introduction to programming with c control statements.ppt

control statements

1. Slide 1 of 64Lecture C
Lesson 3: Conditions and Loops
Unit 1: The if Statement

2. Slide 2 of 64Lecture C
The if Statement
•The Java if statement has the following syntax:
if (boolean-condition)
statement;
•If the Boolean condition is true, the statement is
executed; if it is false, the statement is skipped
•This provides basic decision making capabilities

3. Slide 3 of 64Lecture C
Tempreture
class Temperature {
static final int THRESHOLD = 65;
public static void main(String[] args) {
InputRequestor input = new InputRequestor();
int temperature =
input.requestInt(“Enter the temperature:” );
System.out.println(“Current temperature “ +
temperature);
if (temperature < THRESHOLD)
System.out.println(“It’s cold in here!”);
}
}

4. Slide 4 of 64Lecture C
If statement flow diagram
if (condition)
statement;
condition
statement
true

5. Slide 5 of 64Lecture C
Boolean Expressions
•The condition of an if statement must evaluate to a true
or false result
•Java has several equality and relational operators:

•More complex Boolean expressions are also possible
OperatorMeaning
==equal to
!=not equal to
<less than
<=less than or equal to
>greater than
>=greater than or equal to

6. Slide 6 of 64Lecture C
Block Statements
•Several statements can be grouped together into a block
statement
•Blocks are delimited by braces
•A block statement can be used wherever a statement is
called for in the Java syntax
if (boolean-condition){
statement1;
statement2;
…
}

7. Slide 7 of 64Lecture C
Example - Temperature2
class Temperature2 {
static final int THRESHOLD = 65;
public static void main(String[] args) {
InputRequestor input = new InputRequestor();
int temperature =
input.requestInt(“Enter the temperature:” );
System.out.println(“Current temperature “ +
temperature);
if (temperature < THRESHOLD) {
System.out.println(“It’s cold in here!” );
System.out.println(“But we’ll survive.” );
}
}
}

8. Slide 8 of 64Lecture C
If .. Else Statement
•An else clause can be added to an if statement to
make it an if-else statement:
if (condition)
statement1;
else
statement2;
•If the condition is true, statement1 is executed; if the
condition is false, statement2 is executed

9. Slide 9 of 64Lecture C
Example - Temperature3
class Temperature3 {
static final int FREEZING_POINT = 32;
public static void main(String[] args) {
InputRequestor input = new InputRequestor();
int temperature =
input.requestInt(“Enter temperature:”);
if (temperature <= FREEZING_POINT)
System.out.println(“It’s freezing!”);
else
System.out.println(“Above freezing.”);
}
}

10. Slide 10 of 64Lecture C
If/else flow diagram
if (condition)
statement1;
else
statement2;
condition
statement2
true
statement1

11. Slide 11 of 64Lecture C
Nested If statements
•Since an “If” statement is a statement, it can appear inside
another if statement.
if (condition1)
if (condition2)
statement;
•It can also appear in an “else” clause
if (condition1)
statement1;
else if (condition2)
statement2;

12. Slide 12 of 64Lecture C
Nested If Example
// Reads 2 integers and compares them
class CompareExample {
public static void main(String[] args) {
InputRequestor input = new InputRequestor();
int a = input.requestInt( “First number:”);
int b = input.requestInt( “Second number:”);
if (a != b){
if (a > b)
System.out.println(a+ ” is greater”);
else
System.out.println(b+ ” is greater”);
}else
System.out.println(“the numbers are equal” );
}
}

13. Slide 13 of 64Lecture C
Checking your Input
•When requesting input from the user, keep in mind that the input may be invalid.
•It is good practice to check the validity of user input

int numberOfItems =
input.requestInt(“Enter number of items:” );
if (numberOfItems < 0) {
System.out.println(
“Number of items must be positive!” );
} else {
double price = numberOfItems * ITEM_PRICE;
System.out.println(“The total price is:“ +price);
}

14. Slide 14 of 64Lecture C
Lesson 3: Conditions and Loops
Unit 2: Boolean Expressions

15. Slide 15 of 64Lecture C
Logical Operators
•Boolean expressions may be combined using logical
operators
•There are three logical operators in Java:
•They all take Boolean operands and produce Boolean
results
•Logical NOT is unary (one operand), but logical AND and
OR are binary (two operands)
OperatorOperation
!Logical NOT
&&Logical AND
||Logical OR

16. Slide 16 of 64Lecture C
Logical NOT
•The logical NOT is also called logical negation or logical
complement
•If a is true, !a is false; if a is false, then !a is true
•Logical expressions can be shown using truth tables
a!a
falsetrue
truefalse

17. Slide 17 of 64Lecture C
Logical AND
•The expression a && b is true if both a and b are true,
and false otherwise
•Truth tables show all possible combinations of all terms
aba && b
falsefalsefalse
falsetruefalse
truefalsefalse
truetruetrue

18. Slide 18 of 64Lecture C
Logical OR
•The expression a || b is true if a or b or both are true,
and false otherwise
aba || b
falsefalsefalse
falsetruetrue
truefalsetrue
truetruetrue

19. Slide 19 of 64Lecture C
Logical Operators
•Logical operators are used to form more complex logical
expressions
•Logical operators have precedence relationships between
themselves and other operators
if (a<1 || a%2!=0)
System.out.println(
“The input should be an even even number!” );

20. Slide 20 of 64Lecture C
Logical Operators
•Full expressions can be evaluated using truth tables
a < 1a%2!=0a<1 || a%2=0
falsefalsefalse
falsetruefalse
truefalsefalse
truetruetrue

21. Slide 21 of 64Lecture C
Boolean variables
•Boolean expressions can be assigned to Boolean
variables
•Boolean variables are Boolean expressions

boolean b, c;
b = (x > 17);
c = (x>17) && (x<60);
boolean b, c;
b = (x > 17);
c = b && (x<60);
if (c)
System.out.println(“x is in range”);

22. Slide 22 of 64Lecture C
Example - RightTriangle
// Receives the length of the edges of a triangle
// and determine if this is a right triangle
class RightTriangle {
public static void main(String[] args) {
InputRequestor input = new InputRequestor();
float a = input.requestInt( “Edge1:”);
float b = input.requestInt( “Edge2:”);
float c = input.requestInt( “Hypotenuse:”);
boolean test = a*a+b*b == c*c;
if (test)
System.out.println(“It’s a right triangle” );
else
System.out.println(“It’s not a right triangle” );
}
}

23. Slide 23 of 64Lecture C
Lesson 3: conditions and loops
Unit C3: The while Statement

24. Slide 24 of 64Lecture C
The while statement
•A while statement has the following syntax:
while (condition)
statement;
•If the condition is true, the statement is executed; then
the condition is evaluated again
•The statement is executed over and over until the
condition becomes false
•If the condition of a while statement is false initially, the
statement is never executed
•Therefore, we say that a while statement executes zero
or more times

25. Slide 25 of 64Lecture C
While statement flow diagram
while (condition)
statement;
condition
statement
true

26. Slide 26 of 64Lecture C
Example - Counter
// Counts from 1 to 5
class Counter {
static final int LIMIT = 5;
public static void main(String[] args) {
int count = 1;
while (count <= LIMIT) {
System.out.println(count);
count = count + 1;
}
System.out.println(“done”);
}
}

27. Slide 27 of 64Lecture C
Examples - Factors
// Gets an integer and prints its factors
class FactorsExample {
public static void main(String[] args) {
InputRequestor input = new InputRequestor();
int a = input.requestInt( “Enter a number:”);
int i = 1;
System.out.println(“The divisors of “+a+” are:”);
while (i <= a) {
if (a%i == 0) {
System.out.println(i);
}
i = i + 1;
}
}
}

28. Slide 28 of 64Lecture C
Infinite Loops
•The body of a while loop must eventually make the
condition false
•If not, it is an infinite loop, which will execute until the user
interrupts the program
•This is a common type of logical error -- always double
check that your loops will terminate normally

29. Slide 29 of 64Lecture C
Example - Forever
// This program contains an infinite loop
class Forever {
static final int LIMIT = 25;
public static void main(String[] args) {
int count = 1;
while (count <= LIMIT) {
System.out.println(count);
count = count - 1;
}
}
}

30. Slide 30 of 64Lecture C
Lesson 3: conditions and loops
Unit 4: More conditionals

31. Slide 31 of 64Lecture C
The Conditional Operator
•Java has a conditional operator that evaluates a Boolean
condition that determines which of two expressions is
evaluated
•The result of the chosen expression is the result of the
entire conditional operator
•Its syntax is:
condition ? expression1 :
expression2
•If the condition is true, expression1 is evaluated; if it is
false, expression2 is evaluated

32. Slide 32 of 64Lecture C
The Conditional Operator
•It is similar to an if-else statement, except that it is an
expression that returns a value
•For example:

•If a is greater that b, then a is assigned to max;
otherwise, b is assigned to max
•The conditional operator is ternary, meaning it requires
three operands
int max = (a > b) ? a : b;

33. Slide 33 of 64Lecture C
The Conditional Operator
•Another example:
•If count equals 1, "Dime" is printed, otherwise
"Dimes" is printed
System.out.println ("Your change is " + count +
((count == 1) ? "Dime" : "Dimes”));

34. Slide 34 of 64Lecture C
Another Selection Statement
•The if and the if-else statements are selection
statements, allowing us to select which statement to
perform next based on some Boolean condition
•Another selection construct, called the switch statement,
provides another way to choose the next action
•The switch statement evaluates an expression, then
attempts to match the result to one of a series of values
•Execution transfers to statement list associated with the
first value that matches

35. Slide 35 of 64Lecture C
The switch Statement
•The syntax of the switch statement is:
switch (expression) {
case value1:
statement-list1
case value2:
statement-list2
case …
}

36. Slide 36 of 64Lecture C
The switch Statement
•The expression must evaluate to an integral value, such
as an integer or character
•The break statement is usually used to terminate the
statement list of each case, which causes control to jump
to the end of the switch statement
•A default case can be added to the end of the list of
cases, and will execute if no other case matches

37. Slide 37 of 64Lecture C
The switch Statement
/**
* A client that enables you to connect to the
* bank server and make remote banking operations...
*/
public class BankClient {
public static final int VIEW_BALANCE = 1;
public static final int VIEW_SAVINGS = 2;
public static final int CASH_TRANSFER = 3;
public static final int VIEW_LAST_OPERATIONS = 4;
// ...

38. Slide 38 of 64Lecture C
The switch Statement
// Inside the main loop of the client:
int option =
InputRequestor.requentInt( “Enter your choice:” );
switch(option) {
case VIEW_BALANCE:
showBalance();
break;
case VIEW_SAVINGS:
showSavings();
break;
default:
output.showMessage(“No such option!”);
}

39. Slide 39 of 64Lecture C
Lesson 3: conditions and loops
Unit 5: Shorthand Operators

40. Slide 40 of 64Lecture C
Shorthand Operators
•Many operations are very commonly used
•Java has shorthand notations for these
increment and decrement operators
assignment operators
x = x + 1;
sum = sum + x;

41. Slide 41 of 64Lecture C
The Increment and Decrement Operators
•The increment operator (++) adds one to its integer or
floating point operand
•The decrement operator (--) subtracts one
•The statement
is essentially equivalent to

count++;
count = count + 1;

42. Slide 42 of 64Lecture C
The Increment and Decrement Operators
•The increment and decrement operators can be applied
in prefix (before the variable) or postfix (after the variable)
form
•When used alone in a statement, the prefix and postfix
forms are basically equivalent. That is,

is equivalent to

count++;
++count;

43. Slide 43 of 64Lecture C
The Increment and Decrement Operators
•When used in a larger expression, the prefix and postfix
forms have a different effect
•In both cases the variable is incremented (decremented)
•But the value used in the larger expression depends on
the form
ExpressionsOperationValue Of expression
count++add 1old value
++countadd 1new value
count--subtract 1old value
--countsubtract 1new value

44. Slide 44 of 64Lecture C
The Increment and Decrement Operators
•If count currently contains 45, then

assigns 45 to total and 46 to count
•If count currently contains 45, then

assigns the value 46 to both total and count
total = count++;
total = ++count;

45. Slide 45 of 64Lecture C
The Increment and Decrement Operators
•If sum contains 25, what does this statement print?

•Prints the following result:
25 27 27 27
•sum contains 26 after the line is complete
System.out.println (sum++ + " " +
++sum + " " +
sum + " " +
sum--);

46. Slide 46 of 64Lecture C
Assignment Operators
•Often we perform an operation on a variable, then store
the result back into that variable
•Java provides assignment operators that simplify that
process
•For example, the statement

is equivalent to

sum += value;
sum = sum + value;

47. Slide 47 of 64Lecture C
Assignment Operators
•There are many such assignment operators, always
written as op= , such as:
OperatorExampleEquivalent to
+=x+=yx = x + y
-=x-=yx = x - y
*=x*=yx = x * y
/=x/=yx = x / y
%=x%=yx = x % y

48. Slide 48 of 64Lecture C
Assignment Operators
•The right hand side of an assignment operator can be a
complete expression
•The entire right-hand expression is evaluated first, then
combined with the additional operation
•Therefore
result /= total-MIN;
is equivalent to

result /= total-MIN;
result = result / (total-MIN);

49. Slide 49 of 64Lecture C
Lesson 3: conditions and loops
Unit C6: More Repetition

50. Slide 50 of 64Lecture C
More Repetition Constructs
•In addition to while loops, Java has two other
constructs used to perform repetition:
the do statement
the for statement
•Each loop type has its own unique characteristics
•You must choose which loop type to use in each situation

51. Slide 51 of 64Lecture C
The do Statement
•The do statement has the following syntax:
do
statement
while (condition);
•The statement is executed until the condition becomes
false
•It is similar to a while statement, except that its
termination condition is evaluated after the loop body

52. Slide 52 of 64Lecture C
The do Statement
•The key difference between a do loop and a while loop
is that the body of the do loop will execute at least once
•If the condition of a while loop is false initially, the body
of the loop is never executed
•Another way to put this is that a while loop will execute
zero or more times and a do loop will execute one or
more times

53. Slide 53 of 64Lecture C
Do Statement Example
// Gets an integer and prints its factors
class AvgExample {
public static void main(String[] args){
InputRequestor input = new InputRequestor();
double x, sum=0, count=-1;
do {
x = input.RequestDouble( “Next number:”);
sum += x;
count++;
} while (x != 0);
// 0 is a flag indicating end of input
System.out.println(“The average is “+sum/count);
}
}

54. Slide 54 of 64Lecture C
The do Statement flow diagram
statement
condition
false
true

55. Slide 55 of 64Lecture C
The for Statement
•Many loops have a common pattern, captured by the for
statement
•The syntax of the for loop is
for (intialization; condition; increment)
statement;
•This is equivalent to
initialization;
while (condition) {
statement;
increment;
}

56. Slide 56 of 64Lecture C
The for Statement: examples
•Examples:
for (int count=1; count < 75; count++) {
System.out.println (count);
}

for (int num=1; num <= max; num = num * 2) {
System.out.println (“Next power of 2: “ + num);
}

57. Slide 57 of 64Lecture C
The for Statement
•The initialization is always performed once
•The condition of a for statement is tested prior to
executing the loop body (like in the while statement)
•Therefore, a for loop will execute zero or more times
•For loops are well suited for cases where the number of
iterations is known beforehand
•The increment is executed after each iteration of the loop

58. Slide 58 of 64Lecture C
Omitting parts in a for Statement
•Each expression in the header of a for loop is optional
If the initialization is left out, no initialization is performed
If the condition is left out, it is always considered to be true, and
therefore makes an infinite loop
If the increment is left out, no increment operation is performed
•Both semi-colons are always required
for (;;) {// an infinite loop
System.out.println ( “beep”);
}
// compute a value count
for (; count < max ; count ++ ) {
System.out.println (count);
}

59. Slide 59 of 64Lecture C
The for Statement flow diagram
statement
condition
false
true
initialization
increment

60. Slide 60 of 64Lecture C
Multiplication Table Example
class MultiplicationTable {
public static void main(String[] args){
for(int j=1 ; j <= 10 ; j++) {
for(int k=1 ; k <= 10 ; k++)
System.out.print(j*k);
System.out.println();
}
}
}

61. Slide 61 of 64Lecture C
The break and continue statements
•The break statement, which we used with switch
statements, can also be used inside a loop
•When the break statement is executed, control jumps to
the statement after the loop (the condition is not
evaluated again)
•A similar construct, the continue statement, can also be
executed in a loop
•When the continue statement is executed, control
jumps to the end of the loop and the condition is
evaluated

62. Slide 62 of 64Lecture C
Break and Continue Example
class AvgExample2 {
public static void main(String[] args){
InputRequestor in = new InputRequestor();
double x, sum = 0; count = 0;
while(true){
x = in.RequestDouble();
if (x == 0)
break;
if (x < 0) {
System.out.println(“Only positive numbers!” );
continue;
}
sum += x ;
count ++ ;
} // continued on next page

63. Slide 63 of 64Lecture C
Break and Continue Example (2)
System.out.println(“The average is “+sum/count);

}
}

64. Slide 64 of 64Lecture C
Why do We Need Indentation?
class Mystery {
public static void main(String[] args) {
InputRequestor in = new InputRequestor();
int dimension =
in.requestInt(“Please enter the dimension” );
for (int j = 0; j < dimension; j++) {
for (int k = 1; k < dimension - j; k++) {
System.out.print(" ");
}
for (int k = 0; k < j; k++) {
System.out.print("*");
}
System.out.println();
}}}