C++ Compare two multidimensional arrays - c++

Question

I have two arrays:
unsigned char channeltab1[7][12]; //array which I receive from socket(Array is the same as below)
unsigned char equal_channeltab1[7][12] //arrays which I wants to compare with channeltab1
{
{0x10, 0x0f, 0x02, 0x02, 0x01, 0x00, 0x00, 0x06, 0x10, 0x0e, 0xff, 0xef},
{0x10, 0x0f, 0x02, 0x02, 0x02, 0x00, 0x00, 0x06, 0x10, 0x0e, 0xff, 0xef},
{0x10, 0x0f, 0x02, 0x02, 0x03, 0x00, 0x00, 0x06, 0x10, 0x0e, 0xff, 0xef},
{0x10, 0x0f, 0x02, 0x02, 0x04, 0x00, 0x00, 0x06, 0x10, 0x0e, 0xff, 0xef},
{0x10, 0x0f, 0x02, 0x02, 0x05, 0x00, 0x00, 0x06, 0x10, 0x0e, 0xff, 0xef},
{0x10, 0x0f, 0x02, 0x02, 0x06, 0x00, 0x00, 0x06, 0x10, 0x0e, 0xff, 0xef},
{0x10, 0x0f, 0x02, 0x02, 0x07, 0x00, 0x00, 0x06, 0x10, 0x0e, 0xff, 0xef},
};
I want compare this arrays, but function strcmp work only with one-dimensional arrays.
I try use strncmp:
for(int x = 0; x<7; x++)
{
for(int i =1; x<12;i++)
{
if (strncmp (reinterpret_cast<const char*>(channeltab1[x][i]),reinterpret_cast<const char*>(equal_channeltab1[x][i]),2) == 0)
{
/*...*/
}
else
{
/*...*/
}
}
}
But when application run this instruction says:
Memory fault
If i use:
for(int x = 0; x<7; x++)
{
if (strncmp (reinterpret_cast<const char*>(channeltab1[x]),reinterpret_cast<const char*>(equal_channeltab1[x]),2) == 0)
{
/*..*/
}
else
{
/*..*/
}
}
They are not the same for program.
What should I do?

Answer 1

Following may help:
bool is_equal(const unsigned char (&lhs)[7][12], const unsigned char (&rhs)[7][12])
{
for (int i = 0; i != 7; ++i) {
if (memcmp(lhs[i], rhs[i], 12) != 0) {
return false;
}
}
return true;
}
or even (thanks to Hanno Binder)
bool is_equal(const unsigned char (&lhs)[7][12], const unsigned char (&rhs)[7][12])
{
return memcmp(lhs, rhs, sizeof(lhs)) != 0;
}

Answer 2

You can simply use memcmp
(sizeof(channeltab1) == sizeof(equal_channeltab1)
&& (memcmp(channeltab1, equal_channeltab1, sizeof(equal_channeltab1)) == 0)
Note you have to make sure theirs sizes are equal.

Answer 3

I would suggest you to implement a new comparing function rather then trying to use these C functions. You can start from something like this:
bool checkEquals(unsigned char** a, unsigned char** b, size_t outterSize, size_t innerSize)
{
for (size_t i = 0; i < outterSize; ++i)
{
for (size_t j = 0; j < innerSize; ++j)
{
if (a[i][j] != b[i][j])
{
return false;
}
}
}
return true;
}

Answer 4

"strncmp" is used for standard "C" strings comparsion (or ASCII ones) not for data as it's in your case (data and arrays are always the same in "C++" language). So you should use 'memcpy' instead. So your code will look something like this:
for(int x = 0; x<7; x++)
{
if (memcmp((void*)channeltab1[x],(void*)equal_channeltab1[x], 12 * sizeof(unsigned char)) == 0)
{
/*equal*/
}
else
{
/*not equal*/
}
}

Answer 5

Use std::Array
std::array<unsigned char, 7*12> channeltab1;
std::array<unsigned char, 7*12> equal_channeltab1 = {
0x10, 0x0f, 0x02, 0x02, 0x01, 0x00, 0x00, 0x06, 0x10, 0x0e, 0xff, 0xef,
0x10, 0x0f, 0x02, 0x02, 0x02, 0x00, 0x00, 0x06, 0x10, 0x0e, 0xff, 0xef,
0x10, 0x0f, 0x02, 0x02, 0x03, 0x00, 0x00, 0x06, 0x10, 0x0e, 0xff, 0xef,
0x10, 0x0f, 0x02, 0x02, 0x04, 0x00, 0x00, 0x06, 0x10, 0x0e, 0xff, 0xef,
0x10, 0x0f, 0x02, 0x02, 0x05, 0x00, 0x00, 0x06, 0x10, 0x0e, 0xff, 0xef,
0x10, 0x0f, 0x02, 0x02, 0x06, 0x00, 0x00, 0x06, 0x10, 0x0e, 0xff, 0xef,
0x10, 0x0f, 0x02, 0x02, 0x07, 0x00, 0x00, 0x06, 0x10, 0x0e, 0xff, 0xef
};
and to compare, you can now do:
if (channeltab1 == equal_channeltab1){
//do stuff
}
note, an array[x*y] is the same as an array[x][y]

Answer 6

Multidimensional arrays are store in continuous memory so actually memcmp should work, memcmp avoids mistakes in the cast.
memcmp(channeltab1, equal_channeltab1, sizeof(channeltab1))
Edit: strncmp doesn't work because there could be zero's in the values.