Initialize a list of lists in C++

Question

I want to initialise a list of lists (as in Python) in C++. Suppose the list is: [['a','b'],['c','d']]

New to C++, and mostly worked in Python so not sure what to do.

std::vector<vector<string>> dp {{'a',b'}};

I have tried this but doesn't seem to work.

no matching function for call to 'std::vector<std::vector<std::__cxx11::basic_string<char> > >::vector(<brace-enclosed initializer list>)'|

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It really looks like you could use [a good book or two](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list/388282#388282). — Some programmer dude, Aug 12 '19 at 12:11

Ted Lyngmo · Accepted Answer · 2019-08-12T12:14:37.507

11

It looks similar in C++, but string literals should be surrounded by " not ' (which is for character literals).

#include <iostream>
#include <string>
#include <vector>

int main() {
    std::vector<std::vector<std::string>> dp{{"hello", "world"},
                                             {"goodbye", "for", "now"}};
    for(const auto& v : dp) {
        for(const std::string& s : v) {
            std::cout << s << " ";
        }
        std::cout << "\n";
    }
}

Output:

hello world
goodbye for now

edited Aug 12 '19 at 12:14

answered Aug 12 '19 at 12:04

Ted Lyngmo

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Oh, I used single quotes! Thanks! – mayaa Aug 12 '19 at 12:12
1

The single quotes themselves are not an error. For instance, `vector> dp {{'a',b'}, {'c','d'}};` is OK. C++ has stronger typing than Python; your element initializers' type needs to be similar ("convertible") to the container element type. – MSalters Aug 12 '19 at 12:21
1

@MSalters Yes, but OP needs to be extra careful in this specific case. Char is implicit convertible to int, so if you accidentally mixes notation then it can go wrong without producing an error: `vector dp {'c', "d"}` compiles happily and produces a vector with 99 'd's (since 'c' = to 99 in the ascii table). – Frodyne Aug 12 '19 at 12:40

score 0 · Answer 2 · answered Aug 12 '19 at 12:05

0

Are you after something like this?

    std::vector<std::vector<std::string>> dp = {
        {"a", "b"},
        {"c", "d"},
        {"e","f"}
    };

answered Aug 12 '19 at 12:05

Frodyne

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Initialize a list of lists in C++

2 Answers2