Presented byPresented by

Joaquin VilaJoaquin Vila

Prepared byPrepared by

Sally ScottSally Scott

ACS 168ACS 168Problem Solving Using the ComputerProblem Solving Using the Computer

ACS 168ACS 168Problem Solving Using the ComputerProblem Solving Using the Computer

Week 12

Boolean Expressions, Switches, For-Loops

Chapter 7

Week 12

Boolean Expressions, Switches, For-Loops

Chapter 7

Boolean Expressions

Either true or false Use relational operations

==, < , >, <= , >= , !=, &&, || type bool

can declare variables that have values of true or false

Evaluation of Boolean Expressions

Truth tables

ANDExp1 Exp2 Exp1&&Exp2 T T T T F F F T F F F F

ORExp1 Exp2 Exp1||Exp2 T T T T F T F T T F F F

NOTExp !(Exp) T F F T

Precedence Rules Unary operators: +, -, ++, --, !Binary operators *, /, %Binary operators +, -Boolean operators: <, >, <=, >=Boolean operators: ==, !=Boolean operator: &&Boolean operator: ||

low

high

How expressions are evaluated Evaluation

Leftmost expression evaluated first Others only as needed Consider:

1. a < b && b != c where a = 12, b = 6, c = 8

2. if ((j > 0) && (k/(j +1) > 10)) where j = -1

Integers as boolean values True converts to 1 False converts to 0 Any non-zero integer is true Consider:

if (!time > limit) where time = 36 and limit = 60

Given: a = 12, b = 6, c = 8

1. (!(a < b) && !(b > c))

2. ((a > b) && (a > c) || (b > c))

Given: x = 5, y = 3, z = 8:         1. x < 15 && !y < 0

        2. x > z || y < z

         3. x < y < z

         4. x = y < z

         5. x < y && y < z

Functions that return a Boolean Value

Can be used in if-else statementif (((rate >= 10) && (rate < 20)) || (rate == 0)){ …}

better way:if (appropriate(rate)) { …}

bool appropriate (int rate){ … if (((rate >= 10) && (rate < 20)) || (rate == 0)) …}

Nested if statementsif (expression) statement if true;else if (expression) statement if true; else statement if false;

if (expression) statement if true;else if (expression) statement if true;else if (expression) statement if true;else statement if false;

Rule: else matches nearest unmatched if that precedes it

Consider this:

if (num <= 10) if (num >= 3) cout << num << “ is between 3 and 10” << endl;else cout << num << “ is less than 3” << endl;

if (num <= 10) /* braces allow else to go with first if */{ if (num >= 3) cout << num << “ is between 3 and 10” << endl;} else cout << num << “ is less than 3” << endl;

Multiple-Way Selection

Switch statement alternate to nested-if statement called case statement in pseudocode permits up to 10 branches use when selection based on single variable or

simple expression good for menu choices

Switch exampleswitch (num % 10){ case 0: cout << “One’s digit is zero” << endl; break; case 1: cout << “One’s digit is one” << endl; break; case 2: cout << “One’s digit is two” << endl; break; default: cout << “One’s digit is ” << num % 10 << endl;}

Exampleswitch (prize) {

case ‘d’: cout << “You win the diamond ring!” << endl;

break; case ‘c’:

cout << “You win the new car!” << endl; break;

case ‘s’: cout << “You win the $25,000 savings bond!” << endl; break;

case ‘t’: cout << “You win the trip to Belize!” << endl; break;

default: cout << “This is not a winning ticket. Sorry.” << endl;

}

Switch Example for Calling Functions

switch (choice){case 1:

show_assignment();break;

case 2:calc_grade();break;

case 3:give_hints();break;

default:cout << “Not a valid choice.” << endl;cout << “Choose again.” << endl;

}

break The break statement causes an immediate exit

from the switch Without the break, execution falls through from

each case to the next This enables us to specify multiple values that

should cause the same code to be executed without having to repeat the code

But we must be careful to include the break where it belongs

A break is recommended, though not required, at the end of the last case

Switch exampleswitch (CountFrom) { case 5: cout << “5” << endl; case 4: cout << “4” << endl; case 3: cout << “3” << endl; case 2: cout << “2” << endl; case 1: cout << “1” << endl; cout << “Ignition” << endl; cout << “Blast off!” << endl;}

Selection in PseudocodeIf gender = male then

If marital_status = single then

If age > 18 and age < 26 then

print “all criteria met”

Else

print “wrong age”

Endif

Else

print “married male”

Endif

Else

print “wrong gender”

Endif

CASE OF order_item donuts: add 1 to d_count pretzels: add 1 to pr_count pizza: add 1 to pi_count other: print “invalid order” add 1 to error_countENDCASE

Practice Problem

A program is required to read a customer’s id number, a purchase amount and a tax code. The tax has been validated and will be one of the following:

0 tax exempt (0%)1 state sales tax only (3%)2 city and state sales tax (5%)3 special sales tax (7%)

The program must then compute the sales tax and the total amount due and print the customer’s id, purchase amount, sales tax and total amount due.

lec. notes p.111

Practice Problem Write C++ code to ask for a student’s exam

score out of 100. Your program is then to match the exam score to a letter grade and print the grade to the screen. The letter grade is to be calculated as follows:

90 and above A80-89 B70-79 C60-69 Dbelow 60 F

lec. notes p.110

Loop Design

3 kinds of loops pretesting

while loop: while (! infile.eof()) post-testing

do-while loop: counting loop

for loop

Counting Loops in C

for (initialization; condition; modification) {…statement block

}

sum = 0;for (i = 1; i < 11; i++) {

sum += i;}

initialization – assignment statement to set loop control variable

condition – determines when loop will exit

modification – defines how loop control variable will change

for vs. while loop

i = 0;

while (i < 6){ print (“%d”, i); i = i + 1; }

for (i = 0; i < 6; i = i + 1) { printf (“%d”, i); }

These are the same:

initialization

condition

increment

Example

Declaring variables in the for loop

for (int r = 7; r > 3; r--)

cout << r << endl;

Increment and Decrement Operator As a statement

count++;

In an expression

number = 5;

ans = 2 * (number++);

cout << ans << “ “ << number;

ans = 2 * (++number);

cout << ans << “ “ << number;

Counting Loop Pseudocode

Find Sum Initialize sum to 0 Ask how many numbers Get this_many REPEAT this_many times Read next_num Add next_num to sum END loop Print sumEND

start algorithm

end algorithm

loop

Ways to terminate an input loop

1. Run out of data in file (eof)

2. Ask user if want to continue

3. End with sentinel value

4. Know the number of times loop is to be executed and use for loop or while loop

Problem 1

Write a function to sum the numbers 1 to 10 and display the resulting sum.

lec. notes p.113

Problem 2 - Post-testing Practice

•Write a code segment to prompt for and read the cost of an item until the user types a valid (nonnegative) value. Use a do-while loop.

lec. notes p.112

Problem 3

Write a program that sums a sequence of integers. Assume that the first integer read specifies the number of values remaining to be entered. Your program should read only one value at a time. Use a for loop.

lec. notes p.113

Practice Problem 4

Write nested for loops to print the pattern on the right. Each output statement should print a single asterisk.

*******************************************************

lec. notes p.114

Practice Problem 5

Write a for loop that computes the number of integer values that can be divided evenly by 11 or by 13 in the range of start through end, where start and end are integer variables.

lec. notes p.114

Tracing Problem

int i = 5, j = 5;

while (( i > 0) && (--j > 0))

{

if (i-- % 5 == 0)

{

cout << "1 - i: " << i <<", j: " << j << endl;

}

else

{

cout << "2 - i: " << --i <<", j: " << j << endl;

}

}

lec. notes p.112