How do you calculate a (base * power) sans math.pow?

If the exponent is a nonnegative integer, just multiply in a loop, just like you did on paper when you first learned about exponents before you were taught the concepts of negative and fractional exponents.

For the general case, I don't nkow. I'm not enough of a math-head, but I would suppose there's an algorithm or approximation formula out there.

jverda at 2007-7-9 6:24:30 >

> I've been tasked with trying to figure out how to do

> an exponent question without using the math.pow(3,2)

> method. How would I do that?

Are we talking about integers? Like jverd suggested, use a for loop to calculate the product. And if the (integer) power value is negative just return 1/product.

prometheuzza at 2007-7-9 6:24:30 >

You do it using recursion:

public double pow(double b, double e)

{

if (e < 0){ b = 1.0/b; e*=-1;}

if (e == 0) return 1;

return b * pow (b, e - 1);

}

Carlemagnea at 2007-7-9 6:24:30 >

> I can't use pow, pow is a math method.

public static double iCantUsePow(double b, double e)

{

if (e < 0){ b = 1.0/b; e*=-1;}

if (e == 0) return 1;

return b * iCantUsePow (b, e - 1);

}

Done.

Carlemagnea at 2007-7-9 6:24:31 >

> Somehow I've got to increment the product into a

> total correct?

>

> so base = 6

>

> power = 1

>

> product = 6

>

> total = total + product

So, if someone tells you to compute 2 to the 5th power on paper, you'd write down 2 + 2 + 2 + 2 + 2?

You do it in a loop by doing the same thing N times.

jverda at 2007-7-21 17:29:59 >

> Man this is frustrating

>

> total = 1

> for(power =1; power >0; power++)

Can you explain what that loop control statement means? How many times will it execute? What will be power's value each time?

http://java.sun.com/docs/books/tutorial/java/nutsandbolts/for.html

jverda at 2007-7-21 17:29:59 >

> If I'm trying to input two numbers, the base and the

> power that it wishes to be raised to

I know.

>, don't I need to

> initialize a power variable?

Yes, but what does that have to do with anything? I'm not sure which post you're addressing here. You'll want a power variable. We're not saying don't have one. We're saying you're using it incorrectly.

jverda at 2007-7-21 17:29:59 >

Hey I read it. And I understand how a for statement works. It's just tough trying to describe what problems I'm having with this problem. I have been reading java all day and my brain is a little fried right now. I really appreciate the help, and I'm trying to work through this myself. Right now I'm trying to figure out how exactly the for statement should read.

Shadowplaya at 2007-7-21 17:29:59 >

> And I understand how a for statement

> works.

The for loop you posted suggests that you don't. I say this not to denigrate you, but to give you a dose of reality. I notice you didn't answer my question about what the control clause means. I posted that question to guage how much you actually do understand about the syntax and usage of for, so that I could tailor my advice appropriately.

> It's just tough trying to describe what

> problems I'm having with this problem. I have been

> reading java all day and my brain is a little fried

> right now.

It might be best to take a break then. Continuing to beat your head against a problem when you're already frustrated is often counterproductive.

jverda at 2007-7-21 17:29:59 >

> A control clause indicates whether the loop should be

> terminated, I believe.

>

> So in the statement

>

> for(int a; a > 10; a ++)

>

> the (a > 10) is the control clause and CONTROLS

> whether the for statement continues or the program

> flow exits out of the loop and goes to the next

> statement.

>

> Am I far off track?

You're right, but I was looking for something more specfic--what each part of your specific for (blah; blah; blah) does and how it relates to your program.

So, in the one you provided:

> for(int ix; ix > 10; ix ++)

* It won't compile because you didn't give ix a value in the initialization part.

* Let's say you did the standardfor (int ix = 0; ...). So now we've initialized ix to 0.

* What does ix > 10 mean? When does it get executed?

* What does ix++ mean? When does it get exeucted?

* Based on the above two points, what can you say about how many times the loop body will execute?

jverda at 2007-7-21 17:30:05 >

> > A control clause indicates whether the loop should

> be

> > terminated, I believe.

> >

> > So in the statement

> >

> > for(int a; a > 10; a ++)

> >

> > the (a > 10) is the control clause and CONTROLS

> > whether the for statement continues or the program

> > flow exits out of the loop and goes to the next

> > statement.

> >

> > Am I far off track?

>

> You're right, but I was looking for something more

> specfic--what each part of your specific for (blah;

> blah; blah) does and how it relates to your program.

>

> So, in the one you provided:

> > for(int ix; ix > 10; ix ++)

> * It won't compile because you didn't give ix a value

> in the initialization part.

>

> * Let's say you did the standardfor (int ix = 0;

> ...). So now we've initialized ix to 0.

>

> * What does ix > 10 mean? When does it get executed?

>

> * What does ix++ mean? When does it get exeucted?

>

> * Based on the above two points, what can you say

> about how many times the loop body will execute?

Ok, I understand that I didn't initialize the ix in the for statement.

The ix > 10 means that the loop will execute until ix becomes 11 or more, and the loop will execute 10 times because the ix++ is a post increment, meaning it won't increment until one full time through the loop.

I'm sure this is obvious but what I'm not sure of is what should go in the for loop. I believe it's the power that I'm raising the base to.

So I'm wondering if I should initialize a separate value called 'a' that initializes at ...zero? and increments until a is greater than the power variable.

Shadowplaya at 2007-7-21 17:30:05 >

So I've written this down on paper and now I'm just trying to iron out the bugs but I think this will work.

while (base > 0) // satisfies that no negative numbers are entered

{

for(int a =1; a > power; a++)

{

total *= base;

}

}

Any feedback? like I said I'm still trying to compile the code correctly

Shadowplaya at 2007-7-21 17:30:05 >

Ok this is what I have so far. It isn't working but I think I'm close. BTW, it's not that I'm not reading your posts, it's just that sometimes I start writing and by the time I post, your reply is already out there.

import java.util.Scanner;

public class Homework614

{

public static void main( String args[] )

{

Scanner input = new Scanner(System.in);

int base;

int power;

int total = 1;

System.out.print( "Please enter a number");

base = input.nextInt();

System.out.print( "Please enter the power you wish to raise" );

power = input.nextInt();

if (base > 0)

{

for (int a = 1; a <= power; a++)

{

total = total * base;

}

}

System.out.printf( "The total is %d ", total);

}

}

AAARRRGGGGGHHHHH!!! It's obvious that my for loop here is the problem. I'm under the impression that you need two variables, a counter (in this case -a and what that variable will use as the condition to exit out of the loop.

Shadowplaya at 2007-7-21 17:30:05 >

> Yeah it is working. I was having problems because I

> was forgetting, like an idiot, that if I tell it to

> raise five to the 5th power, then it doesn't mean 5

> x5, it means 5 X 5 X 5 X 5 X 5.

D'OH! :-)

> Thank you very much for your help. I need to give

> you all the credit since you figured out the logic

> for me. The total = total * base along with the for

> loop were the keys and you helped me a lot with the

> total part.

No proble. I hope you'll be able learn not only the Java language issues involved but also the problem-solving and general programming.

> I appreciate you trying to talk me through the

> question. It only took five hours :).

You're welcome. It gets easier as you get more used to a certain way of thinking and get comfortable with the basics of the language. Starting out is hard because you need to learn both the principles and the langugae (and maybe tools, like a compiler or IDE, environment, like maybe Linux...)

jverda at 2007-7-21 17:30:05 >

> Well, I'm going to keep posting if I have questions.

That's what the forum is for. Just a couple of suggestions:

* Really try to figure it out yourself before you post. I'm not saying you're not doing that, but after a long "conversation" like this, it can be very tempting to just lapse into, "Okay, here's the next problem. How do we do it?"

* If you've moved onto a different problem and have a different question, start a new thread.

> I hope you guys don't find me annoying and decide to

> ignore me. :)

There's usually somebody around, but we all answer what we want when we want for our own reasons, so don't assume you'll always get the quick turnaround you got here. (And resist the temptation to tell us how urgent it is as your deadline approaches. That's a major buzzkill for those of us who answer here as a low-stress escape from our own itme pressures.)

jverda at 2007-7-21 17:30:05 >

> import java.util.Scanner;

>

> public class Homework614

> {

>

> public static void main( String args[] )

> {

> Scanner input = new Scanner(System.in);

>

> int base;

> int power;

> double total = 1.0;

>

>

> System.out.print( "Please enter a number");

>

> base = input.nextInt();

> System.out.print( "Please enter the power you wish

> sh to raise" );

> power = input.nextInt();

>

> if (base != 0 && power >= 0)

> {

>

> for (int a = 1; a <= power; a++)

> {

> total = total * base;

> }

> }

>

> else if (base != 0 && power < 0)

> {

> double b = 1.0 / base;

> power = power * -1;

>

> for (int a = 1; a <= power; a++)

> {

> total = total * b;

> }

> }

>

> else if (base == 0 && power != 0)

> {

> total = 0;

> }

>

> else

> {

> //Do nothing (total = 1);

> }

>

>

>

> System.out.println( "The total is " + total);

> }

>

> }

>

>

>

> Why do you use printf instead of print or println?

I don't know. I tried it with both. Does it really matter?

Shadowplaya at 2007-7-21 17:30:09 >