CS 1400 Chapter 7 ARRAYS

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 xyzw

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! z

Array Indexes… Array indexes can also be ordinal variables (integers) for (int n=0; n<5; n++) z[n] = n*10; z

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}; z w 0 1 3

Array Examples Declaration examples; const int SIZE = 10; int costs[SIZE]; 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++]?

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

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

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’};

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: answers.txt: Fred Jan Example output: student: Fred score 3 Jan score 4

Pseudocode… 1.Read in the correct answers array from “key.txt” 2.repeat for all students a)Read in student’s answers from “answers.txt” b)Compare against correct answers and determine score c)Output student score

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];

2. Loop through all students char name[30]; ifstream fin_ans; … fin_ans.open (“e:\\answers.txt”); fin_ans >> name; while (!fin_ans.eof()) {… fin_ans >> name; }

a) read in student’s answers int answers[5]; … for (int n=0; n<5; n++) fin_ans >> answers[n];

b) compare… int score = 0; … for (int n=0; n<5; n++) if (answers[n] == key[n]) score++; c) output score cout << name << “ score “ << score << endl;

#include using namespace std; int main() {int key[5]; ifstream fin_key; int answers[5]; int score = 0; fin_key.open (“e:\\key.txt”);// read in correct answers for (int n=0; n<5; n++) fin_key >> key[n];

fin_ans.open (“e:\\answers.txt”); fin_ans >> name; while (!fin_ans.eof())// loop through all students { for (int n=0; n<5; n++) fin_ans >> answers[n]; // read in student’s answers for (int n=0; n<5; n++) if (answers[n] == key[n])// compare score++; cout << name << “ score “ << score << endl; fin_ans >> name; }

Linear Search Search array list with size elements: 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; }

Example using a linear search Suppose a disk file contains a database of 100 part numbers and part costs: inventory.txt:partcost …

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: Order total (including tax): $

Pseudocode… 1.read database into parallel arrays. 2.query user for part number and quantity 3.use a linear search to find associated cost 4.calculate order total 5.display

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++) fin >> parts[n] >> costs[n];

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

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]; }

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

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); Arrays are always passed by reference

Arrays as function parameters Array parameters are declared with empty brackets; A function with an array parameter usually needs 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!

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

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; }

char arrays revisited… Again, char arrays are handled in a special way for input and output. On input, the system will store a NULL character ‘\0’ at the end of a string On output, the system will output chars until the NULL character is encountered

Example; char word[10]; cin >> word; cout << “The word is: “ << word << endl; word Fred\0

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 Style

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

Passing multi-dimensional arrays When passing a multi-dim. array, all but the first dimension of the corresponding parameter must be stated explicitly in the receiving function 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 }

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];

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

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];

Example A file exists containing a row of daily rainfalls for each of 12 months (12 rows). Each row ends with -1 as a sentinel. For example; … Write a program to output a) the average rainfall for the year and b) the average rainfall for each month.

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…

Assignment Modify the previous Linear Searching program to add the options of adding and deleting employees. When an employee is added, a new line should be added to the database for the ID, extension, room, and department of that new employee. When an employee is deleted, the database line for that employee should be removed. HINT: Rather than delete a row in the parallel arrays of the database within the program, simply mark the ID as not used with a number like -1. Now, when searching the database, simply skip over the employees with IDs of -1. When the database is written back out to disk (at the end of the program), don’t write the lines for employees with IDs of -1.

Suggested menu: Enter choice:(E)mployee ID search (T)elephone extension search (R)oom number search (D)epartment code search (A)dd an employee de(l)ete an employee E(x)it program and update file:

Quiz Show the output of this segment int ages[4] = {21, 36}; for (int n=0; n<4; n++) cout << ages[n] << endl; for (int n=0; n<3; n++) ages[n+1] = ages[n] + 1; for (int n=0; n<4; n++) cout << ages[n] << endl;