looping problem
Hey all,
i am trying to sort out the numbers which are not prime number from 1 to 100 and just don't get the right output. I guess I need to know why I get the output by the code , both posted below. It looks like every x greater than 3 can pass the condition test in if statement . Thanks for the help.
code:
public class Number {
static String a;
public static void main(String args[ ]) {
for (int x=2;x<101;x++){
for (int y=2;y<100;y++){
if((y<x)&&(x%y)==0){
a="not prime ";
}
}
System.out.println(x+" "+a);
}
}
}
output
2 null
3 null
4 not prime
5 not prime
6 not prime
7 not prime
8 not prime
9 not prime
10 not prime
11 not prime
12 not prime
13 not prime
14 not prime
15 not prime
16 not prime
17 not prime
18 not prime
19 not prime
20 not prime
21 not prime
22 not prime
23 not prime
24 not prime
25 not prime
26 not prime
27 not prime
28 not prime
29 not prime
30 not prime
31 not prime
32 not prime
33 not prime
34 not prime
35 not prime
36 not prime
37 not prime
38 not prime
39 not prime
40 not prime
41 not prime
42 not prime
43 not prime
44 not prime
45 not prime
46 not prime
47 not prime
48 not prime
49 not prime
50 not prime
51 not prime
52 not prime
53 not prime
54 not prime
55 not prime
56 not prime
57 not prime
58 not prime
59 not prime
60 not prime
61 not prime
62 not prime
63 not prime
64 not prime
65 not prime
66 not prime
67 not prime
68 not prime
69 not prime
70 not prime
71 not prime
72 not prime
73 not prime
74 not prime
75 not prime
76 not prime
77 not prime
78 not prime
79 not prime
80 not prime
81 not prime
82 not prime
83 not prime
84 not prime
85 not prime
86 not prime
87 not prime
88 not prime
89 not prime
90 not prime
91 not prime
92 not prime
93 not prime
94 not prime
95 not prime
96 not prime
97 not prime
98 not prime
99 not prime
100 not prime>
Hmmm... do you ever assign anything to a besides "not prime"? Are you surprised it never writes "is prime"?Hmmm... I wonder if Hunter9000 will post something similar...Message was edited by: Hippolyte
You never reset a to anything other than "not prime" when you change x values, so that's all that ever gets printed.
Thanks both of you for the replies.
Back to my last post, I still don't understand why some of x values that are prime number can match the condition set for if statement and make the computer execute this if statement. Any idea?
I am not sure what exactly is the another thing supposed to be assigned to a for solving this looping problem. My try(posted below) did not wok it out.
Thanks again.
public class Number {
static String a;
public static void main(String args[ ]) {
for (int x=2;x<101;x++){
for (int y=2;y<100;y++){
if((y<x)&&(x%y)==0){
a="not prime";
}
else {a="is prime";}
}
System.out.println(x+" "+a);
}
}
}
--
Output
:2 is prime
3 is prime
4 is prime
5 is prime
6 is prime
7 is prime
8 is prime
9 is prime
10 is prime
11 is prime
12 is prime
13 is prime
14 is prime
15 is prime
16 is prime
17 is prime
18 is prime
19 is prime
20 is prime
21 is prime
22 is prime
23 is prime
24 is prime
25 is prime
26 is prime
27 is prime
28 is prime
29 is prime
30 is prime
31 is prime
32 is prime
33 is prime
34 is prime
35 is prime
36 is prime
37 is prime
38 is prime
39 is prime
40 is prime
41 is prime
42 is prime
43 is prime
44 is prime
45 is prime
46 is prime
47 is prime
48 is prime
49 is prime
50 is prime
51 is prime
52 is prime
53 is prime
54 is prime
55 is prime
56 is prime
57 is prime
58 is prime
59 is prime
60 is prime
61 is prime
62 is prime
63 is prime
64 is prime
65 is prime
66 is prime
67 is prime
68 is prime
69 is prime
70 is prime
71 is prime
72 is prime
73 is prime
74 is prime
75 is prime
76 is prime
77 is prime
78 is prime
79 is prime
80 is prime
81 is prime
82 is prime
83 is prime
84 is prime
85 is prime
86 is prime
87 is prime
88 is prime
89 is prime
90 is prime
91 is prime
92 is prime
93 is prime
94 is prime
95 is prime
96 is prime
97 is prime
98 is prime
99 is prime
100 is prime>
Did someone delete the other 1,000+ forum threads on finding
the prime numbers between 1 and 100, lol? ; )
Do a forum search.
http://forum.java.sun.com/thread.jspa?forumID=54&threadID=653503
http://forum.java.sun.com/thread.jspa?forumID=31&threadID=788706
http://forum.java.sun.com/thread.jspa?forumID=31&threadID=613411
http://forum.java.sun.com/thread.jspa?forumID=256&threadID=260595
http://forum.java.sun.com/thread.jspa?forumID=513&threadID=566203
http://forum.java.sun.com/thread.jspa?forumID=54&threadID=592457
http://forum.java.sun.com/thread.jspa?forumID=54&threadID=671365
for (int x=2;x<101;x++){// x is the value you want to check for prime
for (int y=2;y< x ;y++){// y is used to check x for primeosity (primeitude?)
// loop y from 2 to x, and you don't need to do the y<x check in the if statement.
if(( x%y == 0) {// as you loop from 2 to x, if any y value divides x evenly, then you know x is not prime.
a="not prime";
}
/* else {// don't do this here, if you do, then only the last y value you check is used to determine prime or not
a="is prime";
}
*/
}
// if you get to the end of the y loop without setting a to "not prime", then it will be "prime". Either way, print it out, then reset it for the next loop.
System.out.println(x+" "+a);
a = "prime";// use "prime" as the default value since you're assuming it is prime going into the loop. This only gets changed if the x value if found to be not prime.
}
I hope that all made sense :)>
Note:
(1) For loop variables have meaningful names
(2) Checking If(1, 2, EVEN) conditions to simplify algorithm
(3) Divisor loop only has to check up to SQRT(number)
(4) Divisor loop can increment by 2 because number is odd
(5) used break to exit for loop when necessary
public class PrimeNumberFinder{
public static void main(String[] args){
new PrimeNumberFinder();
}
public PrimeNumberFinder(){
for(int number = 1; number <= 20; number++){
boolean isPrime = true;
// 1 is ignored
if(number == 1){
System.out.println("1 is ignored");
continue;
} else
// check if even and not 2
if(number % 2 == 0 && number > 2){
isPrime = false;
} else {
int limit = (int)Math.sqrt((double)number);
for(int divisor = 3; divisor <= limit; divisor += 2){
if((number % divisor) == 0){
isPrime = false;
break;
}
}
}
System.out.println("" + number + (isPrime ? " is PRIME" : " is COMPOSITE"));
}
}
}
The output u r getting is not correct. 5 is prime but ur output is otherwise
For 2,3 u r getting null since value of a is not assigned (the if condn can't be met).For values 4,5 ... the if condition becomes true at some point of looping and since a is static so u are getting not prime with rest
of the numbers till 100.My advice is
use for (int y=2;y<x;y++) in inner loop and use (x%y) only to check for prime>
My suggestion to the OP is that he should use more structure.
public class Number {
public static void main(String args[ ]) {
for (int x = 2; x < 101; x++) {
if (isPrime(x)) {
System.out.println("The number " + x + " is a prime number.");
} else {
System.out.println("The number " + x + " is not a prime number.");
}
}
}
private static boolean isPrime(int n) {
if (n < 2) {
return false;
}
if (n == 2) {
return true;
}
for (int p = 2; p < n; p++) {
if (n % p == 0) {
// the number has a proper divisor, so it cannot be a prime.
// no point in checking the rest.
return false;
}
}
// The number had no proper divisors, so it is prime.
return true;
}
}
That's it! Running on Siemens TC65 (J2ME), to prevent to many for loops it is better to extract the root of the number.
****
public class Calculation {
public static boolean prime(long num) {
long sq_root = (long) Math.sqrt(num);
for (long a = 2; a <= sq_root; a++) {
if (num % a == 0) {
return false;
}
}
return true;
}
}
