C(++) For Loop Equivalents


#1

What is the most efficient way in Ruby to create a conditional loop in
Ruby that looks like this in C(++):

for(int i=0; i*j/2<maxNum; i++)
{
//cool method stuff
}

I was looking at the .upto() method but it doesn’t like the comparison
operator :-/

Thanks!


#2

On 3/19/07, studlee2 removed_email_address@domain.invalid wrote:

operator :-/
Wouldn’t it be better to figure out what you’re trying to do than look
for a general construct?

max = 200
=> 200

j = 35
=> 35

i = 0
=> 0

0.upto(max) { |i| break if (i * j / 2) >= max }
=> nil

i
=> 12

That will do the job.

-austin


#3

What is the most efficient way in Ruby to create a conditional loop in
Ruby that looks like this in C(++):

for(int i=0; i*j/2<maxNum; i++)
{
//cool method stuff
}

In the general case, you can ‘while’ anywhere where you’d normally ‘for’

i=0
while i*j/2<maxNum
//cool method stuff
i++
end

But ‘most efficient way’ to do a conditional loop depends on your
context (and also what you’re trying to make efficient - runtime?
writetime? readtime?).

Dan.


#4

studlee2 wrote:

operator :-/

Thanks
Why not just do:
((maxNum2)/(ij)).times do |n|
#cool method stuff
end


#5

Use the step method:

----------------------------------------------------------- Numeric#step
num.step(limit, step ) {|i| block } => num

  Invokes _block_ with the sequence of numbers starting at _num_,
  incremented by _step_ on each call. The loop finishes when the
  value to be passed to the block is greater than _limit_ (if _step_
  is positive) or less than _limit_ (if _step_ is negative). If all
  the arguments are integers, the loop operates using an integer
  counter. If any of the arguments are floating point numbers, all
  are converted to floats, and the loop is executed _floor(n +
  n*epsilon)+ 1_ times, where _n = (limit - num)/step_. Otherwise,
  the loop starts at _num_, uses either the +<+ or +>+ operator to
  compare the counter against _limit_, and increments itself using
  the +++ operator.

     1.step(10, 2) { |i| print i, " " }
     Math::E.step(Math::PI, 0.2) { |f| print f, " " }

  _produces:_

     1 3 5 7 9
     2.71828182845905 2.91828182845905 3.11828182845905

So your example would become:
0.step(ij/2, 1) # (I’m not too sure about the details of C++ and if
defining i in the beginning would make i
j/2 be 0 no matter what)
Or, you could do
0.upto(i*j/2) # assumes step is one.

Finally, you could do
for i in 0…(i*j/2) do |i|

blah…

end


#6

Raj S. wrote:

I was looking at the .upto() method but it doesn’t like the comparison
operator :-/

Thanks
Why not just do:
((maxNum2)/(ij)).times do |n|
#cool method stuff
end

He would need to initialize i before your code as it could be any value,
not just 0. By the same token the increment could be any number, not
just 1.
A further problem with your method is that the loop is executed until
ij/2 is greater than maxNum. As you don’t know if the loop somehow
modifies the value of j you can not optimize it out. A simple example
would be where the program reads some numbers from a file and adds them
to j. i is incremented by one each time a number is read. You want the
program to exit when i
j/2 is larger than maxNum.


#7

Peña wrote:

 1  #---------------------------
12  i=0
23  def for_simple (init=0,incr=1)
34  using our own simple for()...  note advantage,
45  here we initialize to 2 and increment by 2
56  again, here we initialize to 200 and decrement by 20

C:\family\ruby>ruby test.rb
9
7
again, here we initialize to 200 and decrement by 20
60
40
20

C:\family\ruby>

Hth.
kind regards -botp

Now is it possible, like in Scheme, to assign a variable to a string and
have Ruby evaluate the string? For example, f = “i*j/2”. This would
allow one to make a more generic version of your methods. Ideally you
would have for_opt (i=0, f, inc=1) and have the method evaluate each of
the variables if necessary. This would allow one to copy working C code
without having to remember to do a lot of extraneous coding.


#8

From: Michael W. Ryder [mailto:removed_email_address@domain.invalid] :

He would need to initialize i before your code as it could be

any value,

not just 0. By the same token the increment could be any number, not

just 1.

fwiw, this is a simple example (intended for nubies only).

C:\family\ruby>cat -n test.rb
1 #---------------------------
2 #emulate c++ for (the better)
3 #for(int i=0; ij/2<maxNum; i++)
4 # {
5 # //cool method stuff
6 # }
7 #---------------------------
8
9 # using normal while loop
10 puts “using while…”
11 j=20
12 i=0
13 maxnum=100
14 while i
j/2 < maxnum do
15 puts i
16 i+=1
17 end
18
19 # let’s make our own for() and call it for_simple :slight_smile:
20 # note, there’s no condition here
21 # we’re leaving it up to yield/blocks since that
22 # is the ideal way, imho
23 def for_simple (init=0,incr=1)
24 i = init
25 loop do
26 yield i
27 i += incr
28 end
29 end
30
31 # now let’s test it
32 puts <<TEXT
33 Test I.
34 using our own simple for()… note advantage,
35 we don’t need to track the counter…
36 TEXT
37
38 for_simple do |i|
39 break unless ij/2 < maxnum
40 puts i
41 end
42
43 puts <<TEXT
44 Test II.
45 here we initialize to 2 and increment by 2
46 again, we don’t need to track the counter…
47 TEXT
48
49 for_simple(2,2) do |i|
50 break unless i
j/2 < maxnum
51 puts i
52 end
53
54 puts <<TEXT
55 Test III.
56 again, here we initialize to 200 and decrement by 20
57 again, we don’t need to track the counter…
58 and note, we change keyword unless to if
59 ruby just simply rocks…
60 TEXT
61
62 for_simple(200,-20) do |i|
63 break if i*j/2 < maxnum
64 puts i
65 end

C:\family\ruby>ruby test.rb
using while…
0
1
2
3
4
5
6
7
8
9
Test I.
using our own simple for()… note advantage,
we don’t need to track the counter…
0
1
2
3
4
5
6
7
8
9
Test II.
here we initialize to 2 and increment by 2
again, we don’t need to track the counter…
2
4
6
8
Test III.
again, here we initialize to 200 and decrement by 20
again, we don’t need to track the counter…
and note, we change keyword unless to if
ruby just simply rocks…
200
180
160
140
120
100
80
60
40
20

C:\family\ruby>

Hth.
kind regards -botp


#9

Is there any reason you can’t use the syntactical sugar “for”? Or maybe
construct a range:

(i…((maxNum*2)/j) ).each_with_index do |stuff| end