operating with vectors

Hi all
I have a2d vector say
vector<vector<int> > V ( 5,vector<int>(5)); // 25 size

Suppose i want to do this operation. if the last element in vector[i] and the first element of v[i+1] differ by a value greater than 20..., i want to push the v[i+1].front() element into v[i].back() position in the array.
how to do it?

i wrote a code like this but got segmentation error. please help me how to design this one!!

Code:

---

int main (void)
{
        vector <vector<int> > v (5,vector<int>(5));
        vector<int>::iterator it;
        int rnd;
        for (int i=0;i<5;i++)
        {
                for ( int j=0;j<5;j++)
                {
                rnd = rand() % 100;
                v[i][j]= rnd;
                //cout<<v[i][j]<<" ";
                }
        }
       
        for (int i=0;i<5;i++)
        {
                if ( ( (v[i].back() )-(v[i+1].front() ) ) >= 20)
                {
                        //cout<<"ok!"<<endl;
                                v[i].push_back(v[i+1].front());
                                v[i].resize(v[i].size()+1);
                                v[i+1].resize(v[i+1].size()-1);               
                }
        }
return 0;
}

---

i know there is a flaw but the logic iw ant to achieve is simple. please suggest how it shud be rewrote.

It seems that you have a segfault because this for loop takes too many steps:

Code:

---

for (int i=0;i<5;i++)
        {
                if ( ( (v[i].back() )-(v[i+1].front() ) ) >= 20)
                {
                        //cout<<"ok!"<<endl;
                                v[i].push_back(v[i+1].front());
                                v[i].resize(v[i].size()+1);
                                v[i+1].resize(v[i+1].size()-1);               
                }
        }

---

Trying to push_back() elements from a vector nonexistent. Actually, you are accessing another, inner vector outside of the outer vector's bounds. To fix it you have to keep i + 1 in bounds. Related to the problem, you need to stop using magic numbers for your for loops and stuff. The size() function is fast and appropriate for controlling for loops.

Also, I can't figure out why you are resizing. What are you trying to do there?

You don't push_back and then resize by size()+1 push_back already increases the size of the container. You should also use pop_front.

OK i got your point. But i tried this way. i gives me a segmentation fault...whats the problem here can any1 help me??
I will be greatful.

Code:

---

#include <iostream>
#include <vector>
#include <time.h>
using namespace std;

int main (void)
{
        time_t tm;
        time(&tm);
        srand(tm);
        vector <vector<int> > v (5,vector<int>(5));
        vector<int>::iterator it;
        int rnd;
        for (int i=0;i<5;i++)
        {
                for ( int j=0;j<5;j++)
                {
                rnd = rand() % 100;
                v[i][j]= rnd;
                //cout<<v[i][j]<<" ";
                }
        }
        int k;
        for (int i=0;i<4;i++)
        {
                while (abs( ( (v[i].back() )-(v[i+1].front() ) ) < 30))
                {
                                k = v[i+1].front();
                                v[i+1].erase(v[i+1].begin() );
                                v[i].push_back(k);       
                }

        }
       
        for (int i=0;i<v.size();i++)
        {
                //cout<<v[i].size()<<" ";
                for(int j=0;j<v[i].size();j++)
                {
                        cout<<v[i][j]<<" ";
                }
                cout<<endl;
        }
        cout<<endl;
return 0;
}

---

This modification works for me:

Code:

---

        for (int i=0;i<4;i++)
        {
                auto BackE = v[i].back();
                auto FrontE = v[i+1].front();
                if (abs(BackE - FrontE) < 30)
                {
                        k = v[i+1].front();
                        v[i+1].erase(v[i+1].begin() );
                        v[i].push_back(k);       
                }
        }

---

I don't know if the program is 100% right, but I believe the output seemed correct.

No i tried IF condition... but u see i need the elements to be pushed into prev array as long as they are less than 30 of difference. if does it once... can you suggest how can handle continous shifts using while?
thanks

If you want to change it so that the same vectors will be compared, lengthened, and shortened respectively, you can do that. It's just that you won't bump the loop count until the comparison between the two elements fails. The abs(v[i].back () - v[i + 1].front ()) < whatever comparison is still only a comparison.

The output might look like this:

Code:

---

vector[0] = ( 37, 20, 32, 70, 63, ).
vector[1] = ( 31, 95, 49, 93, 38, 24, 30, 12, ).
vector[2] = ( 90, 74, ).
vector[3] = ( 36, 83, 27, 79, 74, 70, 51, 60, 55, ).
vector[4] = ( 96, ).

---

Isn't that what you expect?

You also have to be careful of what happens if a vector gets reduced to being empty.

yeah thanks.. thats what i expect... but how to handle it when the vector becomes empty... i want the next set of vector to be pushed forward... can you suggest what to code when the size of v[i] becomes 0.... i want to remove it... how to do it using iterators?

please suggest!!
thanks
regards,
KR

i got the segment fault only becuz the size of the vecotr became zero and the system dint know what to do...
I want help in given telliing the system to move forward when the size of i+1 becomes zero...
regards

Test for v[i + 1].empty()

Hey i tried the problem using this.. can you tell me if the flow of logic is right in my steps?.. would appreciate any help...

Code:

---

do
                {
                        if (abs( v[i].back() - v[i+1].front() ) < 5)
                        {
                                k = v[i+1].front();
                                v[i].push_back(k);
                                v[i+1].erase(v[i+1].begin() );
                                i++;       
                        }
                }while ((i<5) && (!v[i].empty()) ); // 5 is the size of the vector of vectors.. 5*5 as i said before...

---

Quote:

---

Originally Posted by Elysia

This modification works for me:

Code:

---

        for (int i=0;i<4;i++)
        {
                auto BackE = v[i].back();
                auto FrontE = v[i+1].front();
                if (abs(BackE - FrontE) < 30)
                {
                        k = v[i+1].front();
                        v[i+1].erase(v[i+1].begin() );
                        v[i].push_back(k);       
                }
        }

---

I don't know if the program is 100% right, but I believe the output seemed correct.

---

Auto? Isn't that C++0x only?
I love the feature though, it's going to make some things a lot easier!

Yes, it is. There are available compilers that supports it.

Quote:

---

Originally Posted by Elysia

Yes, it is. There are available compilers that supports it.

---

Yes, though I don't think it's a good idea suggesting to use it to people who don't fully know what they're doing...

It's part of the language. It's as good as any to teach newbies how to use these new features IMHO.

Quote:

---

Originally Posted by Elysia

It's part of the language. It's as good as any to teach newbies how to use these new features IMHO.

---

It's part of the language that is fairly unportable still and only when using specific flags to make compilers features that are still in beta mode. I've had g++ crash on me before for using the auto keyword (on a function, using it properly though).

So I can't advise anyone on using it yet until C++0x is a bit more widely developed.

I'll be damned if I have to wait on some buggy compiler to catch up before I can use the long-awaited C++0x. Not had a single crash (compiler crash) with VS2010. Therefore, I recommend that everyone use it (that is, if they can) until such time that more compilers mature.
You may disagree with this mythology, but it's my of doing things. So unless a newbie cannot, for some reason, use C++0x, I will suggest code that uses it. It also helps them prepare for the upcoming standard.

Hi
this algo is correct,
But you see sometimes the entire 2nd row if pushed into 1st row if all elements in 2nd row pass a test. So forloop is not advisable. I use while loop onle here.....

SO i want a better way to check for pushing elements and resizing the master array of 5 into 4 or lesser bsaed on the element shift. Its simple loic but i cant get the psuedo code...please help me!!!!
thanks
regards
KR

I agree with EvoEx on this one. The features are proposed for inclusion in a future standard and, given the way the standardisation process works, there is a risk a number of those proposed features will not be ratified, or there will be some changes to the manner in which they are eventually ratified. It is therefore misleading to suggest the features are "part of the language".

I certainly wouldn't advocate that any novice use such features. Experienced programmers, who are both willing to accept and able to manage the risks, may wish to use them (eg as a learning exercise, contributing to testing of the compiler).

hey
thanks for the help...
to run it continously.. i have to use a flag like a bool value.. inside the while and operations....
it helped
cheers!
regards
KR