c++ error in 2D array index

Question

I am working on a C++ program and the issue is in the below code. (NB: code has been simplified for readability but the essence is still there). In the first part i am just creating a 2D array with a few simple conditions

    rows= 5/2;
    cols= 4/2;

    char** array;
    if(rows%2 !=0 ){
     array = new char*[rows+1];
    }else {
         array = new char*[rows];
    }
    for(int k = 0; k < rows; k++)
    {
        if(columns%2 !=0){
            array[k] = new char[cols+1];
        }else{
            array[k] = new char[cols];
        }

    }

So far so good. the code works perfectly. the next part is where the issue is,

  for(int k = rows; k<5; k++){
        for (int l=0; l< 2; l++){
            array[k-rows][l]=Array2[k][l];            
        }
    }

so basically this code is just retrieving a small part of a larger array (array2) and inserting it into array. but An error keeps coming up at array[k-rows][l]=Array2[k][l]; which says Thread 1:Exc_BAD_ACCESS(code = 1, address = 0xe). (i am using xcode 7.2)

4 times array. That is not the issue. I also tested this with array[k-rows][l]='X' which removes any uncertainty with array2 and still the error occurs. — user134132523
– user134132523, Commented Dec 23, 2015 at 4:54
Why don't you step through the loop and see what the values of rows, k, and l are, and what the values of array[k-rows] and array[k-rows][l] are? That should help you quickly narrow down which part of the expression is going wrong and probably point to why. — user1118321
– user1118321, Commented Dec 23, 2015 at 5:13
array[k-rows] accesses out of bounds . Your array is 2x2, but k-rows takes values 0, 1, 2 (the last of those being out of bounds). You probably want to do the %2 check on 5 BEFORE diving it by 2. — M.M
– M.M, Commented Dec 23, 2015 at 5:24
Why don't you use (a) std::vector and (b) one vector of size rows*cols. That would be considerably easier. — JohnB
– JohnB, Commented Dec 23, 2015 at 6:06

Rabbid76 · Accepted Answer · 2015-12-23 06:13:05Z

2

With rows= 5/2; you set rows to 2. Than you allocate 2 rows array = new char*[rows];. But you iterate from 2 to 4 for(int k = rows; k<5; k++) and write to rows 0 to 2 array[k-rows][l]. You never allocated row with index 2.

rows= 5/2; // now rows is 2
...
if(rows%2 !=0 ) // 2%2!=0

Try this:

rows= 5;
cols= 4;

int allocRows = ( rows%2==0 ) ? rows/2 : rows/2+1;
rows=rows/2;

int allocCols = ( cols%2==0 ) ? cols/2 : cols/2+1;
cols=cols/2;

array = new char*[allocRows];
for(int k = 0; k < allocRows; k++)
    array[k] = new char[allocCols];

... or this ...

int origRows = rows; 
rows= 5/2;
...
if(origRows%2 !=0 )
...

edited Dec 23, 2015 at 6:13

answered Dec 23, 2015 at 5:24

Rabbid76

212k30 gold badges164 silver badges206 bronze badges

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

4 Comments

user134132523 Over a year ago

i am allocating 3 rows array = new char*[rows+1]; due to the conditions in the first piece of code

1201ProgramAlarm Over a year ago

@john.s Or rows = (rows + 1) / 2; cols = (cols + 1) / 2; instead of the if/else, since both will be positive.

user134132523 Over a year ago

@Gernot1976 the way you did it is correct, however i do want to iterate from 2 to 4, the way you solved it would iterate from 3 to 4.

user134132523 Over a year ago

would this iterate from 2 to 4? i think i still need to do the if else block since i have other stuff to put in the block

Collectives™ on Stack Overflow

c++ error in 2D array index

1 Answer 1

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related