1. Control Structures
Control structures control the flow of program execution.
3 types of control structures: sequence, selection repetition
A sequence control structure uses
compound statements (blocks) to
specify sequential flow.
A compound statement, a block of
codes, is used to specify sequential flow. {
statement1;
statement2;
statement3; }
Sequence .

2. Selection
Use if or switch statements to select one from many alternatives.
if selection structure
if (grade >= 60)
cout << “Passes” << endl;
if structure
Grade >= 60 T
print”Passed” F

3. If/else structure if / else selection structure if / else structure
if (grade >= 60)
cout << “Passes” << endl;
else
cout << “Failed” << endl;
if / else structure
Grade >= 60 T
print”Failed”
print”Passed” F

4. if / else if / else Selection Structure
if ( grade >= 90 )
cout << “A grade” << endl;
else if (grade >= 80)
cout << “B grade” << endl;
else if ( grade >= 70)
cout << “C grade” << endl;
else if ( grade >= 60)
cout << “D grade” << endl;
else
cout << “F grade” << endl;

5. Conditions In general, condition has the following form: p1 op p2
Where p1 = variable
p2 = variable / constant
op = relational ( <, >, <=, >=) or
equality operator (==, != ).
Example: x <= 100
2. num != SENTINEL

6. Logical Operators (&&, ||, !)
With the help of logical operators you can form more complicated conditions or logical expressions.
3 logical operators are && (and), || (or), ! (not)
Truth table for && operator
Op2
Op1 && Op2
Op1 T F
When both of the
operands are true,
the result will be true.

7. Logical Operator || (or)
When both operands are false, the result will be false, otherwise true.
Truth table for || operator T F
Op1
Op2
Op1 || Op2

8. Logical operator ! (not)
Has a single operand.
Yields the logical complement or negation.
Example: !(flag) it evaluates to 1 if flag = 0.
Truth Table for !Operator T F
Op1
!Op1

9. Operator Precedence Operator Precedence function calls highest ! + - &
! &
* / %
+ -
< <= >= >
== !=
&& ||
= lowest

10. Short-Circuit Evaluation
C++ evaluates only part of the expression.
An expression a || b is true if a is true.
C++ stops evaluating when it determines a =1.
An expression a && b is false if a is false.
C++ stops evaluating when it determines a = 0.
This technique is called short-circuit evaluation.

11. Example !flag || (y + z >= x - z)
This expression is of the form a || b.
a = !flag
C++ would stop evaluating when it determines a = !flag = 1.
From the truth table, the value of the expression does not depend on b if a = 1.
!flag || (y + z >= x - z) = 1. x y z
flag
3.0
4.0
2.0

12. Complementing a Logical Expressions
Temp is in the range 60 to 70 F, inclusive.
Logical expression: <= temp && temp <= 70
Evaluation: (temp = 65): 1 && 1 => 1
(temp = 45): 0 && 1 => 0
Temp is outside the range 60 to 70 F (complement).
Logical expression: ! (60 <= temp && temp <= 70)
Alternative 60 > temp || temp > 70
Evaluation: (temp = 65): ! (1 && 1) => 0
(temp = 45): ! (0 && 1) => 1
Alternative (temp = 65): (0 || 0) => 0
(temp = 45): (1 || 0) => 1

13. De Morgan Theorem  60 > temp || temp > 70 Rule 1
Using De Morgan theorem, you can simplify the logical expression.
Rule 1: ! (expr1 && expr2)  !expr1 || !expr2
Rule 2: ! (expr1 || expr2)  !expr1 && !expr2
Example:
! (60 <= temp && temp <= 70)
 60 > temp || temp > Rule 1
!(temp < = 30 || (condition == ‘R’)
 temp > 30 && condition != ‘R’ Rule 2

14. if and if /else statements
One alternative:
if ( x != 0.0)
product = product * x;
Double alternatives:
if (temp > 32.0)
cout << “Above freezing” << endl;
else
cout << “Freezing” << endl;

15. Nested if statements Multiple alternatives if (x < 0.0) {
cout << “negative”;
absx = -x; }
else if (x == 0.0)
cout<< “zero”;
absx = 0.0;
else
cout << “positive”;
absx = x;
if (x < 0.0) {
cout << “negative”;
absx = -x; }
else
if (x == 0.0)
cout << “zero”;
absx = 0.0;
cout << “positive”;
absx = x;

16. Nested if vs.sequence of ifs
More readable and efficient
if (x < 0.0) {
cout << “negative”;
absx = -x; }
if (x == 0.0)
cout << “zero”;
absx = 0.0;
if (x > 0.0)
cout << “positive”;
absx = x;
Sequence
of ifs
Nested if
if (x < 0.0) {
cout << “negative”;
absx = -x; }
else
if (x == 0.0)
cout << “zero”;
absx = 0.0;
cout << “positive”;
absx = x;

17. Switch structure
Switch structure selects one from several alternatives depending on the value of the controlling expression.
The controlling expression can be type int or char, but not type double.
First the expression is evaluated, then the list of case labels is searched until one matches the expression value.
Statements following the matching case level are executed until a break statement is encountered.
The break causes an exit from the switch statement.
Execution continues with the statement that follows the closing brace of the switch statement body.
If no case level matches the controlling expression value, the statement following the default label are executed. If there is no default label, the entire switch statement body is skipped.

18. Switch structure switch (grade) {
case ‘A’ : cout << “Excellent” << endl;
break;
case ‘B’ : cout << “Good” << endl;
case ‘C’ : cout << ”O.K.” << endl;
case ‘D’ : cout << “Poor” << endl;
case ‘F’ : cout << “Failed” << endl;
default : cout << “Invalid letter grade” << endl; }

19. Flowchart for switch structure
Case A
Excellent
break
Case B
Good
break
Case C
O.K.
break
Case D
Poor
break
Case F
Failed
break
invalid

20. Switch structure
How many lines of output will be produced by the following program fragment ?
int x;
x = 0;
switch (x) {
case 0 : cout << "got 0“ << endl;
case 1 : cout << "got 1“ << endl;
case 2 : cout << "got 2“ << endl;
default : cout << "do not have 0, 1 or 2“ << endl; }

21. Switch and if /else if /else structure{
case 5 : c = c + b;
case 2 : c = c + 2*b;
break;
case 3 : c = 7;
case 6 : break;
case 7 : c = c + 9;
case 4 : case 1 : c *= c;
default : c %= 2; }
if (a == 5) {
c = c + b;
c = c + 2*b; }
else if (a == 2) c = c +2*b;
else if (a == 3) c = 7;
else if (a == 6) ;
else if (a == 7) c = c + 9;
else if ((a == 4) || (a == 1) c * = c;
else c % = 2;

22. if /else and switch structures
if / else structure
Switch structure
Switch (i) {
case 1 :
case 2 : j = k - 2;
k += 3;
break;
case 3 : j = k + 3;
++k;
j = k - 2;
k += 3;
case 4 : j = k + 3;
default : j = k + 5; }
if ((i > 0) && ( i < 5)) {
if ( i > 2)
j = k + 3;
++k; }
if ( i < 4)
j = k - 2;
k += 3;
else
j = k + 5;

23. Repetition & Loop Statement
A type of program control structure.
A loop is a group of instructions the computer executes repeatedly while some loop-continuation condition remains true.
Be sure to verify that a loop’s repetition condition will eventually become false, otherwise an infinite loop may result.
Three C++ loop control structures are:
while
do / while
for

24. Flow Diagram of Loop ChoiceNo
No loop
required
Any steps
repeated ?
Yes
Use of the conditional loop:
- sentinel-controlled
End-of-file-controlled
Flag-controlled
- input validation
- general conditional
Know in
advance how
many times to
repeat ? No
Yes
Use a counting loop

25. Counter-Controlled Repetition
A control variable is used to count the number of repetitions.
The control variable is incremented (or decremented) each time the group of instructions is performed.
When the value of the control variable indicates that the correct number of repetitions has been performed, the loop terminates.
Counter controlled repetition requires:
Name of a control variable (loop counter).
Initial value of the control variable.
Increment (or decrement) by which the control variable is modified each time through the loop.
Condition that tests for the final value of the control variable.

26. Counter-Controlled repetition with the while Loop
#include <iostream>
int main() {
int counter = 1; // intitialization
while (counter <= 10) // repetition condition
cout << counter << endl;
++counter; // increment }
return 0;

27. Flowchart for while structure
++counter <= 10
cout << counter
<< endl; T F

28. Do/While Repetition Structure
#include <iostream>
int main() {
int counter = 1; do
cout << setw(2) << counter ;
} while (++counter <= 10)
cout << endl;
return 0; }
Do-while always executes
at least once.
Use a do-while only when there is
a possibility of zero loop iterations.
output

29. Flowchart for do / while Structure
++counter <= 10
cout <<
counter; T F

30. Counter-Controlled repetition with the for Loop
#include <iostream>
int main() {
int counter;
for (counter = 1; counter <= 10; counter++)
initialization repetition condition increment
cout << counter << endl;
return 0; }

31. Flowchart of a for structure
Establish initial value
of control variable
counter = 1
Test if final value
of control variable
has been reached
counter <= 10 T
cout <<
counter;
counter++ F
Body of loop
Increment the
control variable

32. The Break Statement #include <iostream> Output int main() {
int x;
for (x = 1; x <= 10; x++)
if (x == 5)
break; // break loop only if x == 5
cout << setw(2) << x; }
cout << endl;
cout << “Broke out loop at x == “ << x << endl;
return 0;
Output
Broke out of loop at x == 5

33. The Continue Statement
#include <iostream>
int main() {
int x;
for (x = 1; x <= 10; x++)
if (x == 5)
continue; // skip remaining code in loop
// only if x == 5
cout << x; }
cout << endl;
cout << “Used continue to skip printing the value 5” << endl;
return 0;
Output
Used continue to skip printing the value 5

34. Sentinel-Controlled Loop
Sentinel values are used to control repetition when
the precise number of repetitions is not known in advance.
The loop includes statements that obtain data each time the loop is performed.
Sentinel value indicates “end of data”.
Sentinel is entered after all regular data items have been supplied to the program.
Sentinels must be distinct from regular data items.

35. Sentinel-Controlled while Loop
#include <iostream>
const SENTINEL = - 99;
int main() {
int sum = 0,
score;
cout << “Enter first score or “ << SENTINEL << “to quit”<< endl;
cin >> score;
while (score != SENTINEL)
sum += score;
cout << “Enter next score or “<< SENTINEL<< “to quit”<< endl;
cin>> score; }
cout << "Sum of exam scores is “ << sum << endl;
return (0);

36. End-Of-File-Controlled Loop
#include <iostream>
#include <fstream>
int main() {
char inchar;
ifstream myinfile;
myinfile.open("C:input.dat");
if (!myinfile)
cout <<"cannot open file"<< endl;
return 1; }

37. End-Of-File-Controlled Loop
myinfile.get(inchar);
while (myinfile) {
cout <<inchar; }
return 0;

38. Flag-Controlled Loop #include <iostream> int main() { int num;
bool found = false;
while (!found)
cout << “Enter a number” << endl;
cin >> num;
if (num == 4)
cout << “Target found”<< endl;
found = true; }

39. Validating input using do-while statement
Get data value.
If data value isn’t in the acceptable range,
go back to first step. do {
cout << “Enter a letter from A through E>”;
cin >> letter_choice;
} while (letter_choice < ‘A’ || letter_choice > ‘E’);

40. General Conditional Loop
Initialize loop control variable.
As long as exit condition hasn’t been met,
continue processing.
for ( radiation_lev = init_radiation;
radiation_lev > min_radiation;
radiation_lev /= 2.0) {
if (radiation_lev > SAFE_RAD)
cout << “Unsafe”<< endl;
else
cout << “Safe” << endl; }