# C(++) For Loop Equivalents

**URL:** <https://www.ruby-forum.com/t/c-for-loop-equivalents/89377>\
**Category:** Ruby\
**Created:** [March 20, 2007, 1:31am UTC](https://www.ruby-forum.com/t/c-for-loop-equivalents/89377 "2007-03-20T01:31:30Z")\
**Posts on this page:** 9\
**Page:** 1

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**Author:** ![studlee2](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/278dde/32.png) [@studlee2](https://www.ruby-forum.com/u/studlee2)\
**Post date:** [March 20, 2007, 1:31am UTC](https://www.ruby-forum.com/t/c-for-loop-equivalents/89377/1 "2007-03-20T01:31:30Z")

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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!

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<div class="post-metadata">

**Author:** ![studlee2](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/278dde/32.png) [@studlee2](https://www.ruby-forum.com/u/studlee2)\
**Post date:** [March 20, 2007, 1:37am UTC](https://www.ruby-forum.com/t/c-for-loop-equivalents/89377/2 "2007-03-20T01:37:31Z")

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On 3/19/07, studlee2 [removed\_email\_address@domain.invalid](mailto: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

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**Author:** ![studlee2](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/278dde/32.png) [@studlee2](https://www.ruby-forum.com/u/studlee2)\
**Post date:** [March 20, 2007, 2:20am UTC](https://www.ruby-forum.com/t/c-for-loop-equivalents/89377/3 "2007-03-20T02:20:34Z")

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> 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.

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<div class="post-metadata">

**Author:** ![studlee2](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/278dde/32.png) [@studlee2](https://www.ruby-forum.com/u/studlee2)\
**Post date:** [March 20, 2007, 6:05am UTC](https://www.ruby-forum.com/t/c-for-loop-equivalents/89377/4 "2007-03-20T06:05:53Z")

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studlee2 wrote:

> operator :-/
> 
> Thanks  
> Why not just do:  
> ((maxNum_2)/(i_j)).times do |n|  
> #cool method stuff  
> end

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<div class="post-metadata">

**Author:** ![studlee2](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/278dde/32.png) [@studlee2](https://www.ruby-forum.com/u/studlee2)\
**Post date:** [March 20, 2007, 2:28am UTC](https://www.ruby-forum.com/t/c-for-loop-equivalents/89377/5 "2007-03-20T02:28:35Z")

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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(i_j/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

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<div class="post-metadata">

**Author:** ![studlee2](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/278dde/32.png) [@studlee2](https://www.ruby-forum.com/u/studlee2)\
**Post date:** [March 20, 2007, 6:30am UTC](https://www.ruby-forum.com/t/c-for-loop-equivalents/89377/6 "2007-03-20T06:30:55Z")

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Raj S. wrote:

> > I was looking at the .upto() method but it doesn’t like the comparison  
> > operator :-/
> > 
> > Thanks  
> > Why not just do:  
> > ((maxNum_2)/(i_j)).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  
i_j/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.

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**Author:** ![studlee2](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/278dde/32.png) [@studlee2](https://www.ruby-forum.com/u/studlee2)\
**Post date:** [March 20, 2007, 8:01pm UTC](https://www.ruby-forum.com/t/c-for-loop-equivalents/89377/7 "2007-03-20T20:01:28Z")

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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.

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**Author:** ![studlee2](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/278dde/32.png) [@studlee2](https://www.ruby-forum.com/u/studlee2)\
**Post date:** [March 20, 2007, 11:26am UTC](https://www.ruby-forum.com/t/c-for-loop-equivalents/89377/8 "2007-03-20T11:26:12Z")

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From: Michael W. Ryder [[mailto:removed\_email\_address@domain.invalid](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; i_j/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 🙂  
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 i_j/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

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**Author:** ![studlee2](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/278dde/32.png) [@studlee2](https://www.ruby-forum.com/u/studlee2)\
**Post date:** [March 20, 2007, 9:45pm UTC](https://www.ruby-forum.com/t/c-for-loop-equivalents/89377/9 "2007-03-20T21:45:46Z")

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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