How to Multiply Two Positive Integers Using Recursion | Comp Sci Questions

[courtrigrad]

Hello all

If we want to write a recursive method that multiplies two positive integers using repeated additions, we know that: $$a\times b = a + (a\times(b-1))$$ Would this be correct:

// pre: a and b are positive
// post: returning the product of a and b.
public int mult(int a, int b )

a*b = a +(a * (b-1))
if( b ==1)
return(a);
else
  return mult(a + a*(b-1)))

Also if you had:

public void hello(int n )
{
   System.out.println("hello n = ",n);
   if(n > 1)
      hello(n-1);
   System.out.println("goodbye n = ", n);

How would you find the output when a 3 is passed? I got a bumch of hellos and goodbyes, but I got it wrong


Thanks

 

[courtrigrad]

are these right?

 

[learningphysics]

// pre: a and b are positive
// post: returning the product of a and b.
public int mult(int a, int b )

a*b = a +(a * (b-1))

The above line is unnecessary.

if( b ==1)
return(a);
else
  return mult(a + a*(b-1)))

Your second return line here is wrong. You're calling mult which should have two arguments...

 

[learningphysics]

Also if you had:

public void hello(int n )
{
   System.out.println("hello n = ",n);
   if(n > 1)
      hello(n-1);
   System.out.println("goodbye n = ", n);

How would you find the output when a 3 is passed? I got a bumch of hellos and goodbyes, but I got it wrong


Thanks

I think it should be:
System.out.println("hello n="+n);

Instead of a comma you need a "+".