CS 1400 March 5, 2007

1. CS 1400March 5, 2007 Chapter 6 ARRAYS

2. Array variables • Simple variables can hold single values int x; // x can hold one integer float y; // y can hold one float • Array variables can hold multiple values int z[5]; // z can hold 5 integers float w[3]; // w can hold 3 floats x y z w

3. z 0 1 2 3 4 Array Access… • Individual cells of an array variable are accessed using an index: z[2] = 55; z[3] = 921; z[4] = z[2]+1; • Cells of an array are numbered beginning with zero! 55 921 56

4. z 0 1 2 3 4 Array Indexes… • Array indexes can also be ordinal variables (integers) for (int n=0; n<5; n++) z[n] = n*10; 0 10 20 30 40

5. 11 22 33 44 55 z 0 1 2 3 4 Array initialization… • Like simple variables, array variables can be intialized when declared: int z[5] = {11, 22, 33, 44, 55}; (If insufficient initializing constants are given, the rest default to zero.) float w[3] = {1.0}; w 1.0 0.0 0.0 0 1 3

6. Array Examples • Declaration examples; int costs[10]; float rates[100]; int ages [n]; Can’t declare size with a variable! • Initialization examples; int costs [5] = {99, 88, 77, 66, 55}; float rates [100] = {1.0}; • References examples; costs[n] = 21; cout << rates[5]; ages[n]++; how is this different from ages[n++]?

7. Array indexes… • Indexes may be constants (integers) or ordinal variables • Indexes may also be simple integer expressions; cout << costs[n+2]; • There is no array bounds checking. • Array cell indexes always start at 0 (zero) • In an executable statement, only array cells may be referenced costs = rates * 2; exception: char arrays holding words or strings exception: passing an array reference to a function

8. No bounds checking? int cost[5]; for (int n=0; n<7; n++) cin >> cost[n]; // a long walk on a short pier!! cost … 0 1 2 3 4 5 6 … These cells belong to cost Who do these belong to?? other variables? program instructions?

9. char arrays… • Note that char arrays are a special case! • can be input or output as “strings” char word[30]; cin >> word; • can also used as arrays if (word[0] == ‘a’) word[0] = ‘A’; cout << word[0] << word[5]; • can be inialized with strings or with characters char name[30] = “Bobby”; char name2[30] = {‘B’, ‘o’, ‘b’, ‘b’, ‘y’, ‘\0’};

10. Example: test grading • Write a program to grade a test. A test has five (5) numeric answers. • assume: the test key is in key.txt • assume: the test is in answers.txt Example inputs: key.txt: 21 14 7 9 18 answers.txt: Fred 21 15 7 9 17 Example output: student: Fred score 3

11. Pseudocode… • Read in the correct answers array from “key.txt” • Read in student’s answers from “answers.txt” • Compare against correct answers and determine score • Output student score

12. 1. Read in correct answers int key[5]; ifstream fin_key; … fin_key.open (“e:\\key.txt”); for (int n=0; n<5; n++) fin_key >> key[n];

13. 2. Read in student’s answers int answers[5]; char name[30]; ifstream fin_ans; … fin_ans.open (“e:\\answers.txt”); cin >> name; for (int n=0; n<5; n++) fin_ans >> answers[n];

14. 3. Compare… int score = 0; … for (int n=0; n<5; n++) if (answers[n] == key[n]) score++; • Output score cout << “score is “ << score;

15. int main() { int key[5], answers[5], score = 0; char name[30]; ifstream fin_key, fin_ans; fin_key.open (“e:\\key.txt”); for (int n=0; n<5; n++) fin_key >> key[n]; fin_ans.open (“e:\\answers.txt”); fin >> name; for (int n=0; n<5; n++) fin_ans >> answers[n]; for (int n=0; n<5; n++) if (answers[n] == key[n]) score++; cout << “score for “ << name << “ is “ << score << endl; }

16. Linear Search • Search array list withsizeelements: find the position of variable value. bool found = false; int position = -1; for (int n=0; n<size && !found; n++) { if (list[n] == value) { position = n; found = true; } }

17. Example using a linear search • Suppose a disk file contains a database of 100 part numbers and part costs: inventory.txt: part cost 413 121.78 214 22.98 456 234.56 786 9.56 177 86.99 356 198.22 …

18. POS terminal program • Write a program to query the user for a part number and a quantity for an order. Calculate and display the total price of the order (including 20% markup and 6.5% sales tax) Enter part number and quantity: 177 32 Order total (including tax): $ 3557.54

19. Pseudocode… • read database into parallel arrays. • query user for part number and quantity • use a linear search to find associated cost • calculate order total • display

20. 1. Read database into parallel arrays int parts[100]; float costs[100]; char word[30]; ifstream fin; … fin.open (“inventory.txt”); fin >> word >> word; // skip over titles for (int n=0; n<100; n++) cin >> parts[n] >> costs[n];

21. 2. Query user for part number and quantity int partnumber, quantity; … cout << “Enter part number and quantity: “; cin >> partnumber >> quantity;

22. 3. Linear search… bool found = false; float thiscost = -1.0; … for (int n=0; n<100 && !found; n++) { if (partnumber == parts[n]) { found = true; thiscost = costs[n]; } }

23. 4. Calculate total float total; … total = quantity * thiscost * 1.2 * 1.065; 5. Display cout << fixed << setprecision(2); cout << “Order total (including tax): $ “ << total;

24. Passing Arrays to Functions… • Arrays (of any size) can be passed to functions by using just the array name as a function argument. float ages[100]; float weights[200] MyFunc(ages, 100); MyFunc(weights, 200);

25. Arrays as function parameters • Array parameters are declared with empty brackets; • A function with an array parameter may need to know the size of the array or the number of elements to be considered; void MyFunc (float array[ ], int size); Array parameters are always reference parameters!

26. Examples; • A function to find the maximum value of an array • A function to calculate the average value of an array • A function to count the number of negative values in an array

27. Linear Search as a function… int LinearSearch (int list[], int size, int value) { bool found = false; int position = -1; for (int n=0; n<size && !found; n++) { if (list[n] == value) { position = n; found = true; } } return position; }

28. Multi-dimension arrays • Arrays can contain sub-arrays. An array of rows of cells is a 2-d table: Consider a table of chair prices; Covering material 0 1 2 0 1 2 3 Style

29. declaration with initialization… float chair_costs [4] [3] = {{ 67.83, 61.99, 59.70 }, { 44.56, 42.68, 39.98 }, { 89.99, 83.56, 80.15 }, { 55.67, 51.34, 49.99 }}; … int style, covering; cout << “enter a style and covering: “; cin >> style >> covering; cout << “cost is: “ << chair_costs[style] [covering];

30. Passing multi-dimensional arrays • When passing a multi-dim. array, all but the first dimension of the corresponding parameter must be stated explicitly in main: cout << SumChairCosts (chair_costs) << endl; Function: float SumChairCosts (costs[ ] [3]) { float sum = 0; for (int row=0; row<4; row++) for (int col=0; col<3; col++) sum += costs[row][col]; return sum }

31. Higher dimensioned arrays are possible, but rarely used • Consider a table of chairs • there are 4 styles • each style has 3 coverings • each covering has 5 colors • each color has 7 shades; float chair_costs [4] [3] [5] [7]; … int style, covering, color, shade; cout << “enter a style, covering, color and shade: “; cin >> style >> covering >> color >> shade; cout << “cost is: “ << chair_costs[style][covering][color][shade];

32. 2-d char arrays • Since strings are stored in arrays of char, a 2-d char array is an array of strings! char names[5][10] = {“Fred”, “Jane”, “Billy”, “Annie”, “Jason”}; cout << names[n] << endl; // refers to row n or string n cout << names[n][m] << endl; // refers to char in row n, cell m for (int thisname = 0; thisname<5; thisname++) cin >> names[thisname]; // input 5 new names

33. Common Errors… • Attempting to copy one array into another with a single assignment statement; • Taking a long walk on a short pier; int array1[10], array2[10]; array1 = array2; // illegal!! int array[10]; for (int n=0; n<=10; n++) // illegal!! cin >> array[n];

34. Interesting questions… • Can an array have different kinds of cells? • Can an array size be changed while a program is running? • Can each row of a 2-d array have a different number of cells? • not yet…