displaying Array Contents cout and cin can be used to display values from and store values into an array element. int T[5]; // 5-element array cout << "Enter first T score "; cin >> T[0];

  11. Array Subscripts • Array subscript can be an integer constant, integer variable, or integer expression • Examples: Subscript is cin >> T[3]; int constant cout << T[i]; int variable cout << T[i+j]; int expression cin >> T[T[0]]; int expression

  12. Inputting and Displaying All Array Elements To access each element of an array • Use a (for) loop • The loop control variable sequence must be valid subscripts, e.g., 0,1,2,… • A different array element will be referenced each time through the loop * for (i = 0; i < 5; i++) sum = sum + T[i];

  13. num 25 [0] [1] [2] WARNING!! No Bounds Checking • C++ does not check that an array subscript is in range • An invalid array subscript can cause program to overwrite other memory • Example:* const int CAPACITY = 3; int num[CAPACITY]; num[4] = 25; • C++ Segmentation error!!

  14. Off-By-One Errors Most often occur when a program accesses data one position beyond the end of an array, or misses the first or last element of an array. Don’t confuse the ordinal (order) number of an array element (first, second, third) with its subscript (0, 1, 2)

  15. 8.5 Processing Array Contents • Array elements can be • treated as ordinary variables of the same type as the array • used in arithmetic operations, in relational expressions, etc. • Example: if (principalAmt[3] >= 10000) interest = principalAmt[3] * intRate1; else interest = principalAmt[3] * intRate2;

  16. Using Increment and Decrement Operators with Array Elements int T[15];int Tsize; When using ++ and -- operators, don’t confuse the element with the subscript T[i]++; // increments T[i], but has // no effect on i. T[i++]; // increments i, but has // no effect on T array.

  17. 8.4 Array Initialization • Can be initialized during program execution with assignment statements T[0] = 79; T[1] = 82; // etc. • Can be initialized at array definition with an initialization list * int T[5] = {79,82,91,77,84};

  18. Partial Array Initialization • If an array is initialized at definition with fewer values than the size declarator of the array, remaining elements will be set to 0. * int T[5] = {79, 82}; • Initial values used in order; cannot skip over elements to initialize noncontiguous range.

  19. Implicit Array Sizing • Can determine array size by the size of the initialization list * short quizzes[]={12,17,15,11}; • Must use either array size declarator or initialization list when array is defined

  20. STOP / START HERE

  21. Copying One Array to Another • Assignment operator = not defined for arrays (but for int, float, etc.). int S[5], T[5]={1,5,3,2,7}; int Tsize = 5; // contains 5 vals. S = T; // won’t work • Instead: use a loop to copy element-by-element: * for (int k=0; k<Tsize; k++) S[k] = T[k];

  22. Are Two Arrays Equal? • Also, relational operators not defined for arrays: if (T2 == T1) cout << “EQUIVALENT”; //Invalid • Instead: • Sizes must match AND corresponding elements must match. • Method 1: Use a flag and while loop: int k = 0; bool Equivalent = (T1size == T2size); while (Equivalent && k<T1size) { if(T1[k] != T2[k])Equivalent = false; k++; } if (Equivalent) cout << “EQUIVALENT”;

  23. Are Two Arrays Equal? • Also, cannot compare in a single expression: if (T2 == T1) • Sizes AND corresponding elements must match. • Method 1a: Use a flag and while loop: int k = 0; bool Equal = (T1size == T2size); while (Equal && k < CAPACITY) { Equal = (T1[k] == T2[k]); k++; } if (Equivalent) cout << “EQUIVALENT”;

  24. Are Two Arrays Equal? • Also, cannot compare in a single expression: if (T2 == T1) • Instead: • Sizes must match AND corresponding elements must match. • Method 2: Use a flag and for loop: bool Equal = (T1size == T2size); for (int k=0; Equal && k<T1size; k++) { Equal = (T1[k] == T2[k]); }

  25. STOP / START HERE

  26. Sum, Average of Array Elements • Use a simple loop to add together array elements float average, sum = 0; for(int j=0; j<Tsize; j++) sum += T[j]; • Once summed, average can be computed average = sum/Tsize;

  27. Largest Array Element • Use a loop to examine each element and find the largest element (i.e., one with the largest value) * int largest = T[0]; for (int m = 1; m < CAPACITY; m++) { if (T[m] > largest) largest = T[m]; } cout << "Highest score is " << largest; • A similar algorithm exists to find smallest.

  28. Partially-Filled Arrays • The exact amount of data may not be known when a program is written. • Programmer estimates maximum data capacity, and declares the array accordingly. • Program must keep track of how many array elements are actually used – to avoid exceeding array capacity (segmentation error) * int T[CAPACITY]; int Tsize = 0; // Size of array T, initialized to empty.

  29. C-Strings and string Objects Both contain an array of characters. Both can be manipulated character by character. string city; cout << "Enter city name: "; cin >> city; cout << “Third letter is ‘” << city[2];

  30. 8.6 Using Parallel Arrays • Parallel arrays: two or more arrays that contain related data • Subscript is used to relate arrays • elements at same subscript are related • The parallel arrays do not have to hold data of the same type • Example: Each spreadsheet column is stored as a separate, parallel array.

  31. name average grade 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 Parallel Array Example const int CAPACITY = 5; string name[CAPACITY]; // student name float average[CAPACITY]; // course average char grade[CAPACITY]; // course grade

  32. Displaying Parallel Array Contents int size = 0; * ... //-| Processing to obtain size rows of data. ... //-| Display size rows of data. for (int i = 0; i < size; i++) cout << " Student: " << name[i] << " Average: " << average[i] << " Grade: " << grade[i] << endl;

  33. 8.13 Arrays of Structured Variables • Records can be used as array elements * struct Student { int studentID; string name; short year; double gpa; }; Student Class[30];// Holds 30 // Student records.

  34. Arrays of Structured Variables • Use array subscript [ ] to access a specific record in the array • Then use dot . operator to access a members of that record cin >> Class[25].studentID; cout << Class[i].name << " has GPA " << Class[i].gpa << endl;

  35. Number of cols Number of rows 8.9 Two-Dimensional Arrays • Can define one array for multiple sets of data • Like a table in a spreadsheet • Use two size declarators in definition int exam[4][3];

  36. Two-Dimensional Array Representation columns r o w s int exam[4][3]; Must use two subscripts to access element exam[2][2] = 86;

  37. Initialization at Definition • Two-dimensional arrays are initialized row-by-row int exam[2][2] = { {84, 78}, {92, 97} }; • exam[0]is row 1, a 2-element int array.

  38. 2D Array Traversal • Use nested loops, outer one for row and inner one for column, to visit each array element in a row-by-row fashion. for (int r=0; r<NROWS; r++) //outer for (int c=0; c<NCOLS; c++) //inner process exam[r][c] …

  39. 2D Array Traversal • Use nested loops, outer one for column, and inner one for row, to visit each array element in a column-by-column fashion for (int c=0; c<NCOLS; c++) for (int r=0; r<NCOLS; r++) process exam[r][c] …

  40. STOP / START HERE

  41. Array Element as Function Argument • Passing a single array element (e.g., X[3]) to a function is no different than passing a regular variable. • The receiving function does not need to know that the value it receives is coming from an array displayValue(score[i]); // function call void displayValue(int item) // function header { cout << item << endl; ) // function body }

  42. 8.8 Arrays as Function Arguments • To declare a function with a one-dimensional array parameter, use empty [] to indicate the argument is a 1-D array. • To pass an array argument to a function, just use the array name // Function prototype void showScores(int[]); // Function header void showScores(intT[]) // Function call showScores(T);

  43. Modifying Arrays in Functions • Array parameters are passed by reference • Changes made to array in a function are made to the actual array in the calling function • Must be careful that an array is not inadvertently/mistakenly changed by a function

  44. Passing an Entire 1-D Array • Use the array name, without any brackets, as the argument • Often necessary to also pass the array size so the function knows how many elements to process void showScores(int[],int); // function prototype. void showScores(int A[],int Asize) // function header. showScores(T, 5); // function call.

  45. Passing a 2-Dimensional Array to a Function • Use empty [] for row and a size declarator for col in the prototype and header // Prototype, where #cols is 2 void getExams(int[][2], int); // Header void getExams (intEx[][2],introws) • Use array name as argument in function call getExams(exam, 2);

  46. 8.10 Arrays with Three or More Dimensions • Can define arrays with any number of dimensions short rectSolid [2][3][5]; double timeGrid[3][4][3][4]; • When used as parameter, specify size of all but first dimension void getRectSolid(short [][3][5]);

  47. 8.7 The typedef Statement • Creates an alias for a data type. • DOES NOT create a data type. • Format: typedef existingType newTypeName; • Example: typedef float Money; Money Paycheck[12]; // float array.

  48. Uses of typedef • Can make code more readable • Creates alias (alternate name) for a data type • Can give a name to an array-based data type (element type, capacity (specified or unspecified). // Define yearArray as a data type // that is an array of 12 ints typedef int yearArray[12]; // Create two 12-element int arrays. yearArray highTemps, lowTemps;

  49. Using typedef for Array Parameter • Can use typedef to simplify function prototype and heading // Make intArray an integer array // of unspecified size typedef int intArray[]; // Function prototype void showScores(intArray, int); // Function header void showScores(intArray T, int size) • This aliased array type has unspecified capacity.

  50. Using typedef with aTwo-Dimensional Array Can use typedef for simpler notation typedef int intExamsARRAY[][2]; // Indefinite // Nx2 array. // Function prototype void getExams(intExamsARRAY, int); // Function header void getExams(intExamsARRAY exam,introws) WARNING: Implementor/caller of function must remember that typedef refers to an array.

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