Java Reference
In-Depth Information
26 low++;
27 high--;
28 }
29
30
if
(isPalindrome)
31 System.out.println(s +
" is a palindrome"
);
32
else
33 System.out.println(s +
" is not a palindrome"
);
34 }
35 }
update indices
Enter a string: noon
noon is a palindrome
Enter a string: moon
moon is not a palindrome
The program uses two variables,
low
and
high
, to denote the position of the two charac-
ters at the beginning and the end in a string
s
(lines 14, 17). Initially,
low
is
0
and
high
is
s.
length() - 1
. If the two characters at these positions match, increment
low
by
1
and decre-
ment
high
by
1
(lines 26-27). This process continues until (
low >= high
) or a mismatch
is found (line 21).
The program uses a
boolean
variable
isPalindrome
to denote whether the string
s
is pal-
indrome. Initially, it is set to
true
(line 19). When a mismatch is discovered (line 21),
isPal-
indrome
is to
false
(line 22) and the loop is terminated with a break statement (line 23).
This section presents a program that displays the first fifty prime numbers in five lines,
each containing ten numbers.
Key
Point
An integer greater than
1
is
prime
if its only positive divisor is
1
or itself. For example,
2
,
3
,
5
, and
7
are prime numbers, but
4
,
6
,
8
, and
9
are not.
The problem is to display the first 50 prime numbers in five lines, each of which contains
ten numbers. The problem can be broken into the following tasks:
Determine whether a given number is prime.
■
For
number
=
2
,
3
,
4
,
5
,
6
, . . ., test whether it is prime.
■
Count the prime numbers.
■
Display each prime number, and display ten numbers per line.
■
Obviously, you need to write a loop and repeatedly test whether a new
number
is prime.
If the
number
is prime, increase the count by
1
. The
count
is
0
initially. When it reaches
50
,
the loop terminates.
Here is the algorithm for the problem:
Set the number of prime numbers to be printed as
a constant NUMBER_OF_PRIMES;
Use count to track the number of prime numbers and
set an initial count to 0;
Set an initial number to 2;
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