irregular (non-consecutive) iteration (for loop)

[Nobody]

hi there,

suppose I want to iterate for a specific variable e.g. i , but for non
regular (or consecutive) values. For example i=0,1,2,4,5,7,8 etc
how can I do that with a for loop?

MY solution which is not that elegant involves if statements (or switch
statements) in the body of the loop: e.g.
for (int i=0; i<=8; i++)
{
if ((i!=3) && (i!=6))
{
// execute whatever
}

//else do nothing

}
is there a more elegant/compact/precise way of doing the above?

Thank you
** Posted from http://www.teranews.com **

[Gianni Mariani]

Nobody wrote:

hi there,

suppose I want to iterate for a specific variable e.g. i , but for non
regular (or consecutive) values. For example i=0,1,2,4,5,7,8 etc
how can I do that with a for loop?

MY solution which is not that elegant involves if statements (or switch
statements) in the body of the loop: e.g.
for (int i=0; i<=8; i++)
{
if ((i!=3) && (i!=6))
{
// execute whatever
}

//else do nothing

}
is there a more elegant/compact/precise way of doing the above?

const int nums[] = {0,1,2,4,5,7,8};

for (int in = 0; in < NumElem(nums); ++ in ) {
int i = nums[in];
}

[Sam]

Nobody writes:

hi there,

suppose I want to iterate for a specific variable e.g. i , but for non
regular (or consecutive) values. For example i=0,1,2,4,5,7,8 etc
how can I do that with a for loop?

MY solution which is not that elegant involves if statements (or switch
statements) in the body of the loop: e.g.
for (int i=0; i<=8; i++)
{
if ((i!=3) && (i!=6))
{
// execute whatever
}

//else do nothing

}
is there a more elegant/compact/precise way of doing the above?

It's just a matter of thinking logically:

static const int loop_iter[]={0,1,2,4,5,7,8};

int ii;

for (ii=0; ii<sizeof(loop_iter)/sizeof(loop_iter[0]); ++ii)
{
int i=loop_iter[ii];

// Now, do whatever you want with "i"
}
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[Mirco Wahab]

Nobody wrote:

suppose I want to iterate for a specific variable e.g. i , but for non
regular (or consecutive) values. For example i=0,1,2,4,5,7,8 etc
how can I do that with a for loop?
MY solution which is not that elegant involves if statements (or switch
statements) in the body of the loop: e.g.
is there a more elegant/compact/precise way of doing the above?

You could overload all necessary operators of a "Range" class
that has an underlying vector (or sth. else). If so, you might
write:
int main()
{
int irregular[] = {1,2,2,2,5};
Range i(irregular, 5); // <== that's our class

for(i=0; i<5; i++) // looks ok ...
cout << i << endl;

return 0;
}
This would indeed show the sequence given in the
int array 'irregular'.

How would such a range class look like? Simple, just
inherit from a std::vector and add the overloads:

class Range : public vector<int>{
int pos;
public: Range(int a[], int n) : vector<int>(a, a+n), pos(0) {}
int operator ++ (int) { return operator[](pos++); }
int operator = (int v) { return pos = v; }
int operator < (int b) { return pos < b; }
int operator () () const { return operator[](pos); }
operator int() const { return operator[](pos); }
};
Of course, to get that working you'll
need the headers:

#include <vector>
#include <iostream>

and you would add here:

using namespace std;

before the class definition.

Regards

Mirco

[Jerry Coffin]

In article <f1******************@news.teranews.com>, no****@nowhere.com
says...

hi there,

suppose I want to iterate for a specific variable e.g. i , but for non
regular (or consecutive) values. For example i=0,1,2,4,5,7,8 etc
how can I do that with a for loop?

MY solution which is not that elegant involves if statements (or switch
statements) in the body of the loop: e.g.
for (int i=0; i<=8; i++)

I'd probably do something like this:

int f(int i) {
if (i==3 || i==6)
return i+2;
return i+1;
}

for (int i=0; i<max; i=f(i))
// whatever

of course, I've only defined f to do exactly what you showed, skipping 3
and 6, but nothing else. If you really meant (for example) to skip all
multiples of three, that's somewhat simpler. In any case, it's purely a
matter of figuring out what you want and writing 'f' to suit.

--
Later,
Jerry.

The universe is a figment of its own imagination.

[Victor Bazarov]

Sam wrote:

[..]
It's just a matter of thinking logically:

static const int loop_iter[]={0,1,2,4,5,7,8};

int ii;

for (ii=0; ii<sizeof(loop_iter)/sizeof(loop_iter[0]); ++ii)
{
int i=loop_iter[ii];

// Now, do whatever you want with "i"
}

And, logically speaking/thinking, why is 'ii' declared outside of the
'for' loop? :-)

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[James Kanze]

On Jul 10, 3:55 pm, Jerry Coffin <[e-mail]:

In article <f1468$48722d2a$[e-mail]>,
[e-mail] says...

suppose I want to iterate for a specific variable e.g. i ,
but for non regular (or consecutive) values. For example
i=0,1,2,4,5,7,8 etc how can I do that with a for loop?

MY solution which is not that elegant involves if statements
(or switch statements) in the body of the loop: e.g.

for (int i=0; i<=8; i++)

I'd probably do something like this:

int f(int i) {
if (i==3 || i==6)
return i+2;
return i+1;
}

for (int i=0; i<max; i=f(i))
// whatever

Which could easily be written without the function:

for ( int i = 0 ; i < max ; i += (i == 3 || i == 6 ? 2 : 1))

(Whether that's an improvement is another question. Written on
a single like, like above, it's actually rather intimidating,
but correctly formatted:

for ( int i = 0 ;
i < max ;
i += (i == 3 || i == 6
? 2
: 1) )

it's not that bad.)

of course, I've only defined f to do exactly what you showed,
skipping 3 and 6, but nothing else. If you really meant (for
example) to skip all multiples of three, that's somewhat
simpler. In any case, it's purely a matter of figuring out
what you want and writing 'f' to suit.

More generally, in C++, he as two choices:

for ( int i = 0 ; i < max ; ++ i ) {
// use f(i) throughout the loop...
}

or

for ( int i = 0 ; i < max ; i = f(i) ) {
// use i throughout the loop...
}

Most other languages (or at least older languages) only offer
the first.

And of course, in the above, f(i) should be understood as any
arbitrary expression taking an i, and returning the same type.
Array[i], for example (so the previous suggestions fall into the
first pattern).

--
James Kanze (GABI Software) email:ja*********@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
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