Concise lists/vectors in C++ - c++

Question

I'm currently translating an algorithm in Python to C++.
This line EXCH_SYMBOL_SETS = [["i", "1", "l"], ["s", "5"], ["b", "8"], ["m", "n"]]
is now
vector<vector<char>> exch_symbols;
vector<char> vector_1il;
vector_1il.push_back('1');
vector_1il.push_back('i');
vector_1il.push_back('l');
vector<char> vector_5s;
vector_5s.push_back('5');
vector_5s.push_back('s');
vector<char> vector_8b;
vector_8b.push_back('8');
vector_8b.push_back('b');
vector<char> vector_mn;
vector_mn.push_back('m');
vector_mn.push_back('n');
exch_symbols.push_back(vector_1il);
exch_symbols.push_back(vector_5s);
exch_symbols.push_back(vector_8b);
exch_symbols.push_back(vector_mn);
I hate to have an intermediate named variable for each inner variable in a 2-D vector. I'm not really familiar with multidimensional datastructures in C++. Is there a better way?
What's happening afterwards is this:
multimap<char, char> exch_symbol_map;
/*# Insert all possibilities
for symbol_set in EXCH_SYMBOL_SETS:
for symbol in symbol_set:
for symbol2 in symbol_set:
if symbol != symbol2:
exch_symbol_map[symbol].add(symbol2)*/
void insert_all_exch_pairs(const vector<vector<char>>& exch_symbols) {
for (vector<vector<char>>::const_iterator symsets_it = exch_symbols.begin();
symsets_it != exch_symbols.end(); ++symsets_it) {
for (vector<char>::const_iterator sym1_it = symsets_it->begin();
sym1_it != symsets_it->end(); ++sym1_it) {
for (vector<char>::const_iterator sym2_it = symsets_it->begin();
sym2_it != symsets_it->end(); ++sym2_it) {
if (sym1_it != sym2_it) {
exch_symbol_map.insert(pair<char, char>(*sym1_it, *sym2_it));
}
}
}
}
}
So this algorithm should work in one way or another with the representation here. The goal is that EXCH_SYMBOL_SETS can be easily changed later to include new groups of chars or add new letters to existing groups. Thank you!

Answer 1

I would refactor, instead of vector<char>, use std::string as internal, i.e.
vector<string> exch_symbols;
exch_symbols.push_back("1il");
exch_symbols.push_back("s5");
exch_symbols.push_back("b8");
exch_symbols.push_back("mn");
then change your insert method:
void insert_all_exch_pairs(const vector<string>& exch_symbols)
{
for (vector<string>::const_iterator symsets_it = exch_symbols.begin(); symsets_it != exch_symbols.end(); ++symsets_it)
{
for (string::const_iterator sym1_it = symsets_it->begin(); sym1_it != symsets_it->end(); ++sym1_it)
{
for (string::const_iterator sym2_it = symsets_it->begin(); sym2_it != symsets_it->end(); ++sym2_it)
{
if (sym1_it != sym2_it)
exch_symbol_map.insert(pair<char, char>(*sym1_it, *sym2_it));
}
}
}
}

Answer 2

You could shorten it by getting rid of the intermediate values
vector<vector<char> > exch_symbols(4, vector<char>()); //>> is not valid in C++98 btw.
//exch_symbols[0].reserve(3)
exch_symbols[0].push_back('i');
etc.
You could also use boost.assign or something similiar
EXCH_SYMBOL_SETS = [["i", "1", "l"], ["s", "5"], ["b", "8"], ["m", "n"]] then becomes
vector<vector<char>> exch_symbols(list_of(vector<char>(list_of('i')('1')('l')))(vector<char>(list_of('s')('5'))(list_of('m')('n'))) (not tested and never used it with nested vectors, but it should be something like this)

Answer 3

For your real question of...
how could I translate L = [A, [B],
[[C], D]]] to C++ ... at all!
There is no direct translation - you've switched from storing values of the same type to storing values of variable type. Python allows this because it's a dynamically typed language, not because it has a nicer array syntax.
There are ways to replicate the behaviour in C++ (e.g. a vector of boost::any or boost::variant, or a user defined container class that supports this behviour), but it's never going to be as easy as it is in Python.

Answer 4

Your code:
vector<char> vector_1il;
vector_1il.push_back('1');
vector_1il.push_back('i');
vector_1il.push_back('l');
Concise code:
char values[] = "1il";
vector<char> vector_1il(&values[0], &values[3]);
Is it fine with you?
If you want to use std::string as suggested by Nim, then you can use even this:
//Concise form of what Nim suggested!
std::string s[] = {"1il", "5s", "8b", "mn"};
vector<std::string> exch_symbols(&s[0], &s[4]);
Rest you can follow Nim's post. :-)

Answer 5

In c++0x the instruction
vector<string> EXCH_SYMBOL_SETS={"i1l", "s5", "b8", "mn"} ;
compiles and works fine. Sadly enough the apparently similar statement
vector<vector<char>> EXCH_SYMBOL_SETS={{'i','1','l'},{'s','5'}, {'b','8'}, {'m','n'}};
doesn't work :-(.
This is implemented in g++ 4.5.0 or later you should add the -std=c++0x option. I think this feature is not yet avaliable in microsoft c (VC10), and I don't know what's the status of other compilers.

Answer 6

I know that this is an old post, but in case anyone stumbles across it, C++ has gotten MUCH better at dealing with this stuff:
In c++11 the first code block can simply be re-written in as:
std::vector<std::string> exch_symbols {"1il", "5s", "8b", "mn"};
This isn't special to string either, we can nest vector like so:
std::vector<std::vector<int>> vov {{1,2,3}, {2,3,5,7,11}};
And here's the entire code in c++14-style, with an added cout at the end:
#include <iostream>
#include <map>
#include <string>
#include <vector>
void add_all_char_pairs (std::multimap<char, char> & mmap, const std::string & str)
{
// we choose not to add {str[i], str[i]} pairs for some reason...
const int s = str.size();
for (int i1 = 0; i1 < s; ++i1)
{
char c1 = str[i1];
for (int i2 = i1 + 1; i2 < s; ++i2)
{
char c2 = str[i2];
mmap.insert({c1, c2});
mmap.insert({c2, c1});
}
}
}
auto all_char_pairs_of_each_str (const std::vector<std::string> & strs)
{
std::multimap<char, char> mmap;
for (auto & str : strs)
{
add_all_char_pairs(mmap, str);
}
return mmap;
}
int main ()
{
std::vector<std::string> exch_symbols {"1il", "5s", "8b", "mn"};
auto mmap = all_char_pairs_of_each_str(exch_symbols);
for (auto e : mmap)
{
std::cout << e.first << e.second << std::endl;
}
}