Section 3 - Selection and Repetition Constructs

Control Structures

1. Sequence

2. Selection

3. Repetition

Selection – choosing between alternatives

Relational Operators

• Used to compare numbers, characters and strings to determine relative order

• Operators:

> Greater than< Less than>= Greater than or equal to<= Less than or equal to== Equal to!= Not equal to

Relational Expressions

• Result is a Boolean value – true or false

• Examples:12 > 5 is true7 <= 5 is false

if x is 10, then x == 10 is true x != 8 is true x == 8 is false

Examples

int i = 5;

int k = 12;

bool p = (i < 10);

bool q = (k > i);

bool r = (i >= k);

bool s = (k <= 12);

Comparison Operators

Operator Name Example Result

< less than 1 < 2 true

<= less than or equal to 1 <= 2 true

> greater than 1 > 2 false

>= greater than or equal to 1 >= 2 false

== equal to 1 == 2 false

!= not equal to 1 != 2 true

Relational Expressions

• Can be assigned to a variableresult = x <= y;

• Assigns 0 for false, 1 for true

• Do not confuse = and ==

The if Statement

• Allows statements to be conditionally executed or skipped over

• Models the way we mentally evaluate situations– “If it is raining, take an umbrella.”

• Syntaxif (expression)

statement;

if statement – what happens

To evaluateif (expression)

statement;

• If (expression) is true, then statement is executed.

• If (expression) is false, then statement is skipped – not executed

if statement – what happens

expression

statement

expressionis true

expressionis false

Expanding the if Statement

• To execute more than one statement as part of an if statement, enclose them in { }

if (score > 90){

grade = “Excellent”;cout << “Well done!\n";

}

• { } creates a block of code

Example

if (radius >= 0) { area = radius * radius * PI; cout << "The area for the circle of " << " radius " << radius << " is " << area;}

The if/else Statement

• Allows choice between statements if (expression) is true or false

• Syntaxif (expression)

statement1; // or blockelse

statement2; // or block

if/else – what happens

expression

statement1

expressionis true

expressionis false

statement2

Example

if (score >= 90.0) grade = 'A';else if (score >= 80.0) grade = 'B';else if (score >= 70.0) grade = 'C';else if (score >= 60.0) grade = 'D';else grade = 'F';

Finding the Max

cout << "Enter two integers: ";int Value1;int Value2;cin >> Value1 >> Value2;

int Max;if (Value1 < Value2) { Max = Value2;}else { Max = Value1;}cout << "Maximum of inputs is: " << Max << endl;

The if/else/if Statement

• Chain of if statements that test in order until one is found to be true

• Also models thought processes:– “If it is raining, take an umbrella,

else, if it is windy, take a hat, else, take sunglasses”

if/else/if format

if (expression) statement1; // or block

else if (expression) statement2; // or block . . // other else ifs .

else if (expression) statementn; // or block

An If-Else-If Statement

if ( nbr < 0 ){ cout << nbr << " is negative" << endl;}else if ( nbr > 0 ) { cout << nbr << " is positive" << endl;}else { cout << nbr << " is zero" << endl;}

Example

The switch Statement

• Used to select among statements from several alternatives

• May be used instead of if/else if statements

switch Statement

switch (weather) { // sunny case 0: take sunglasses; break; // windy case 1: take hat; break; // rainy case 2: take umbrella; break; // not any of the above default: flip a coin!;}

switch statement format

switch (expression) //integer{

case exp1: statement1;case exp2: statement2;...case expn: statementn;default: statementn+1;

}

switch statement requirements

1) expression must be an integer or char variable, or an expression that evaluates to an integer value

2) exp1 through expn must be constant integer expressions or literals, and must be unique in the switch statement

3) default is optional but recommended “catch-all”

The keyword break is optional, but it should be used at the end of each case in order to terminate the remainder of the switch statement. If the break statement is not present, the next case statement will be executed.

switch (switch-expression) {

case value1: statement(s)1;

break;

case value2: statement(s)2;

break;

…

case valueN: statement(s)N;

break;

default: statement(s)-for-default;

}

The default case, which is optional, can be used to perform actions when none of the specified cases matches the switch-expression.

The case statements are executed in sequential order, but the order of the cases (including the default case) does not matter. However, it is good programming style to follow the logical sequence of the cases and place the default case at the end.

switch statement – how it works

1. expression is evaluated2. The value of expression is compared against exp1 through

expn. 3. If expression matches value expi, the program branches to the

statement(s) following expiNote: Must “break out”of switch construct after statement(s)executed.

4. If no matching value is found, the program branches to the statement after default

break statement

• Used to stop execution in the current block

• Also used to exit a switch statement

• Useful to execute a single case statement without “falling through” and executing the statements following it

Example

switch (ch) { case 'a': cout << ch; case 'b': cout << ch; case 'c': cout << ch;}

If ch equals ‘a’, what happens?

switch (ch) { case 'a': cout << ch; break; case 'b': cout << ch; break; case 'c': cout << ch;}

A Switch Statement

switch (ch) {

case 'a': case 'A':

case 'e': case 'E':

case 'i': case 'I':

case 'o': case 'O':

case 'u': case 'U':

cout << ch << " is a vowel" << endl;

break;

default:

cout << ch << " is not a vowel" << endl;

}

cout << "Enter simple expression: ";

int Left;

int Right;

char Operator;

cin >> Left >> Operator >> Right;

cout << Left << " " << Operator << " " << Right

<< " = ";

switch (Operator) {

case '+' : cout << Left + Right << endl; break;

case '-' : cout << Left - Right << endl; break;

case '*' : cout << Left * Right << endl; break;

case '/' : cout << Left / Right << endl; break;

default: cout << "Illegal operation" << endl;

}

Example

Logical Operators - examples

int x = 12, y = 5, z = -4;

(x > y) && (y > z) true

(x > y) && (z > y) false

(x <= z) || (y == z) false

(x <= z) || (y != z) true

!(x >= z) false

Common Error 1: Forgetting Necessary Braces

if (radius >= 0) area = radius * radius * PI; cout << "The area " << " is " << area;

(a) Wrong

if (radius >= 0) { area = radius * radius * PI; cout << "The area " << " is " << area; } (b) Correct

Common Error 2: Wrong Semicolon at the if Line

if (radius >= 0); { area = radius * radius * PI; cout << "The area " << " is " << area; } (a)

Equivalent

Logic Error

if (radius >= 0) { }; { area = radius * radius * PI; cout << "The area " << " is " << area; } (b)

Empty Body

Common Error 4: Redundant Testing of Boolean Values

if (even == true) cout <<"It is even.";

(a)

Equivalent if (even) cout << "It is even.";

(b) This is better

Common Error 3: Mistakenly Using = for ==

if (count = 1) cout << "count is zero" << endl;else cout << "count is not zero" << endl;

Repetition - looping

Suppose that you need to print a string (e.g. "Welcome to C++!") a hundred times.

- It would be tedious to have to write the following statement a hundred times:

cout << "Welcome to C++!" << endl;

So, how do you solve this problem?

Answer: Use one of the three looping constructs available

The while Loop

• Loop: a statement or set of statements that may execute more than one time (repeats)

Syntaxwhile (expression)

statement;

Will execute statement repeatedly while expression evaluates to true

• statement can also be a block of statements enclosed in { }

while Loop

• no ; after (expression)

• while is a pre-test loop – expression is evaluated before the loop executes

• Loop may not be executed at all! – pre-test

• loop must contain code to make expression become false

• Infinite loop: loop that does not stop

Example

int count = 0;while (count < 100) { cout << "Welcome to C++!\n"; count++;}

while Loop Example

int val = 5;while (val <= 8)

cout << val++ << endl;

produces output:5678

Counters

• Counter: variable that is incremented or decremented each time a loop repeats

• Can be used to control execution of the loop (called a loop control variable)

• Must be initialized before entering loop

• May be incremented/decremented either inside the loop or in the loop test

Letting the User Control a Loop

int num = 1, limit;cout << "Table of squares\n";cout << "How high to go? ";cin >> limit;cout << "number square\n";while (num <= limit){ cout << num <<“ “ << num*num << endl;

num++;}

The do-while Loop

• do-while: a post-test loop – execute the loop, then test the expression

Syntaxdo

statement; // or block in { } while (expression);

• Note ; after (expression)

do-while Loop

• Loop always executes at least once – post-test

• Execution continues as long as expression is true, stops repetition when expression becomes false

• Useful in menu-driven programs to bring user back to menu to make another choice

Statement(s)

true

false

Expression

Loop Continuation Condition?

true

Statement(s) (loop body)

false

for Loop

• Useful for counter-controlled loop

Syntaxfor (initialization; test; update)

statement; // or block in { }

Or, re-stated with a counter variable

for (i = initialValue; i < endValue; i++) statement; // or block in { }

int i;for (i = 0; i < 100; i++) { cout << "Welcome to C++!\n";}

Example

for Loop - Example

int sum, num;

for (sum=0, num=1; num <= 10; num++)sum += num;

cout << "Sum of numbers 1 through 10 is" << sum << endl;

Nested Loops

• A nested loop is a loop inside the body of another loop

• Inner (inside), outer (outside) loops

Example

if (i > k) { if (j > k) cout << "i and j are greater than k";}else cout << "i is less than or equal to k";

Example

for (row=1; row<=3; row++) //outer

for (col=1; col<=3; col++) //inner

cout << row * col << endl;

For each repetition of outer loop, inner loop is executed 3 times.

Nested Loops

• Inner loop goes through all repetitions for each repetition of outer loop

• Inner loop repetitions complete sooner than outer loop

• Total number of repetitions for inner loop is product of number of repetitions of the two loops.

• In previous example, inner loop repeats 9 times