Understanding multidimensional string array in C++ - c++

Question

I am new to c++. I can not understand why the following code prints the "r" string. I think the it should be an array of 2X3X4 elements, so by pointing to the arr[0][0][0] i would expect the first char in the first string of the first arr=a, but this prints abcd. Can anyone explain it?
#include <iostream>
using namespace std;
int main()
{
string arr [2] [3] [4]={
{"abcd","efgh","ijkl"},
{"mnop","qrst","xywz"}
};
cout<<arr [1] [0] [1] [1]<<endl;
return 0;
}
Edit:
What makes me confused is the behavior in python. The following python code prints a:
arr=[["abcd","efgh","ijkl"],["mnop","qrst","xwyz"]]
print arr[0][0][0]
It addresses to the first letter of the first string in the first list.
I would think that the equivalent of this in c++ would be:
#include <iostream>
using namespace std;
int main()
{
string arr [2] [3] [4]={
{"abcd","efgh","ijkl"},
{"mnop","qrst","xywz"}
};
cout<<arr[0][0][0]<<endl;
return 0;
}
by pointing to the first letter in the first string of the first array. But this prints the first string abcd. My question is why should i put another [0] in order to get to the a?

Answer 1

Your initializer populates the array as follows:
arr[0][0][0] = "abcd";
arr[0][0][1] = "efgh";
arr[0][0][2] = "ijkl";
arr[1][0][0] = "mnop";
arr[1][0][1] = "qrst";
arr[1][0][2] = "xywz";
All other elements are default-initialized to empty string.
Thus, arr[1][0][1] is the string containing "qrst", and arr[1][0][1][1] is the second character of that string, namely 'r'.

Answer 2

You've confused the standard library string object with the concept of a c-string/string literal, and you've helped yourself with this by avoiding the use of the std:: prefix. If we add this, it starts to make more sense:
std::string arr [2] [3] [4]={
{"abcd","efgh","ijkl"},
{"mnop","qrst","xywz"}
};
What you are declaring here is an array of 2 x 3 x 4 instances of std::string. But what you wrote looks like you thought you were declaring character arrays:
char arr [2] [3] [4] = {
{"abcd","efgh","ijkl"},
{"mnop","qrst","xywz"}
};
would almost have the effect you were trying to achieve -- in this case arr[0][0][0] does point to a rather than the string.
Unfortunately the problem here is that you've specified a final dimension of 4 and then supplied 5-character c-strings to the initializer. Remember:
"abcd"
is equivalent to
{ 'a', 'b', 'c', 'd', 0 }
because c-strings are nul-terminated. So you would need to write
char arr [2] [3] [5] = {
{"abcd","efgh","ijkl"},
{"mnop","qrst","xywz"}
};
or, if what you actually want is specifically arrays of characters, not nul-terminated c-strings:
charr arr[2][3][4] = {
{ { 'a', 'b', 'c', 'd' }, { 'e', 'f', 'g', 'h' }, ...
std::string is a discrete object, not an alias for a c-string.
#include <iostream>
#include <string>
int main() {
std::string arr[2][3] = {
{ "abcd", "efgh", "ijkl" },
{ "mnop", "qrst", "wxyz" }, // who needs 'u' or 'v'?
};
std::cout << "arr[0][0] = " << arr[0][0] << "\n";
std::cout << "arr[0][0][0] = " << arr[0][0][0] << "\n";
}
http://ideone.com/JQrDxr

Answer 3

The array you initialized is probably not what you wanted to initialize.
One dimension string array
string arr [2]= {"abcd","efgh"};
Two dimensional string array
string arr [2][2]= {{"abcd","efgh"}, {"ijkl","mnop"}};
Three dimensional string array
string arr [2][2][2]= {
{
{"abcd","qwer"},
{"efgh","tyui"}
},
{
{"ijkl","zxcv"},
{"mnop","bnmo"}
}
};
so, cout<<arr [1] [0] [1] [1]<<endl; will output 'x'

Answer 4

You can't compare python to c++ because in python a list and a string is nearly the same while in c++ there completely different. In Python it doesn't matter if you write
arr=[["abcd","efgh","ijkl"],["mnop","qrst","xwyz"]]
or
arr=[["a","b","c","d","e","f","g","h","i","j","k","l"],
["m", "n","o","p","q","r","s","t","x","w","y","z"]]
because there meaning the same. in c++ instead it treats a string as one "container" which leads to your observed behavior that it will assign these "containers" to the first indexes instead the individual chars. What you can instead do is
char[2][12] arr = {
{"a","b","c","d","e","f","g","h","i","j","k","l"},
{"m", "n","o","p","q","r","s","t","x","w","y","z"}
};