CS 1400

1. CS 1400 13 Oct 2006 Chap 7

2. Arrays • 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++]?

3. Array indexes… • Indexes may be constants (integers) or 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) for the first cell. • 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

4. char arrays… • Note that char arrays are an exception. • 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];

5. Example: test grading • Write a program to grade a group of true/false tests. Each test has five (5) answers. • assume: the test key is in key.txt • assume: the tests are in students.txt (one line per student) Example: key.txt: T F T F F students.txt: Fred T F F T F Jane T F T T F Bill F T F F F Ann T F T F F

6. Example output: student: Fred score 3 student: Jane score 4 student: Bill score 2 student: Ann score 5 correct answers summary by question: question 1: 3 question 2: 3 question 3: 2 question 4: 2 question 5: 4

7. Pseudocode… • Read in the answers array from “key.txt” • Process each student in turn from “students.txt” • Input this students answers • Compare against correct answers and determine score • Output student score • Tabulate correct answers to summary table • Output summary table of correct answers

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

9. 2. Process each student in turn… char name[32]; ifstream fin_ans; bool done = false; … fin_ans.open (“e:\\answers.txt”); // 2. process each student in turn while (!done) { fin_ans >> name; if (fin_ans.fail()) done = true; else { … // 2.a, b, c, d } … // 3. }

10. 2. Process each student in turn… char answers[5]; … else { for (int n=0; n<5; n++) // 2.a input a student’s answers fin_ans >> answers[n]; … // 2.b, c, d } … // 3.

11. 2. Process each student in turn… else { … // 2.a input student’s answers int score = 0; // 2.b determine score for (j=0; j<5; j++) if (answers[j] == key[j]) score++; … }

12. 2. Process each student in turn… else { … // 2.a input student’s answers … // 2.b determine score cout << “student: “ << name << “ score: “ << score << endl; // 2.c output student’s score … }

13. 2. Process each student in turn… int table[5] = {0, 0, 0, 0, 0}; … else { … // 2.a input student’s answers … // 2.b determine score … // 2.c output student’s score for (m=0; m<5; m++) if (answers[m] == key[m]) // 2.d tabulate scores table[m]++; }

14. 3. Output summary table… … cout << “correct answers summary by question: \n”; for (int p=0; p<5; p++) cout << “ question “ << p << “: “ << table[p] << endl; …