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Compound Statements
If you want to do more than one statement if an IF-
else case, you can form a block of statements, or
compound statement, by enclosing the list of
statements in curly braces ({ }). The statements in
side of the braces will be executed sequentially, and
the block counts as one statement.
Compound Statement Example
if (a > b) { c = a; System.out.println(a);}else { c = b; System.out.println(b);}
IF statement
If there is nothing to do if the test is false, then the
else part of the statement can be omitted, leaving
just an IF statement:
if (n > 0)
n = n + 1;
Nested If statements
If statements can be nested, that is, the statement to
be done when the test is true (or false) can also be
an If statement. Consider printing out three numbers
in order:
An Extended Example
if(a > b)
if(b > c)
System.out.println(a + " " + b + " " + c); else // b <= c
if(a > c)
System.out.println(a + " " + c + " " + b);
else // a <= c
System.out.println(c + " " + a + " " + b);
else // a <= b
if(a > c)
System.out.println(b + " " + a + " " + c);
else // a <= c ...
Multi-way If-else Statement
If the statement that is done when the test is false is
also an If statement, it is customary to write the else
and the if on one line: if(b > c)
System.out.prinln(a + " " + b + " " + c);
else
if(a > c)
is written
if (b > c)
System.out.print(a + " " + b + " " + c);
else if (a > c)
More on Multi-way If Statements
This is common when there are a number of
possibilities based on the value of a variable:
if (bedSoftness > 10)
System.out.println("Too hard!");
else if(bedSoftness > 5)
System.out.println("Just right!");
else
System.out.println("Too soft!");
Dangling Else Problem
Consider the following code:
if (a > b)
if (b > c)
System.out.println(c);
else
System.out.println(b);
Does the else go with if(a > b) or with if(b >
c)?
Answer
It goes with the last if statement that is without an
else, that, is if(b > c).
The Switch Statement
If there are many cases the depend on the exact value
of an int or char variable, rather than on ranges, we
could use a multi-way if statement:if (class == 1)
System.out.println("COSC 150");
else if (class == 2)
System.out.println("COSC 160");
else if (class == 3)
System.out.println("COSC 507");
The Switch Statement (cont'd)
Or, we could use the switch statement, which
implements multi-way branches based on the value
of an expression.
switch (class) {
case 1: System.out.println("COSC 150"); break;
case 2: System.out.println("COSC 160"); break;
case 3: System.out.println("COSC 507"); break;
default: System.out.println("Illegal value"); break;
}
The Switch statement (cont'd)
What the switch statement does depends on the
controlling expression, given in parentheses. If the
value matches one of the cases, that code is done.
Without the break statement, the next case is also
done. The break statement forces the program to
bypass the rest of the cases. The default case is
done if the value doesn't match any of the cases.
The Break Statement
The use (or non-use) of the break statement gives the
programming greater flexibility. You decide if the next case
is to be done or not. The reason you might want the next
case to be done is that often different values should be
processed the same way:switch (inputCommand) {
case 'A': case 'a':
System.out.println("command a"); break;
case 'C': case 'c':
System.out.println("command c"); break;
}
Multi-way If vs. Switch
The switch statement is more compact than the
multi-way if statement and should be used when
the code to be done is based on the value of an int
or char expression. However, the switch
statement cannot be floating point expressions and
so the multi-way if statement must be used. The
switch statement may be used for strings in Java SE
7 and later.
Enumerated Types
Often, a problem uses a set of named literals
(constants), e.g., the months of the year, or the days
of the week. Good programming practice says that
we should use mnemonic names rather that numbers
to represent such items. Instead of creating twelve
(or seven) constants, we can use an enumerated type
– that is create a set of new literals to represent the
items.
Enumerated Type Examples
public enum Month { JAN, FEB, MAR, APR, MAY, JUN, JUL, AUG, SEPT, OCT, NOV, DEC };
public enum Day { SUNDAY, MONDAY, TUESDAY,
WEDNESDAY, THURSDAY, FRIDAY, SATURDAY };
Using an Enumerated Type
Enumerated types can be used with the Switch
statement:Month m;
switch (m) {
case JAN: <code> break;
case FEB: <code> break;
...
case DEC: <code> break;
}
The Conditional Operator
The conditional operator (my personal favorite of all
the operators) is a trinary operator (three operands)
which evaluates a test (Boolean expression) and
based on the value of the expression, takes its value
from one of two other expressions (the true-case
value or the false-case value). It is similar to an if-
else statement, but can be used in an expression.
Conditional Operator Example
System.out.println(a > b ? a : b);
is more succinct than the if-else statement
if (a > b)
System.out.println(a);
else
System.out.println(b);
Reading in a char via Scanner
The Scanner class has a nextInt() method, and a
nextDouble() method, but no nextChar() method.
To read in a character, you must read in a String
(using the next() method) and then extract the first
character. The code is:
char c = sc.next().charAt(0);
Exercise
Write a Java program that reads in a char from the
user and then a number (double). If the char is 'C' or
'c', then the number is the radius of a circle. If the
char is 'S' or 's', then the number is the side of a
square. If the letter is 'G' or 'g' then the number is
the radius of a sphere. The program should then
print out the area of the object.
Exercise (cont'd)
Your program should: Read in a char shape Read in a double d Use a multi-way if statement or a switch statement
to distinguish the choicesCalculate the area (vol) of the object and print it out
Circle = PI * d2; Square == d2; Sphere: 4/3 * PI * d3
Loops
Use to repeat stepsDifferent kinds:
Count-controlled: when you know ahead of time how many times to loop
Sentinel-controlled: when reading until a special value
General Condition: Arbitrary loopThree Java statements to implement loops: while, do-while, and for.
Count-Controlled Loops
If you know the number of times you want to repeat a section of code, you can use a count-controlled loop. A for-statement is usually used. The syntax is:
for(int i = 0; i < 10; i++) <loop body>
Note the use of semicolons to separate the parts of the for statement. If the loop body is more than one statement, braces may be used.
Anatomy of a For Statement
Here is a close-up of the For statement: int i = 0; // initialization i < 10; // test i++ // increment
The variable i is the loop index. First it is initialized
to 0. Then before the loop body is executed, the
test is checked. If the test evaluates to true the loop
body is executed and the increment is done.
Anatomy of a For Statem't (cont'd)
If the test evaluates to the false, the loop is done, and
the program continues with the next line following
the loop. Note: The loop index is only valid inside
of the loop. Once the loop is finished, that i
becomes meaningless.
Syntax
The syntax of a For statement is:for(<initialization>; <test>; <increment>)
<loop body>
Any of the the parts may be omitted. For example,for(;;)
<loop body>
is an infinite loop.
Examplefor (int i=0; i<10; i++)
System.out.println(i + "^2 = " + i*i);
would print out:0^2 = 0
1^2 = 1
2^2 = 4
3^2 = 9
4^2 = 16
5^2 = 25
6^2 = 36
7^2 = 49
8^2 = 64
9^2 = 81
Note on Example
The index of loop goes from 0 to 9, inclusive, and
not from 0 to 10. It is a Java idiom to begin loops
at 0, since arrays start at 0. Thus to repeat code ten
times, the test is “i < 10” and not “i <= 10.”
Potential Pitfall
It is a bad idea to change the value of the loop index
inside of the loop. For example, for (int i=0; i<10; i++) {
System.out.println(i + "^2 = " + i*i);
i++;
}
would print out:0^2 = 0
2^2 = 4
4^2 = 16
6^2 = 36
8^2 = 64
Potential Pitfall (cont'd)
In this case, it would be better to use a different
increment step, e.g., i = i + 2.
Another case where programmers change the index
variable is to jump out the loop prematurely, e.g.,
set i to 10. It is better to use the break statement to
jump out of the loop this way.
Break, Continue, and Return
The break statement can be used to jump out of a
loop prematurely. Control passes to the statement
following the loop. The continue statement will cause the program to
skip over any remaining part of the loop body and
begin the next iteration of the loop. The return statement will exit the method that the
loop is in.
Example
int a = 5, b = 7;
for (int i=0; i<10; i++) {
a = a * b;
if(a > 1000) // too big
break;
}
System.out.println(a);
Nested Loops
Loops, including For statements, may be nested, that
is, one loop is included in the loop body of another
loop. This is useful for processing 2-dimensional
tables. The loop indices must be different
variables.
Example
for(int i=1; i<= 10; i++) {
for(int j=1; j<= 10; j++)
System.out.printf(" %3d", i * j);
System.out.println();
}
will print out the multiplication table.
Exercise
Write a loop that will read in 25 numbers (ints) and
will print out the smallest number read in.
While Statement
In addition to the For statement, Java contains the
While statement, which allows code to be repeated
while an arbitrary condition holds. For example:while(a < b)
a *= 2;
is a while loop. The loop continues as long as a is
less than b, and halts when a becomes greater
than or equal to b.
Using a While for a For
A While statement could be used instead of a For
statement. The For statement:for(int i=0; i<25; i++)
<loop body>
could be re-written as:int i = 0;
while(i < 25) {
<loop body>
i++;
}
Sentinel Controlled Loop
A sentinel-controlled loop reads values until a
special value (the sentinel) is read, which cannot be
a valid. For example, if the program is reading in
grades, then a negative number could be used for
the sentinel, as it is not a valid grade.
Example
total = 0;
grade = sc.nextInt();
while(grade >= 0) {
total += grade;
grade = sc.nextInt();
}
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