Programming Ruby book p.50 (Fibonacci yield example)


#1

here’s the example:

  1. def fib_up_to(max)
  2. i1,i2 = 1,1
  3. while i1 <= max
  4. yield i1
    
  5. i1, i2=i2, i1+i2
    
  6. end
  7. end
  8. fib_up_to(1000) {|f| print f, " "}

I’m familiar with yield passing values to the block and vice-versa. But
I have no clue what is happenening in line 5. Or at least in general
can anyone explain what is going on here in these 9 lines of code?
(I’ve been programming for 10 years so I’m familiar methods, loops, etc.
it’s just this yield thing I don’t completely understand especially line
5.).

cheers,
mike


#2

Mike G. wrote:

  1. fib_up_to(1000) {|f| print f, " "}

Hi Mike,

What’s happening at line 5 is a swap. The code is equivalent to this:

tmp = i1+i2
i1 = i2
i2 = tmp

Hope that helps.

Ola B. (http://ola-bini.blogspot.com)
JvYAML, RbYAML, JRuby and Jatha contributor
System Developer, Karolinska Institutet (http://www.ki.se)
OLogix Consulting (http://www.ologix.com)

“Yields falsehood when quined” yields falsehood when quined.


#3

On Mar 3, 2007, at 12:13 PM, Mike G. wrote:

  1. i1, i2=i2, i1+i2
    

[…]

But I have no clue what is happenening in line 5.

The spacing in your text is a bit misleading.
This makes it a little clearer:

   i1,i2  =  i2,(i1+i2)

This is Ruby’s multiple assignment statement. The
expressions on the right are evaluated into a list and
assigned, in order, to the variables on the left.

Gary W.


#4

Mike G. wrote:

I’m familiar with yield passing values to the block and vice-versa. But
I have no clue what is happenening in line 5.

Line 5 has nothing to do with the yield (which in this case just
displays the
current value of i1). It assigns the value of i2 to i1 and the sum of
the
values of i1 (before the assignment) and i2 to i2.

(I’ve been programming for 10 years so I’m familiar methods, loops, etc.
it’s just this yield thing I don’t completely understand especially line
5.).

Like I said: The yield (in line 4) only displays the value and line 5
has
nothing to do with yield.

HTH,
Sebastian


#5

Ok thanx guys. I just found that multiple assignment is not explained
until p.91 of the text while this example is on page 50 … strange.,

mike


#6

On Sun, Mar 04, 2007 at 02:34:00AM +0900, Gary W. wrote:

This is Ruby’s multiple assignment statement. The
expressions on the right are evaluated into a list and
assigned, in order, to the variables on the left.

I probably would have done it thusly:

i1, i2 = i2, i1 + i2
. . . or:
i1, i2 = [i2, i1 + i2]

. . . or something to that effect. Somehow, it seems more readable to
me. YMMV.

Do my versions for any particular reason seem unlike the typical Ruby
idiom for some reason? I’m curious.


#7

I happen to have this book as well. There’s actually a comment
immediate to the right of the line i1, i2 = 1, 1 and says # parallel
assignment (i1 = 1 and i2 =1)

i1, i2 = i2, i1+i2
is another way of writing
temp = i1
i1 = i2
i2 = temp+i2