Expression must be modifiable lvalue - c++

Question

I'm trying to store a structure of capacity 6 in another structure.
struct eachElement {
float centerX;
float centerY;
int flagMountain;
};
eachElement cn[6];
struct characters {
eachElement each[6];
};
characters chars[1500];
float strtPt = 235.0;
float initializer = strtPt;
float endPt = 120.0;
float holder = endPt;
int count = 0;
int ctr = 0;
int cr = 0;
int countCharacters = 0;
int dup = 0;
while (holder < m_img_height) {
for (float i = initializer ; i < m_img_width - 500; ) {
float j = holder;
int ck = 1;
while (ck < 4) {
cvCircle(image, cvPoint(i, j), 3, cvScalar(0, 255, 0), 1);
cr = ctr++;
cn[cr].centerX = i;
cn[cr].centerY = j;
cn[cr].flagMountain = 1;
cvCircle(image, cvPoint(i + 5, j + 5), 1, cvScalar(0, 255, 0), 1);
j += 9.448;
count++;
ck++;
}
if (count == 6) {
i += 23.811;
count = 0;
ctr = 0;
dup = countCharacters++;
chars[dup].each = cn;
}
else
i += 9.448;
}
holder += 56.686;
}
In this line,
chars[dup].each = cn;
it gives me an error saying expression must be a modifiable lvalue.
Even though I'm assigning it to the same type, I got this error.
Any help would be appreciated.

Answer 1

I don't know what you try to achieve with your code, but you must specify an array index for the each member array to access any of it's innards:
chars[dup].each[0] = cn[0];
// ^^^ ^^^
The array start address cannot be changed
chars[dup].each = cn;
hence the compiler error.
To fix that use std::copy():
std::copy(std::begin(cn),std::end(cn),std::begin(chars[dup].each));

Answer 2

Arrays do not have the copy assignment operator. You have to copy each element of one array into another array.
Thus this statement
chars[dup].each = cn;
is wrong.
You can use standard algorithm std::copy to copy one array into another. For example
#include <algorithm>
//...
std::copy( std::begin( cn ), std::end( cn ), std::begin( chars[dup].each ) );
Or if the compiler does not support function std::begin and std::end then you can just write
#include <algorithm>
//...
std::copy( cn, cn + 6, chars[dup].each );
or
std::copy( cn, cn + sizeof( cn ) / sizeof( *cn ), chars[dup].each );

Answer 3

Your struct has fixed buffer
struct characters {
eachElement each[6];
};
and your assignemt
chars[dup].each = cn;
will be legal (I don't know if will make sense) if the struct were like this:
struct characters {
eachElement * each;
};
The code is hard to understand for me, I cannot propose an equivalent solution for your goal. But such is the background of error message
maybe, given your declaration - but I don't understand the goal - this style would work:
for(int i=1;i<6;i++)
chars[dup].each[i] = cn;