CS Sept 2006 Pick ups from chapters 4 and 5

Condition statements General forms if (expr) { statements } else { statements }

Braces are not required for a single statement But an else is always associated with the closest possible if above it! example: Skill level >1 is advanced, level >2 is advanced master, lower levels are beginners. if (skill > 1) cout << “advanced “; if (skill > 2) cout << “master”; else cout << “beginner”;

Solution – use braces! if (skill > 1) {cout << “advanced “; if (skill > 2) cout << “master”; } else cout << “beginner”;

if-else-if If you are careful, if statements can be chained… –program 4.11 Chained if statements can be useful for menus… –example: menu.cpp

The switch statement General form: switch (integer variable) { case (constant value 1 ): statements break; case (constant value 2 ): statements break; case (constant value 3 ): statements break; … default: statements }

Compound boolean expressions Two boolean expressions can be combined using && (and) || (or) examples: if ((age > 21) && (age < 35)) cout << “You could be drafted!”; if ((age 120)) cout << “Incorrect age!”;

Comparing two strings… The following will have unexpected results: char name1[20], name2[20]; cin >> name1 >> name2; if (name1 == name2) cout << “these names are the same!”; The reasons will become clear later on… if (strcmp (name1, name2) == 0) cout << “these names are the same!”;

Testing for file errors… A file variable may be tested for the failure of the most recent file operation using the member function.fail() Example: ifstream fin; fin.open(“myfile.txt”); if (fin.fail()) cout << “this file could not be opened!”; Alternate example: if (!fin) cout << “this file could not be opened!”;

Other loop statements… The do-while loop do {statements; } while ( expr ) ; This is a post-test loop: The expression is tested at the bottom of the loop and always performs at least one iteration

Example; Input validation – a program must prompt for an age and only accept a value greater than 0 and less than 120. do { cout << “Enter a valid age: “; cin >> age; } while ((age > 0) && (age < 120));

Other loop statements. The for loop for ( initialization ; test expr ; update ) { statements; } This is a pre-test loop: The test expression is tested at the top of the loop. The initialization, test expression and update segments can be any valid C++ expression

Explanation of segments… initialization –This segment is executed once prior to beginning the loop test expr –This segment is executed prior to each iteration. If this segment is false, the loop terminates update –This segment is executed at the bottom of each iteration.

Example; Add 20 numbers input by the user; float num, sum = 0.0; int n; for (n=0; n<20; n++) { cout << “Enter a number: “; cin >> num; sum += num; }

Example variation… The test variable can actually be declared within the loop float num, sum = 0.0; for (int n=0; n<20; n++) { cout << “Enter a number: “; cin >> num; sum += num; }

A programmer has a choice… Convert the following while loop to a for loop int count = 0; while (count < 50) { cout << “count is: “ << count << endl; count ++; } Convert the following for loop to a while for (int x = 50; x > 0; x- - ) { cout << x << “ seconds to launch.\n”; }

Example – loan amoritization A case study (5.14, pg 295) Write a loan amoritization program –inputs: principle, interest_rate, num_years –outputs: monthly payment for each month: month number, interest, principle, remaining_balance

Example execution: Enter loan amount: $ 2500 Enter annual Interest rate: 0.08 Enter number of years: 2 Monthly payment: $ month interest principle balance … (a total of 24 month lines)

Formulas payment = (principle * interest_rate/12 * term) / (term-1) where term = (1 + interest_rate/12) num_years * 12 monthly_interest = (annual_rate / 12) * balance principle = payment – monthly_interest

Pseudocode… 1.Prompt and input loan amount, annual interest rate, and number of years 2.Calculate and display monthly payment 3.Print report header 4.For each month in the loan period: a)calculate the monthly interest b)calculate the principle c)display the month number, interest, principle, and balance d)calculate the new balance

for ( int month = 1 ; month <= num_years * 12 ; month++) {}{} d) c) b) a) First refinement…

Example – loops within loops input table of grades Student count: 64 John (9 scores): Fred (13 scores): Emily (8 scores): …

Pseudocode – rough outline 1. Input number_of_students 2. Repeat for number_of_students iterations; a-b) Input student_name and count_of_grades c-d) sum grades for this student e) Calculate average f) Output student_name and average

Pseudocode (more detailed)… 1. Input number_of_students 2. Repeat for number_of_students iterations; a) Input student name b) Input count_of_grades c) initialize sum to 0. d) Repeat for count_of_grades iterations; i– Input grade ii– sum grade e) Calculate average f) Output student_name and average

1 2 for ( int student = 0 ; student <= num_of_students ; student++) {}{} c) b) a) First refinement… e) f) d)

c) Second refinement… e) f) … for (int count=0; count<=num_of_grades; count++) { } … ii-- i--