which one runs faster ??

[onkar]

which one runs faster ??

for(i=0;i<100;i ++)
for(j=0;j<10;j+ +)
a[i][j]=0;

OR

for(j=0;j<10;j+ +)
for(i=0;i<100;i ++)
a[i][j]=0;

[Joachim Schmitz]

"onkar" <on*******@gmai l.comschrieb im Newsbeitrag
news:11******** **************@ i38g2000prf.goo glegroups.com.. .

which one runs faster ??

for(i=0;i<100;i ++)
for(j=0;j<10;j+ +)
a[i][j]=0;

OR

for(j=0;j<10;j+ +)
for(i=0;i<100;i ++)
a[i][j]=0;

time it and you'll know, for your platform, compiler and optimazation
options

Bye, Jojo

[Tor Rustad]

onkar wrote:

which one runs faster ??

for(i=0;i<100;i ++)
for(j=0;j<10;j+ +)
a[i][j]=0;

OR

for(j=0;j<10;j+ +)
for(i=0;i<100;i ++)
a[i][j]=0;

Why do you expect there is a difference? The inner loop looks identical.
--
Tor <torust [at] online [dot] no>

[keith]

On 30 May, 12:23, onkar <onkar....@gmai l.comwrote:

which one runs faster ??

for(i=0;i<100;i ++)
for(j=0;j<10;j+ +)
a[i][j]=0;

OR

for(j=0;j<10;j+ +)
for(i=0;i<100;i ++)
a[i][j]=0;

Short Answer: It Depends.

Less Short Answer: It depends on a bunch of things, none of which is
directly related to standard C.

Pragmatic Answer: Try both and see.

What I'd Probably Do<tm>: memset(a, 0x00, sizeof a[0][0] * 1000);

Thought Experiment: What would each of your methods do if every array
element happened to be around half a memory page in size? Assuming
you are on a system with virtual memory, of course.

[mark_bluemel]

On May 30, 12:23 pm, onkar <onkar....@gmai l.comwrote:

which one runs faster ??

Yes.

>
for(i=0;i<100;i ++)
for(j=0;j<10;j+ +)
a[i][j]=0;

OR

for(j=0;j<10;j+ +)
for(i=0;i<100;i ++)
a[i][j]=0;

1. You haven't told me what a[rows][columns] is or what the cache line
size on your machine. If a is a 2-D array of bytes, and the cache line
was 1000 bytes, then it probably won't matter.

2. This is not really about C as such. However, as C is a Row-Major
language (see http://en.wikipedia.org/wiki/Row-major_order), it's
normally taken to be better to vary the column index faster than the
row index, or more generally to order index variation from the right.

[jacob navia]

onkar wrote:

which one runs faster ??

for(i=0;i<100;i ++)
for(j=0;j<10;j+ +)
a[i][j]=0;

OR

for(j=0;j<10;j+ +)
for(i=0;i<100;i ++)
a[i][j]=0;

memset(a,0,1000 *sizeof(a[0]));

That will be optimized in assembly by the
system provider, and it will be faster than
your loops.

[Eric Sosman]

jacob navia wrote:

onkar wrote:

>which one runs faster ??

for(i=0;i<100; i++)
for(j=0;j<10;j ++)
a[i][j]=0;

OR

for(j=0;j<10;j ++)
for(i=0;i<100; i++)
a[i][j]=0;

memset(a,0,1000 *sizeof(a[0]));

That will be optimized in assembly by the
system provider, and it will be faster than
your loops.

It will also be wrong. R-O-N-G, wrong. (Hint: On the
assumption that the original loops do not invoke undefined
behavior, is sizeof a[0] == sizeof a[0][0] possible?)

Even after the obvious repair it could still be wrong.
No, I'm not talking about exotic machines where all-bits-zero
is not "zero" for the type in question. (Hint: which elements
of `unsigned char a[120][150]' are affected by the loops and
which are affected by memset?)

--
Eric Sosman
es*****@acm-dot-org.invalid

[Robbie Hatley]

"onkar" <on*******@gmai l.comwrote:

which one runs faster ??

for(i=0;i<100;i ++)
for(j=0;j<10;j+ +)
a[i][j]=0;

OR

for(j=0;j<10;j+ +)
for(i=0;i<100;i ++)
a[i][j]=0;

(The following assumes that "a" has been allocated as a
single contiguous block of memory, and that the dimensions
of a are [100][10]. You didn't state either. If these
assumptions are not true, the following does not hold.)

Your first set of nested loops might run faster, because it
involves a single traverse through the memory, one byte
increments in the same direction. The second jumps back
and forth.

A compiler might translate the first to machine laguage as
"rep stosl" (writing all the bytes in a single rapid-fire
burst).

The second, however, might get tranlated to nested loops,
which would be slower.

--
Cheers,
Robbie Hatley
East Tustin, CA, USA
lonewolf aatt well dott com
triple-dubya dott tustinfreezone dott org

[Guru Jois]

On May 30, 4:23 pm, onkar <onkar....@gmai l.comwrote:

which one runs faster ??

for(i=0;i<100;i ++)
for(j=0;j<10;j+ +)
a[i][j]=0;

OR

for(j=0;j<10;j+ +)
for(i=0;i<100;i ++)
a[i][j]=0;

check it yourself using clock()
help - use man page for clock

Bye
Guru Jois

[Tak]

On 5ÔÂ30ÈÕ, ÏÂÎç7ʱ23·Ö, onkar <onkar....@gmai l.comwrote:

which one runs faster ??

for(i=0;i<100;i ++)
for(j=0;j<10;j+ +)
a[i][j]=0;

OR

for(j=0;j<10;j+ +)
for(i=0;i<100;i ++)
a[i][j]=0;

I thought them run in the same time, haha , I am new at c/c++;