array members working with them

Question

3.00/5 (1 vote)

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Hi.
i have the code below:

C++

int dynamicbinomial(int n, int k)
{
	int* b = new int[n,k];
	for(int i=0; i<= n; i++)
	{
		for(int j=0; j<= min(i,k); j++)
		{
			if(j==0 || j==i)
				b[i,j] = 1;
			else if(j==1 || j==i-1)
				b[i,j] = i;
			else
			        b[i,j] = b[i-1,j-1] + b[i-1,j];			
		}
	}
	return b[n,k];
}

it works correctly but in the last line:

C++

b[i,j] = b[i-1,j-1] + b[i-1,j];

when it reaches that line it does like it:
b[i,j] = b[0,j-1] + b[1,j-1] + b[2,j-1] + b[3,j-1] + ... + b[i-1,j] ;

it shows the sum of all numbers in line (i-1) form 0 to j

if you enter 5 as n and 3 as k you must have a matrix as below:

but it makes a matrix like it:

1
1 1
1 2 1
1 3 3
1 4 7
1 5 12

traced it this way:

1- i= 0 j= 0 k= 3 min( i,k )= 0 b[i,j]= b[0,0]= 1
2- i= 1 j= 0 k= 3 min( i,k )= 1 b[i,j]= b[1,0]= 1

3- i= 1 j= 1 k= 3 min( i,k )= 1 b[i,j]= b[1,1]= 1

4- i= 2 j= 0 k= 3 min( i,k )= 2 b[i,j]= b[2,0]= 1

5- i= 2 j= 1 k= 3 min( i,k )= 2 b[i,j]= b[2,1]= 2

6- i= 2 j= 2 k= 3 min( i,k )= 2 b[i,j]= b[2,2]= 1

7- i= 3 j= 0 k= 3 min( i,k )= 3 b[i,j]= b[3,0]= 1

8- i= 3 j= 1 k= 3 min( i,k )= 3 b[i,j]= b[3,1]= 3

9- i= 3 j= 2 k= 3 min( i,k )= 3 b[i,j]= b[3,2]= 3

10- i= 3 j= 3 k= 3 min( i,k )= 3 b[i,j]= b[3,3]= 1

11- i= 4 j= 0 k= 3 min( i,k )= 3 b[i,j]= b[4,0]= 1

12- i= 4 j= 1 k= 3 min( i,k )= 3 b[i,j]= b[4,1]= 4

13- i= 4 j= 2 k= 3 min( i,k )= 3 b[i,j]= b[4,2]= 7

14- i= 4 j= 3 k= 3 min( i,k )= 3 b[i,j]= b[4,3]= 4

15- i= 5 j= 0 k= 3 min( i,k )= 3 b[i,j]= b[5,0]= 1

16- i= 5 j= 1 k= 3 min( i,k )= 3 b[i,j]= b[5,1]= 5

17- i= 5 j= 2 k= 3 min( i,k )= 3 b[i,j]= b[5,2]= 12

see line 13 ?

Posted 2-Jun-13 6:20am

m.r.m.40

Updated 2-Jun-13 7:09am

v4

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Comments

Sergey Alexandrovich Kryukov 2-Jun-13 12:26pm

Did you try to run it step by step using the debugger and check up each step?
—SA

m.r.m.40 2-Jun-13 12:44pm

yes alexandrovix,
but couldn't get anything of it.

Sergey Alexandrovich Kryukov 2-Jun-13 14:19pm

Sorry, this isn't my name. Anyway, your "couldn't get..." is not informative. The ability to debug code is more important than your particular problem...
—SA

m.r.m.40 2-Jun-13 18:23pm

Please help.
it took 1 day for me to solve it. I'm still working on it, but yet no result.
Thank you,

2 solutions

Solution 1

Have you looked closely at your code?
Let's just shrink it a bit...

C#

int* b = new int[n, k];
for (int i = 0; i <= n; i++)
    {
    for (int j = 0; j <= Math.Min(i, k); j++)
        {
        b[i, j] = 1;
        }
    }

how many elements are there in the array, and how many are you assigning "1" to?

Posted 2-Jun-13 6:32am

OriginalGriff

Comments

m.r.m.40 2-Jun-13 13:10pm

n * k
at maximum

OriginalGriff 2-Jun-13 13:33pm

Are you sure about that?
How many times do you go round each loop if n and k are both one?

m.r.m.40 2-Jun-13 13:42pm

look griff i cant get what you mean.
could you please be straight?

OriginalGriff 2-Jun-13 14:36pm

int [4] declares an array of four integers.
for (i = 0; i <= 4; i ++)
Executes 5 times, when i equals 0,1,2,3, and 4
Your two loops run off the end, and start to corrupt data...

m.r.m.40 2-Jun-13 18:19pm

this is what i want:
when input is 5 it means make an array from 0 to 5.
and also the for loop is needed to act 6 times if n is 5.

OriginalGriff 3-Jun-13 3:09am

Then you need to change your code to do that, and try again. When you start "running off the ends" of your arrays, you will always cause problems, which can be very unpredictable.

I would suggest that you have a good hard look at the algorithm you are using - I suspect you may have misinterpreted it a bit somewhere.

singhgurpreet1583 3-Jun-13 4:04am

In that case you should allocate more memory using new.

m.r.m.40 2-Jun-13 13:43pm

all program is working as its supposed to.
but that line.
although i wrote exactly as the binomial factor using 2d array algorithm.

singhgurpreet1583 3-Jun-13 4:05am

Allocating memory for 2d array is not correct. I have printed the address of the memory you are allocated. The same address is printed for all the 5 rows. Needs to work on allocating memory using new operator for 2d array.

There is no problem with the last line b[i,j] = b[i-1,j-1] + b[i-1,j];
The only problem is that when you compute 4 value for your iteration:
12- i= 4 j= 1 k= 3 min( i,k )= 3 b[i,j]= b[4,1]= 4
It is copied at the same address at b[i-1, j-1].

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singhgurpreet1583 · Accepted Answer · 2013-06-02T22:13:00

I have modified your function. Hope it helps. Make sure to release the allocated memory.

C#

int dynamicbinomial(int n, int k)
{
    int **b = new int*[n];
    for(int i = 0; i < n; ++i) {
    b[i] = new int[k];
    }

    // Print the address of the allocated array
    for (int i=0; i<n; ++i)
    {
        for (int j=0; j<k; j++)
        {
            std::cout<< &b[i][j] << " ";
        }
        std::cout<<"\n";
    }

    for(int i=0; i< n; i++)
    {
        for(int j=0; j< std::min(i+1,k); j++)
        {
            if(j==0 || j==i)
                b[i][j] = 1;
            else if(j==1 || j==i-1)
                b[i][j] = i;
            else
            {
                b[i][j] = b[i-1][j-1] + b[i-1][j];
            }

            // Print the values and address
            std::cout<<b[i][j] << " " <<std::cout<< &b[i][j] << " ";
        }
        std::cout<<"\n";
    }
    return b[n][k];
}