Control Statements

Define the way of flow in which the program statements should take place.

Control Statements

Implement decisions and repetitions. There are four types of controls in C:

Bi-directional control (if…else)

Multidirectional conditional control (switch) Loop controls

Unconditional control (goto)

for … loop while … loop do … while loop

Use, when there are two possibilities (alternatives) could happen.

If … else Statement

The if comes in two forms: if … if … else

Program should take different actions in the zero case and non-zero case.

E.g. 1 Did the user enter a zero or not?

Program should give real-valued solns for non-negative values of and

E.g. 2 = b2 – 4ac of a quadratic equation is non-negative or negative.

complex-valued solns for negative values of .

if statement tests a particular condition

If Statement

Syntax of the ‘if’ statement

//Single Statement

if ( test_expression )

statement;

//Multiple Statements

if ( test_expression ) {

statement_1;

statement_2;

…

}

if the condition evaluates as true (or non-zero) an action or set of actions is

executed Otherwise, the action(s) are ignored.

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Note: The test_expression must beenclosed within parentheses.

Flow Chart – if …Start

Test Expn

Body of if

End

True

False

End

if … else statement allows either-or condition by using an ‘else’ clause

If … else Statement

Syntax of the ‘if … else’ statement //Single Statement

if (test_expression)

statement_1;

else

statement_2;

//Multiple Statements

if (test_expression) {

statement(s);

}

else {

statement(s);

}

if the condition evaluates as true (or non-zero) action(s) in if clause is/are executed

Otherwise, the action(s) in else clause is/are executed

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Flow Chart – if … elseStart

Test Expn

Body of if

End

True

False

End

Body ofelse

Examples1) //Single Statement

if (x<0)

printf(“Error: Negative number”);

2) //Single Statement

if (mark < 40)

printf(“Very Bad: You failed the examination”);

else

printf(“Congratulations: You passed the examination”);

3) //Multiple Statements

if (op = = ‘e’ || op = = ‘E’) {

printf(“Bye: Use MyProg again”);

exit(0);

}

Examples …4) //Multiple Statements

if (delta >= 0) {

printf( “Equation has real roots\n”);

printf (“Root 1 = %f\n”, -b + sqrt( delta ) / ( 2 * a ));

printf (“Root 2 = %f\n”, -b - sqrt( delta ) / ( 2 * a ));

}

else { // delta is negativeprintf(“Equation has imaginary roots\n”);printf(“Root 1 = ( %f, %f)\n, -b, sqrt( - delta ) / ( 2 * a ));printf(“Root 2 = ( %f, %f)\n, -b, -sqrt( - delta ) / ( 2 * a ));

}

“if … else” statement can be nested within an another “if …else”.

More on “if … else”

E.g.

if ( op = = ‘e’ || op = = ‘E’ ) {

printf(“Do you want to exit? (1 - Yes/0 – N0)”);

scanf(“%d”, doExit); //doExit is an ‘int’ variable

if ( doExit = = 1 ) {

cout << “Bye: Use MyProg again”;

exit(0);

}

}

Nested within an “if …”statement

Watch out the indentation in the inner “if...” statement.

More on “if … else” … Matching the else clause

if ( a = = b )

if ( b = = c )

printf(“a, b and c are the same”);

else

printf( “a and b are different”);

The “else” clause is matched with the inner “if”.

If a = b = 3 and c = 2, what is the output? Output: a and b are different But a and b are same. Hmmm ! Then, What’s wrong with the code?

How would we correct it?

More on “if … else” … Matching the else clause …

//Correct Version

if ( a = = b ) {

if ( b = = c )

printf( “a, b and c are the same”);

}

else

printf( “a and b are different”);

Now “else” clause is matched with the outer “if”. Use braces { } to match the else clause

correctly. Note: In cases like this, braces are compulsory even we have a single statement.

“If … else” Ladder Computer programs, like life, may present

more than two selections. Can extend “if …else” to meet that need.

//Compute student’s grade

if ( mark >= 70 )

grade = ‘A’;

else

if ( mark >= 55 )

grade = ‘B’;

else

if ( mark >= 40 )

grade = ‘C’;

else

grade = “F’;

//Revise format

if ( mark >= 70 )

grade = ‘A’;

else if ( mark >= 55 )

grade = ‘B’;

else if ( mark >= 40 )

grade = ‘C’;

else

grade = “F’;

The Switch Statement Can use in C to select one of several

alternatives. E.g.

Screen menu that asks the user to select one of following four choices.

1: Add 2. Subtract

5. Exit

3: Multiply 4. Division

Useful when the selection is based on a value of a single variable (controlling variable) a value of a simple expression (controlling

expression).

The Switch Statement … General form:

switch ( controlling variable/expression ) {case const_1:

statement(s);break;

case const_2:statement(s);break;…

case const_nstatement(s);break;

default:statement(s);

} “break” statement is not necessary

after default statements

The Switch Statement … Switch statement works as follows:

If the control variable/expression evaluated as

const_1 statements under the case const_1 executed.

const_2 statements under the case const_2 executed.

const_n statements under the case const_n executed.

other value statements under the default executed.

“break” statement in each case causes exit from the switch statement.

Flow Chat - Switch Statement

Start

Control Variable

End

Body of case const_1

const_1

Body of case const_2

const_2

Body of case const_n

const_n

Body of default

Other Value

…

The Switch Statement … The value of this controlling variable or

expression may be of type int or char or long But not double or float.

The “default” statement is optional. If you omit it and there is no match, the program

jumps to the next statement following the switch.

When the program jumps to the particular case statement, it executes statements under it.

What would happen if one of the “break” statements omit?

Then it sequentially executes the following case statements until it reaches to a break statement.

Switch and if … else Both the “switch” and “if … else” statements

select a one from list of alternatives. “if … else” can handle ranges. But switch isn’t

designed to handle ranges. Each “switch case” label must be single valued.

If all the alternatives can be identified with integer constants.

When … “switch” statement?

Hmmm ! We can use “if … else” with integer constants. Why then a switch statement?

The “case label value” must be a constant.

More efficient in terms of code size and execution speed.

The Switch Statement … Rule of thumb

E.g. 1switch ( op ) {

case 1: //Addresult = num1 + num2;break;

case 2: //Subtractresult = num1 – num2;break;

case 3: //Multiplyresult = num1 * num2;break;

case 4: // Divisionif ( num2 != 0 )

result = num1 / num2;break;

case 5: // Exitexit(0);

default:cout << “Invalid key\n”;

}

Use switch statement if you have three or more alternatives.

See the Indentation

The Switch Statement … E.g. 2

switch (op) {case ‘a’:case ‘A’:

result = num1 + num2;break;

case ‘s’:case ‘S’:

result = num1 – num2;break;

case ‘m’:case ‘M’:

result = num1 * num2;break;

case ‘d’:case ‘D’:

if ( num2 != 0 )result = num1 / num2;

break;case ‘e’:case ‘E’:

exit(0);default:

cout << “Invalid key\n”;}

Loops Many jobs that are required to be done with

the help of a computer are repetitive in nature. E.g. Calculation of salary of different casual

workers in a factory. The salary is calculated in the same manner for each

worker (salary = no of hours worked * wage rate).

for … loop

C++ provides three kinds of loop controls

Such type of repetitive calculations can easily be done using loops.

while … loop do … while loop

For Loop Use to repeat a statement or a block of

statements a specified number of times. E.g. Calculation of salary of 1000 workers.

In advance, the programmer knows the loop must repeat 1000 times.

The usual parts of a for loop handle these steps: Setting an initial value to loop control variable(s)

Performing a test to see if the loop should continue

Executing the loop actions

Updating the loop control variable(s)

Setting curWorker (loop control variable) to 0.

Testing curWorker < 1000

Compute salary for the current worker

Move to the next worker

For Loop …

//Single Statement

for ( initialization; test_expn; update_expn )

statement;

General form:

//Multiple Statement

for ( initialization; test_expn; update_expn ) {

statement_1;

statement_2;

…

statement_n;

}

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For Loop … Initialization

Loop evaluates initialization just once (as soon as the loop is entered).

Typically, programs use this expression to initialize the loop control variable (E.g. curWorker = 0;)

This variable will also be used to count the loop cycles.

Test expression

If the test_expn (E.g. curWorker < 1000) is true (or non-zero), the loop body will be executed.

Otherwise the loop will be terminated.

For Loop … Update expression

is evaluated at the end of the loop, after the body has been executed.

Typically, it is used to increase or decrease the value of the loop control variable (E.g. curWorker++).

E.g.for ( int curWorker = 1; curWorker < 1000; curWorker++ )

salary[curWorker] = hoursWorked[curWorker] * wageRate;

InitializationTest expression Update expression

Loop body

Flow Chart – For Loop

Start

Initialization

Test Expn

Body of for

Update Expn

End

True

False

While Loop Use when we do not know the exact number

of repetitions before the loop execution begins. E.g. 1 Withdraw money from your bank account as

long as your bank balance is above Rs. 1000/-.

//Single Statement

while ( test_expression )

statement;

//Multiple Statements

while ( test_expression ) {

statement_1;

statement_2;

…

statement_n;

}

General form:

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While Loop … First a program evaluates the

test_expression. If the test_expression evaluates to a non-zero value (true), the program executes the

statement(s) in the body.

After finishing with the body, the program returns to the test_expression and reevaluates it.

If the test_expression is again non-zero, the program executes the body again.

This cycle of testing and execution continues until the test_expression evaluates to 0 (false).

Note: While loop does not execute its body if the test_expression is initially 0 (false).

Flow Chart – While Loop

Start

Test Expn

Body of while

End

True

False

While Loop … E.g.

while ( balance > 1000 ) {cout << “Input your withdraw amount: “;cin >> withdrawAmt;if ( balance – withdrawAmt <= 1000 )

cout << “Sorry: Balance is not sufficient\n”;else

balance = balance – withdrawAmt;}

Test expression

Loop body

Do … While Loop The While loop evaluates its test_expression

at the beginning of the loop.

But there are situations where you need to execute the body at least once.

Loop will not never execute if the test_expression is zero (false).

In such situations, you should use a do…while. Do … while loop executes its body first.

Next, it evaluates the test_expression. If the test_expression is non-zero (true)

executes the body again. This cycle of testing and execution continues

until the test_expression evaluates to 0 (false).

Do … while Loop (cont.)

//Single Statement

do

statement;

while ( test_expression );

//Multiple Statements

do {statement_1;statement_2;…statement_n;

} while ( test_expression ) ;

General form:

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E.g.do {

cout << “Enter the numerator: “; cin >> num;cout << “Enter the denomenator: “; cin >> den;cout << “Quotient is “ << num / den << “\n”;cout << “Remainder is “ << num % den << “\n”;cout << “Do another? (y/n): ”; cin >> doAgain;

} while (ch != ‘n’);

Flow Chart – Do …While Loop

Body ofdo…while

Start

True

Test Expn EndFalse

Break Statement Exits out of the current loop and transfers to

the statement immediately following the loop.

E.g.while ( i > 0 ) {

cout << “Count = “ << i++ << “\n”;if ( i = = 5 ) // breaks out the loop when i = 5

break; }cout << “End of program”;

Output

Count = 0Count = 1Count = 2Count = 3Count = 4End of program

Continue Statement The break statement takes you out of the

bottom of a loop. Sometimes you need to go back to top of the

loop, when something happens unexpectedly .

Occurs an runtime error as num / den is not defined.

E.g.do {

cout << “Enter the numerator: “; cin >> num;cout << “Enter the denomenator: “; cin >> den;cout << “Quotient is “ << num / den << “\n”;cout << “Remainder is “ << num % den << “\n”;cout << “Do another? (y/n): ”; cin >> doAgain;

} while (ch != ‘n’);

What is the output if the user keyed a zero (0) as den?

Continue Statement … Use a continue statement to move to the top

of the loop when the user inputs zero as den. E.g.

do {cout << “Enter the numerator: “; cin >> num;cout << “Enter the denomenator: “; cin >> den;if ( den = = 0 ) {

cout << “Illegal denomenator\n”;continue;

}cout << “Quotient is “ << num / den << “\n”;cout << “Remainder is “ << num % den << “\n”;cout << “Do another? (y/n): ”;cin >> doAgain;

} while (ch != ‘n’);