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

.

Selection• Use if or switch statements to select one from

many alternatives.

• if selection structure if (grade >= 60)

cout << “Passes” << endl;

Grade >= 60T

print”Passed”

F

if structure

If/else structure

• if / else selection structure if (grade >= 60)

cout << “Passes” << endl;

else

cout << “Failed” << endl;

Grade >= 60T

print”Failed” print”Passed”F

if / else structure

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;

Conditions In general, condition has the following

form:

p1 op p2

Where p1 = variable

p2 = variable / constant

op = relational ( <, >, <=, >=) or

equality operator (==, != ).

Example: 1. x <= 100

2. num != SENTINEL

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

T T T

T F F

F T F

F F F

When both of the operands are true, the result will be true.

Op1 Op2 Op1 && Op2

Logical Operator || (or)• When both operands are false, the result will be false,

otherwise true.• Truth table for || operator

T T T

T F T

F T T

F F F

Op1 Op2 Op1 || Op2

Logical operator ! (not)• Has a single operand.

• Yields the logical complement or negation.

• Example: !(flag) it evaluates to 1 if flag = 0.

T F

F T

Op1 !Op1

Truth Table for !Operator

Operator Precedence

Operator Precedence

function calls highest

! + - &

* / %

+ -

< <= >= >

== !=

&&

||

= lowest

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.

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.

3.0 4.0 02.0

x y z flag

Complementing a Logical Expressions• Temp is in the range 60 to 70 F, inclusive. Logical expression: 60 <= 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

De Morgan Theorem• 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 > 70 Rule 1

!(temp < = 30 || (condition == ‘R’)

temp > 30 && condition != ‘R’ Rule 2

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;

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; } else { cout << “positive”; absx = x; }

Nested if vs.sequence of ifs 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;

}

if (x < 0.0) { cout << “negative”; absx = -x; }else if (x == 0.0) { cout << “zero”; absx = 0.0; } else { cout << “positive”; absx = x; }

More readable and efficient

Nested ifSequence of ifs

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.

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

Case A

Flowchart for switch structure

Case B

Case C

Case D

Case F

Excellent break

Good

O.K.

Poor

Failed

break

break

break

break

invalid

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;

}

Switch and if /else if /else structure

switch (a){ case 5 : c = c + b; case 2 : c = c + 2*b; break; case 3 : c = 7; break; case 6 : break; case 7 : c = c + 9; break; case 4 : case 1 : c *= c; break; default : c %= 2; break;}

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;

switchif / else if /else

if /else and switch structures

if ((i > 0) && ( i < 5)) { if ( i > 2) { j = k + 3; ++k; } if ( i < 4) { j = k - 2; k += 3; } } else j = k + 5;

Switch (i){ case 1 : case 2 : j = k - 2; k += 3; break; case 3 : j = k + 3; ++k; j = k - 2; k += 3; break; case 4 : j = k + 3; ++k; break; default : j = k + 5; break;}

if / else structure Switch structure

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

Flow Diagram of Loop Choice

Any stepsrepeated ?

Know in advance how many times to repeat ?

Use a counting loop

No loop required

Use of the conditional loop:- sentinel-controlled- End-of-file-controlled- Flag-controlled- input validation- general conditional

No

No

Yes

Yes

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.

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;

}

Flowchart for while structure

while structure

++counter <= 10 cout << counter<< endl;

T

F

Do/While Repetition Structure #include <iostream>

int main()

{

int counter = 1;

do

{

cout << setw(2) << counter ;

} while (++counter <= 10)

cout << endl;

return 0;

} 1 2 3 4 5 6 7 8 9 10

output

Do-while always executes at least once.

Use a do-while only when there isa possibility of zero loop iterations.

Flowchart for do / while Structure

++counter <= 10

cout << counter;

T

F

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;

}

Flowchart of a for structure

counter <= 10

counter = 1

cout <<counter;

counter++

Establish initial value of control variable

Test if final valueof control variablehas been reached

Body of loop Increment the control variable

T

F

The Break Statement #include <iostream> 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; }

1 2 3 4Broke out of loop at x == 5

Output

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; }

1 2 3 4 6 7 8 9 10Used continue to skip printing the value 5

Output

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.

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); }

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;

}

End-Of-File-Controlled Loop

myinfile.get(inchar);

while (myinfile)

{

cout <<inchar;

myinfile.get(inchar);

}

return 0;

}

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; } }}

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’);

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; }