L04 - Control Structures

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Lecture 4

Control Structures

MIT AITI 2004


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What are Control Structures?

Java will execute your code in a specific sequence

Control structures are a way to alter the natural

sequence of execution in a Java program

In other words, they act as direction signals to control

the patha program takes

Control structures include:

block statements

decision statements

loops


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Decision St

atem

ents


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If Statements

The if decision statement executes a statementconditionally

if (expression) {statement;

}

next_statement;

The expression must produce aboolean value, eithertrue orfalse

If expression returns true, statement is executed andthen next_statement

Ifexpression

returnsfalse

,statement

is not executedand the program continues at next_statement


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executestatement

executenext_statement

expression istrue?

if (expression)statement

next_statementyes

no


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If-Else Statements

The basic if statement can be extended by adding theelse clauseif (expression) {statement1;

}else{statement2;

}next_statement;

Again, the expression must produce aboolean value

Ifexpression returns true, statement1 is executeda

ndthen next_statement is executed.

Ifexpression returns false, statement2 is executed andthen next_statement is executed.


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expression istrue?

executestatement1

executestatement2

executenext_statement

if (expression){statement1

} else {statement2

}next_statement

noyes


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Here is an example of chained if-else statements:

if (grade == 'A')System.out.println("You got an A.");

else if (grade == 'B')System.out.println("You got a B.");

else if (grade == 'C')System.out.println("You got a C.");

elseSystem.out.println("You got an F.");


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Switch Statements

The switch statement enables you to test severalcases generated by a given expression. The expression must produce a result of type char,byte, short or int, but not long, float, ordouble.

For example:switch (expression) {

case value1:statement1;

case value2:

statement2;default:

default_statement;}

Every statement after the true case is executed


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y

y

n

n

switch (expression){

case value1:

// Do value1 thing

case value2:

// Do value2 thing

...

default:

// Do default action

}

// Continue the program

expression

equa

lsvalue1? Do value1 thing

Do value2 thing

Do default action

expressionequals

value2?

Continue the

program


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Break Statements in Switch Statements

The break; statement tells the computer to exit theswitch statement

For example:switch (expression) {

case value1:statement1;break;

case value2:statement2;

break;

default:default_statement;break;

}


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expression

equalsvalue1?

expression

equalsvalue2?

Dodefault action

Do value1 t

h

ing

Do value2 thing

break

break

break

Continue theprogram

switch (expression){

case value1:

// Do value1 thing

break;

case value2:

// Do value2 thing

break;

...default:

// Do default actionbreak;

}

// Continue the program

y

y

n

n


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Remember the Example

Here is an example of chained if-else statements:

if (grade == 'A')

System.out.println("You got an A.");

else if (grade == 'B')System.out.println("You got a B.");

else if (grade == 'C')

System.out.println("You got a C.");

else

System.out.println("You got an F.");


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Another way to do this same example is to use the

switch statement

Complicated if-else chains can be rewritten with theswitch statement

switch (grade) {case 'A':

System.out.println("You got an A.");

break;case 'B':

System.out.println("You got a B.");break;

case 'C':

System.out.println("You got a C.");break;default:

System.out.println("You got an F.");}


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Loops


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A loop allows you to execute a statement or block of

statements repeatedly.

There are three types of loops in Java:

1. for loops

2. while loops

3. Do-while loops (will not discuss in th

is course)


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The for Loopfor (initialization_expression;

loop_condition;increment_expression){//statement

}

The control of the for loop appear in parentheses and is made up ofthree parts.

1. The first part, the initialization_expression, sets theinitial conditions for the loop and is executed before the loop starts.

2. Loop executes so long as the loop_condition is true and exitsotherwise

3. The third part of the control information, theincrement_expression, is usually used to increment the loopcounter. This is executed at the end of each loop iteration.


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For example:int limit = 5;

int sum = 0;

for(int i = 1; i


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Another example:

for(int div = 0; div


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If there is more than one variable to set up or incrementthey are separated by a comma.

for(i=0, j=0; i*j


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The while Loop

while (expression){statement

}

This while loop executes as long as the given logical

expression between parentheses is true. Whenexpression is false, execution continues with the

statement following the loop block.

The expression is tested at the beginning of the loop, soif it is initially

false, the loop will not be executed at all.


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For example:

int limit = 7;

int sum = 0;int i = 1;

while (i < limit){

sum += i;i++;

}

Wha

t is theva

lue of sum ?21


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Test condition

is true?

Execute loop

statement(?)

Next statement

The while loop

y

n

Initialize count

Test condition

is true?

Execute loop

statement(s)

Increment

count

New statement

The for loop

y

n


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The continue Statement

The continue statement causes the loop to exit its currenttrip through the loop and start overat the first statement ofthe loop. Here are two examples:

Example 1:

int index = 0;while(index


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Example 2:

int sum = 0;for(int i=1; i


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Using the break Statement in Loops

We have seen the use of the break statement in the switchstatement.

In loops, you can use the break statement to exit the current

loop you are in. Here is an example:

int index = 0;

while (index


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Nested Loops

You can nest loops ofany kind one inside another to anydepth. Here is a example:

int totalCount = 0;

while(totalCount


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Final example:

for (int i = 1; i


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POP QUIZ

1. In the switch statement expression must producea result of what type?

2. What must be used to separate each section ofafor

statement.

3. Which statement causes a program to go back to thestatement that began a loop and then keep going fromthere.

4.W

ritea

for loop tha

t outputs 1001 in re

versesequence.

5. Write afor loop that outputs all numbers that aredivisible by 3 between 0-50.

char, byte, short, int


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POP QUIZ

1. In the switch statement expression must producea result of what type?

2. What must be used to separate each section ofafor

sta

tem

ent.

3. Which statement causes a program to go back to thestatement that began a loop and then keep going fromthere.

4. Write afor loop that outputs 1001 in reversesequence.

5. Write afor loop that outputs all numbers that aredivisible by 3 between 0-50.


; (semi-colon)


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POP QUIZ

In the switch statement expression must produce aresult of what type?

What must be used to separate each section ofa for

statement.

Which statement causes a program to go back to thestatement that began a loop and then keep going fromthere.

W

ritea

for loop tha


versesequence.

Write a for loop that outputs all numbers that aredivisible by 3 between 0-50.


; (semi-colon)

continue


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POP QUIZ



statement.


W

ritea

for loop tha


versesequence.



for(int i=100; i>=0;i--) {

System.out.println(i);}

; (semi-colon)

continue


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POP QUIZ



statement.


W

ritea

for loop tha


versesequence.



for(int i=100; i>=0;i--) {

System.out.println(i);}

; (semi-colon)

continue

for(int i=0;i

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L04 - Control Structures