CIS162AB - C++

Flow Control if, while, do-while

Juan Marquez

(03_flow_control.ppt)

CIS162AB 2

Overview of Topics• Pseudocode

• Control Structures

• Flowcharts

• Single and Compound Boolean Expressions

• Single and Compound Statements

• If, if-else, nested ifs

• While, do-while, nested loops

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Pseudocode

• Pseudocode is a mixture of C++ code and English like statements.

• Used when designing algorithms.• When designing, we don’t necessarily want to be

concerned about where semi-colons go. We want to concentrate on the design.

• I’ll use pseudocode throughout the course, so don’t feel compelled to correct my syntax.

• However, if at anytime you are not sure about a command, please be sure to ask for clarification.

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Flow Control

• The order in which statements are executed.

• There are four control structures. 1. Sequence Control Structure

2. Selection Control Structure• Also referred to as branching (if and if-else)

3. Repetition Control Structure (loops)

4. Case Control Structure (switch)

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Flowcharting

• A flowchart is a pictorial representation of an algorithm or logical steps.

• Each step is represented by a symbol and the arrows indicate the flow and order of the steps.

• The shape of the symbol indicates the type of operation that is to occur.

• Flowcharts may help the move visual students learn and understand logic.

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Flowchart Symbols

Begin or End Input or Output

Processing Decision

Branch or Direction of Flow

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1. Sequence Control Structure

• The order statements are placed (sequenced)

cin >> price >> qty;subtotal = price * qty;cout << subtotal;

• The only way to display the subtotal, statements must be in this order.

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Flowchart – Sequence Control

Begin

Input price, qty

subtotal = price * qty;

Output subtotal

End

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2. Selection Control ( if )

Simple if with single statement.

discountRate = 1.00;

if (qty > 25)

discountRate = .98;

subtotal = qty * (price * discountRate);

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Flowchart – if statement

discountRate = 1.00;

If qty > 25

Subtotal = qty * (price * discountRate);

discountRate = .98;True

False

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Selection Control (if-else)• Simple if-else with single statements.

DISCOUNT_RATE = .98;

if (qty > 25)subtotal = qty * (price * DISCOUNT_RATE);

else

subtotal = qty * price;

cout << subtotal;

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Flowchart – if-else statement

DISCOUNT_RATE = .98;

If qty > 25

Subtotal = qty * price;

Subtotal = qty * (price * DISCOUNT_RATE);

Output subtotal

True

False

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Boolean Expressions

• Boolean expressions evaluate to true or false.

• Must be enclosed in parenthesis

if (Boolean expression)

true statements (Yes)

else

false statements (No)

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Compound Statements • Use braces to create a block of statements.

if (hours > 40){

regularPay = 40 * payRate;overtimePay = (hours – 40) * (payRate * OVERTIME_RATE);

}else{

regularPay = hours * payRate;overtimePay = 0;

}

grossPay = regularPay + overtimePay;

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Incorrect if-elseif (hours > 40){

regularPay = 40 * payRate;overtimePay = (hours – 40) * (payRate * OVERTIME_RATE);

}else

regularPay = hours * payRate;overtimePay = 0;

grossPay = regularPay + overtimePay;

• overtimePay would always be set to zero.

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Boolean Expressions

• Simple Expressions– Have one comparison– (hours > 40)

• Compound Expressions– Have more than one comparison– Created by using And and Or operators– And - (qty > 0 && qty < 51)– Or - (qty < 0 || qty > 51)

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And Operator - &&• Both conditions must be true.

if (qty > 0 && qty < 51){

subtotal = qty * price;}

else

cout << “Quantity must be 1 – 50.”;

• What happens if qty = 0, qty = 25, or qty = 60?• A block with a single statement is valid.

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Or Operator - | |• Either condition must be true.

if (qty < 1 || qty > 50){

cout << “Quantity must be 1 -50.”;}

else

subtotal = qty * price;

• What happens if qty = 0, qty = 25, or qty = 60?

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Pipe Character - |

• Where is the pipe character on the keyboard?• On most keyboards

– It is right above the Enter key– Shares the key with the back slash - \– Must hold the shift key go get it– Instead of a solid line, it is shown as a broken line

• For the Or operator, 2 pipe characters must be entered - | |.

• For the And operator, 2 ampersands characters must be entered - &&.

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Notes on Boolean Expressions

• Short-circuit evaluation – if the evaluation of the entire expression can be determined by the result of the first expression, the second expression is not evaluated.

int y =1, x =0;

if (y > 0 | | x > 0) // T or F

• Not operator (!):( y != 0) //This statement is OK, but the not( !(y= =0)) //outside the parentheses can be confusing.

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Booleans Only True or False

• C++ does the math to get to a True or False.• true = 1, false = 0• Non-zeros converted to true, which is a one.

  if ( (5 && 7) + (!6) )

T && T + (!T)

T + F

1 + 0 = 1 = T

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Nested if Statements - Indented

if (qty > 0)

if (qty < 51)

subtotal = qty * price;

else

cout << “Quantity must be < 51”;

else

cout << “Quantity must be > 0”;

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Matching else to if

How are else statements matched with an if?Compiler works it’s way back. When an else is

encounter, it looks back to find an if that has not been matched to an else.

Why do we indent each level?We indent to make programs easier to read.

Indenting has no effect on how compiler matches an else to an if.

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Multiway BranchingIndenting not always practical

if (qty < 26)discount = 0

elseif (qty < 51)

discount = .01else

if (qty < 76)discount = .02

elseif (qty < 101)

discount = .03 … more

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Multiway Branching

if (qty < 26)discount = 0

else if (qty < 51)discount = .01

else if (qty < 76)discount = .02

else if (qty < 101)discount = .03

elsediscount = .04;

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Conditional Operator Expression

if (qty > 25)discountRate = 0.98;

elsediscountRate = 1.00;

discountRate = (qty > 25) ? .98 : 1.00;

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Assignment (=) vs Comparison (==)

• if (x = 12) // always true• if (x == 12) // comparison

• To prevent accidental assignment, state constant first.

• if (12 = x)• Compiler will report this as a syntax error.

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3. Repetition Control (loops)

• loop – a group of statements that are repeated until a certain conditions occurs to stop it.

• while loop

• do-while loop

• for-loop (covered later)

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While Loop Examplecount = 3; //initialize controlling variable

while (count > 0){cout << count;count--;

}

Output:

3 2 1 

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While Loop

• Controlling Boolean expression evaluated before executing loop body.

• Controlling variable must be initialized.• Boolean expression must be true to enter loop body.• No semicolon after Boolean expression.• It is possible that body is not executed at all.• Boolean expression must be false to get out of loop.• Controlling variable must be modified inside loop.• Execution continues with next statement after loop.

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Flowchart – While Loopcount = 3

while count > 0

Output count

count--

Next statement

False

True

Skip or Exit LoopRepresents Loop

Initialization important for While Loop

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Do-while Loop Examplecount = 3;do{cout << count;count--;

}while (count > 0);

Output:

3 2 1

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Do-while Loop

• Controlling Boolean expression evaluated after executing body.

• So, body is always executed at least one time.• Initialization of controlling variable not necessarily

required if it will be initialized inside the loop. • Boolean expression must be false to get out of loop.• Controlling variable must be modified inside loop.• Semicolon required after Boolean expression.• Execution continues with next statement after loop.

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Flowchart – Do-while Loopcount = 3

while count > 0

Output count

count--

Next statement

False - Exit Loop

True

Loop will be executed at least one time, because the condition is at the bottom.

Represents Loop

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Loop Summary

• Incrementor: count++• Decrementor: count--• Infinite loop – expression that always evaluates to true

– Avoid using = = as the operator.

– Controlling variable must be altered within the loop.

– Use Control-C to terminate an infinite loop.

• When to use a while or do-while will become evident as we continue to use and learn each loop.

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Nested Loopscount = 3;while (count > 0){

cout << endl << count;count--;count2 = 1;do{

cout << count2;count2++;

}while (count2 < 4);}

3 1 2 32 1 2 31 1 2 3

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While Loop Error

• Semi-colon after expression creates an infinite loop with a single statement that does nothing.

• DO NOT DO THIS.

while (count > 0);{cout << count;count--;

}

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4. Case Control Structure

• Switch statement covered later.

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Summary

• Control Structures

• Flowcharts

• Single and Compound Boolean Expressions

• Single and Compound Statements

• If, if-else, and nested ifs

• While, do-while, and nested loops