while loop confusion
my code below gets perfect numbers by doing this:
1+2 = 3(prime)3*2 = 6(perfect)
1+2+4 = 7(prime) 7*4 = 28(perfect)
1+2+4+8+16 = 31(prime)31*16 = 496(perfect).
but i don't understand why it only prints out the first three perfect numbers, and not also the fourth, which is 8128.
my while loop is while(sum < 32){
to print out 496, it calclates 31(sum) * 16(num).
so if sum = 31, sum is therefore below 32, so why does the while loop not do another round and print the fourth perfect number? please help.
class PerfectNumbersThree{
publicstaticvoid main(String[] args){
System.out.print("The first four perfect numbers are: ");
testIfPerfect(0);
}
staticvoid testIfPerfect(int num){
int sum = 0, numMultByTwo = 1;
while(sum < 32){
sum = sum + numMultByTwo;
num = numMultByTwo;
numMultByTwo = num*2;
if(isPrime(sum) ){
int perfectNum = sum*num;
System.out.print(perfectNum+" ");
}
}//close while.
}
staticboolean isPrime(int n){
if(n < 2)returnfalse;
if(n == 2)returntrue;
for (int d = 2; d < n; d+=1)
if (n%d == 0)returnfalse;
returntrue;
}
}
output
The first four perfect numbers are: 6 28 496
[2690 byte] By [
mark_8206a] at [2007-11-27 1:06:27]
> while(sum < 64)thanks prome, i'll go try to figure out why it's 64!
> > ...> > thanks prome, i'll go try to figure out why it's 64!Hint:1st = 2 *3 =62nd = 4 *7 =283rd = 16 * 31 = 4964th = 64 * 127 = 8128
isn't 64 the value of num, and not sum?
Why not let your end condition (output *4* perfect numbers) be the condition for your while loop?
Something like this:
static void testIfPerfect(int num)
{
int sum = 0, numMultByTwo = 1;
int perfectNumberCount = 0; // a new variable
while (perfectNumberCount < 4) // this seems more logical
{
sum = sum + numMultByTwo;
num = numMultByTwo;
numMultByTwo = num * 2;
if (isPrime(sum))
{
int perfectNum = sum * num;
System.out.print(perfectNum + " ");
perfectNumberCount++; // increment variable when perf numb found
}
} // close while.
}
Also, your testIfPerfect routine is a little deceiving to my eyes. It kind of makes me believe that it takes a parameter and tests it and only it to see if it is a perfect number. In fact what it does is tests all numbers from 0 to whatever. There are two ways to make this clearer:
1) take the variable num and declare it inside the method, and change the method to a parameter-less form: static void testForPerfectNumbers() {....
or
2) change it to a true function that tests for perfect numbers: static boolean testIfPerfect(int num) {....
then loop through your numbers in a higher level routine such as the main routine.
thanks petes, but i still don't follow the while loop in my code, when 496 gets printed,(31*16) the value of sum is 31, is that correct?
> thanks petes, but i still don't follow the while loop
> in my code,
> when 496 gets printed,(31*16) the value of sum is 31,
> is that correct?
I find that in this situation when I want to know what variables are doing at a certain time is either to sprinkle a few "System.out.println(myVariable);" in my code, or step through it with a debugger. Why not put a few S.o.println()s in the if (isPrime(sum)) block? Let your program do the hard work and tell you what you need to know.
> > thanks petes, but i still don't follow the while
> loop
> > in my code,
> > when 496 gets printed,(31*16) the value of sum is 31,
> > is that correct?
>
> I find that in this situation when I want to know
> what variables are doing at a certain time is either
> to sprinkle a few "System.out.println(myVariable);"
> in my code, or step through it with a debugger. Why
> not put a few S.o.println()s in the if
> (isPrime(sum)) block? Let your program do the hard
> work and tell you what you need to know.
; )
petes1234, that's exactly what has been proposed to Mark in about every thread he started!
@Mark, you seem to be smart guy (from what I've seen so far), but why don't you try to answer those questions yourself by debugging your code? (no offense meant!)
>why don't you try to answer those
> questions yourself by debugging your code? (no
> offense meant!)
i changed my code slightly so i could see the value of the sum variable at the end of the while loop.
i can see the value of sum is 31 by
int perfectNum = sum*num;
System.out.print(sum+" multiplied by "+num+" is ");
System.out.println(perfectNum);
this prints: 31 multiplied by 16 is 496
so sum is 31, num is 16, and perfectNum is 496.
and that's what i don't understand, if the value of sum is below 32 then the while loop should keep on going, but somehow it stops and doesn't print the fourth number.
class PerfectNumbersTest {
public static void main(String[] args) {
System.out.println("The first four perfect numbers: ");
testIfPerfect(0);
}
static void testIfPerfect(int num) {
int sum = 0, numMultByTwo = 1;
while(sum < 32) {
sum = sum + numMultByTwo;
num = numMultByTwo;
numMultByTwo = num*2;
if(isPrime(sum) ) {
int perfectNum = sum*num;
System.out.print(sum+" multiplied by "+num+" is ");
System.out.println(perfectNum);
}
} //close while.
}
static boolean isPrime(int n) {
if(n < 2) return false;
if(n == 2) return true;
for (int d = 2; d < n; d+=1)
if (n%d == 0) return false;
return true;
}
}
The first four perfect numbers:
3 multiplied by 2 is 6
7 multiplied by 4 is 28
31 multiplied by 16 is 496
I don't understand what there's not to understand actually. Run this code:
class PerfectNumbersThree {
public static void main(String[] args) {
testIfPerfect(0);
}
static void testIfPerfect(int num) {
int sum = 0, numMultByTwo = 1;
while(sum < 32) {
sum = sum + numMultByTwo;
num = numMultByTwo;
numMultByTwo = num*2;
if(isPrime(sum)) {
int perfectNum = sum*num;
System.out.println("Perfect number = sum * num = "+
sum+" * "+num+" = "+perfectNum);
}
System.out.println("sum = "+sum);
if(sum >= 32) {
System.out.println("sum >= 32, stop looping!");
}
}
}
static boolean isPrime(int n) {
if(n < 2) return false;
if(n == 2) return true;
for (int d = 2; d < n; d+=1)
if (n%d == 0) return false;
return true;
}
}
Whenever sum is more than 31, the while loop terminates. Since 8128 = 127*64, the 4th perfect number is naturally not printed since 127 (or 64, which ever is your sum variable) is more than 31.
