Concatenating two 3 by 3 arrays into one array in C++

Question

As the title indicates, I want to concatenate two arrays into one larger array. for example:

array1= 1  2  3
        4  5  6
        7  8  9
array1= 10  20  30
        40  50  60
        70  80  90
array3= 1  2  3  10  20  30
        4  5  6  40  50  60
        7  8  9  70  80  90

So, I wrote this code:

#include <iostream>

using namespace std;

int main()
{
    int array1[3][3],array2[3][3],array3[3][6];
    int i,j;

    //Matrix 1 input
    cout<<"Enter matrix 1\n";
    for (i=0;i<3;i++)
    {
        for (j=0;j<3;j++)
        {
            cin>>array1[i][j];
            array3[i][j]=array1[i][j]; //Assigning array1 values to array3
        }
    }

    //array2 input
    cout<<"Enter matrix 2\n";
    for (i=0;i<3;i++)
    {
        for (j=3;j<6;j++)
        {
            cin>>array2[i][j];
            array3[i][j]=array2[i][j]; //Assigning array2 values to array3
        }
    }

    //array3 output
    cout<<"New matrix is\n";
    for (i=0;i<3;i++)
    {
        for (j=0;j<6;j++)
        {
            cout<<array3[i][j]<<"\t";
        }
    cout<<"\n";
    }
}

But when I execute it, I ended up with last row of array2 being the (2,3), (2,4) and (2,5) elements (which is right), but also being the (0,1), (0,2) and (0,3) elemnts (which should be [1 2 3]).

array3= 70  80  90  10  20  30
        4   5    6  40  50  60
        7   8    9  70  80  90

So, what's happening here?

EDIT: I did the following:

for (i=0;i<3;i++)
{
    for (j=0;j<3;j++)
    {
        cin>>array2[i][j];
        array3[i][j+3]=array2[i][j]; //Assigning array2 values to array3 and adding 3 to j
    }

And it went ok. Is the method I used "professional"? }

In your array2 input part, j is [3,6) while array2 is array2[3][3] — justmscs
– justmscs, Commented Jan 23, 2015 at 6:51

John Zwinck · Accepted Answer · 2015-01-23 06:50:52Z

1

This is undefined behavior:

for (i=0;i<3;i++)
    for (j=3;j<6;j++)
        array3[i][j]=array2[i][j];

array2 is 3x3 but you're indexing into it starting with [0][3] which is an error. You could get runtime checking of this sort of thing if you used C++11 std::array and its .at() method instead of raw C arrays.

answered Jan 23, 2015 at 6:50

John Zwinck

252k44 gold badges346 silver badges459 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

user657267 Over a year ago

GCC's -Warray-bounds caught this on compile.

John Zwinck Over a year ago

@user657267: Good point, though on my system it only detects this if compiling with -O2 or higher. A simple g++ -Wall -O2 does the trick.

Serge Ballesta · Accepted Answer · 2015-01-23 07:26:12Z

0

array2 is a 3x3 array and you use j indices between 3 and 6 : you access outside of your matrix anywhere in memory.

You should instead write (eg) :

//array2 input
cout<<"Enter matrix 2\n";
for (i=0;i<3;i++)
{
    for (j=0;j<3;j++)
    {
        cin>>array2[i][j]; // j is correct for array2
        array3[i][j+3]=array2[i][j]; //Assigning array2 values to array3
    }
}

answered Jan 23, 2015 at 7:26

Serge Ballesta

150k13 gold badges137 silver badges267 bronze badges

Collectives™ on Stack Overflow

Concatenating two 3 by 3 arrays into one array in C++

2 Answers 2

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related