1. Flow Control if, while, do-while Juan Marquez (03_flow_control.ppt)
CIS162AB - C++
Flow Control if, while, do-while
Juan Marquez (03_flow_control.ppt)

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
CIS162AB

3. 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. 
CIS162AB

4. Flow Control The order in which statements are executed.
There are four control structures.
Sequence Control Structure
Selection Control Structure
Also referred to as branching (if and if-else)
Repetition Control Structure (loops)
Case Control Structure (switch)
CIS162AB

5. 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.
CIS162AB

6. Flowchart Symbols Input or Output Begin or End Decision Processing
Branch or Direction of Flow
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7. 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.
CIS162AB

8. Flowchart – Sequence Control
Begin
Input price, qty
subtotal = price * qty;
Output subtotal
End
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9. 2. Selection Control ( if )
Simple if with single statement.
discountRate = 1.00;
if (qty > 25)
discountRate = .98;
subtotal = qty * (price * discountRate);
CIS162AB

10. Flowchart – if statement
discountRate = 1.00;
If qty > 25
True
discountRate = .98;
False
Subtotal = qty * (price * discountRate);
CIS162AB

11. 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;
CIS162AB

12. Flowchart – if-else statement
DISCOUNT_RATE = .98;
If qty > 25
Subtotal = qty * (price * DISCOUNT_RATE);
True
False
Subtotal = qty * price;
Output subtotal
CIS162AB

13. 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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14. 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;
CIS162AB

15. Incorrect if-else regularPay = 40 * payRate;
if (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.
CIS162AB

16. Boolean Expressions Simple Expressions Compound 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)
CIS162AB

17. 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.
CIS162AB

18. Or Operator - | | Either condition must be true.
if (qty < 1 || qty > 50) {
cout << “Quantity must be ”; }
else
subtotal = qty * price;
What happens if qty = 0, qty = 25, or qty = 60?
CIS162AB

19. 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 - &&.
CIS162AB

20. 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.
CIS162AB

21. 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 = T
CIS162AB

22. Nested if Statements - Indented
if (qty > 0)
if (qty < 51)
subtotal = qty * price;
else
cout << “Quantity must be < 51”;
cout << “Quantity must be > 0”;
CIS162AB

23. 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.
CIS162AB

24. Multiway Branching Indenting not always practical
if (qty < 26)
discount = 0
else
if (qty < 51)
discount = .01
if (qty < 76)
discount = .02
if (qty < 101)
discount = .03 … more
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25. Multiway Branching if (qty < 26) discount = 0 else if (qty < 51)
CIS162AB

26. Conditional Operator Expression
if (qty > 25)
discountRate = 0.98;
else
discountRate = 1.00;
discountRate = (qty > 25) ? .98 : 1.00;
CIS162AB

27. 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.
CIS162AB

28. 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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29. While Loop Example count = 3; //initialize controlling variable
while (count > 0) {
cout << count;
count--; }
Output:
3 2 1 
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30. 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.
CIS162AB

31. Flowchart – While Loop Initialization important for While Loop
count = 3
while count > 0
False
True
Output count
Represents Loop
Skip or Exit Loop
count--
Next statement
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32. Do-while Loop Example count = 3; do { cout << count; count--;
}while (count > 0);
Output:
3 2 1
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33. 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.
CIS162AB

34. Flowchart – Do-while Loop
count = 3
Output count
Loop will be executed at least one time, because the condition is at the bottom.
count--
Represents Loop
while count > 0
True
False - Exit Loop
Next statement
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35. 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.
CIS162AB

36. Nested Loops count = 3; while (count > 0){
cout << endl << count;
count--;
count2 = 1; do
cout << count2;
count2++;
}while (count2 < 4); } 3
1 2 3 2
1 2 3 1
1 2 3
CIS162AB

37. 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--; }
CIS162AB

38. 4. Case Control Structure
Switch statement covered later.
CIS162AB

39. 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
CIS162AB