Re: executing a for loop or once depending on a test

Victor Bazarov <>
Tue, 17 Feb 2009 10:36:23 -0500
zebulon wrote:

I am trying to avoid redundancy in my code and would like to know if
there is an elegant solution for this.

Let's say I have 2 vectors (va and vb) that may contain Int and String
elements respectively, or be empty. I'd like to generate all
combinations of Int/String so I could use a nested loop :

for (vector<int>::iterator vai = va.begin(); vai != va.end(); ++vai)
    for (vector<string>::iterator vbi = vb.begin(); vbi != vb.end(); +

Just a side observation, if your 'vb' and 'va' do not change during the
execution of these loops, it's better to get the iterator values once
before entering the very first loop. It may look like premature
optimization, but I've seen it with my own three eyes how long a simple
function call can take (relatively speaking). So, consider

  vector<int>::iterator vab = va.begin(), vae = va.end();
  vector<string>::iterator vbb = vb.begin(), vbb = vb.end();
  for (vector<int>::iterator vai = vab; vai != vae; ++vai)
    for (vector<string>::iterator vbi = vbb; vbi != vbe; ++vbi)

        // create the object
        Object* oneobject = new Object();
        // store vai and vbi combination
        // store the object pointer somewhere;

The problem here is that if one of my vectors is empty, no object at
all will be created.

So, what you're saying that it should be created... It's not obvious
from your requirement. If there aren't any ints or strings, there
shouldn't be any combinations, no? Nothing to combine, right?...

So I solved it using some tests:

if (!va.empty() && vb.empty()) //only ints
    for (vector<int>::iterator vai = va.begin(); vai != va.end(); +
        // create object
        // store
else if (va.empty() && !vb.empty()) //only strings
  for (vector<int>::iterator vbi = vb.begin(); vbi != vb.end(); ++vbi)
        // etc...
else if (!va.empty() && !vb.empty()) //int and strings
    for (vector<int>::iterator vbi = vb.begin(); vbi != vb.end(); +
        for (vector<int>::iterator vbi = vb.begin(); vbi != vb.end(); +
            // etc...
else if (!va.empty() && !vb.empty()) //no int no strings
      // etc...

The problem is that there is a lot of code redundency (common parts in
the loops), and if there are more than two vectors, the number of if
statements would be impossible to manage.

Actually, instead of Int and String, if I have pointers to objects, I
know I could use inheritance/polymorphism in order to store them in
the same container. However, the objects can be heterogenous and
inheritence is not suitable.

Is there an elegant way to avoid all the if statements ?

It would seem you're asking for an algorithm. Consider the newsgroup
'comp.programming' for such generic requests. An algorithm is not
language-specific, usually.

What is it you'd like to end up with? Let's say you have collection A,
collection B, and collection C (possibly more). Let's further say that
collection A contains 2 objects, collection B contains one, and
collection C is empty. Let's name objects 'a0', 'a1', and 'b0'. What
combinations are you looking for? Please enumerate *all*.

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