These days, it’s easy to do the right thing.<\/p>\n
Don’t do this:<\/p>\n
std::rand()<\/code>. Ever.<\/li>\n- Don’t use
std::random_shuffle()<\/code> to permute containers. (Too easy to misuse; can use std::rand()<\/code> under the hood.)<\/li>\n- Don’t use any kind of clock for a seed.<\/li>\n
- Don’t use mod (
%<\/code>) to get a random value into a range. It introduces bias.<\/li>\n- Don’t use
std::default_random_engine<\/code> – it’s probably not the best choice and can vary by implementation.<\/li>\n<\/ul>\nDo this:<\/p>\n
\n- Use
std::random_device()<\/code> as a source of entropy. It should be the best randomness your OS can supply.<\/li>\n- Use
std::shuffle()<\/code> to permute containers. Its interface doesn’t permit misuse like std::random_shuffle()<\/code>.<\/li>\n- Use
std::mt19937<\/code> (or std::mt19937_64<\/code>) as an RNG engine. (Or consider PCG, but that’s not standard – yet.)<\/li>\n- Seed the engine with as much seed data as it requires for its internal state (using its
state_size<\/code> member), otherwise you’re throwing away entropy. For std::mt19937<\/code> this is 624<\/em> 32-bit values (not<\/em> just a single 32-bit value). Use std::seed_seq<\/code> to help with initialization.<\/li>\n- RNG engines (particularly
std::mt19937<\/code>) can be expensive to initialize (and have large amounts of internal state), so don’t put them on the stack unnecessarily. Consider using thread_local<\/code>.<\/li>\n- Use
std::uniform_int_distribution<\/code> (and the other distributions) to get your random number into the required range. The distributions are carefully crafted to be unbiased (why you shouldn’t use %<\/code>). Note that they work on closed (inclusive) ranges, not half-open ranges – otherwise the max value would be unreachable.<\/li>\n<\/ul>\nWrong:<\/p>\n
\/\/ init RNG\nsrand(time(NULL));\n\n\/\/ get random number\nint n = rand() % 100;\n<\/pre>\nRight:<\/p>\n
\/\/ init RNG\nstd::array<int, std::mt19937::state_size> seed_data;\nstd::random_device r;\nstd::generate_n(seed_data.data(), seed_data.size(), std::ref(r));\nstd::seed_seq seq(std::begin(seed_data), std::end(seed_data));\nstd::mt19937 gen(seq);\n\n\/\/ get random number\nstd::uniform_int_distribution dis(0,99);\nint n = dis(gen);\n<\/pre>\nEdit: Alternatively, Melissa O’Neill<\/a> has produced randutils<\/a> which wraps C++11 random number generation in a nice interface, and does the right thing. She knows what she’s talking about: her work on randomness is worth your time.<\/p>\n","protected":false},"excerpt":{"rendered":"These days, it’s easy to do the right thing. Don’t do this: Don’t use std::rand(). Ever. Don’t use std::random_shuffle() to permute containers. (Too easy to misuse; can use std::rand() under the hood.) Don’t use any kind of clock for a seed. Don’t use mod (%) to get a random value…<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[22,8],"tags":[],"class_list":["post-1081","post","type-post","status-publish","format-standard","hentry","category-cpp","category-programming"],"_links":{"self":[{"href":"https:\/\/www.elbeno.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1081","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.elbeno.com\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.elbeno.com\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.elbeno.com\/blog\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.elbeno.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1081"}],"version-history":[{"count":7,"href":"https:\/\/www.elbeno.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1081\/revisions"}],"predecessor-version":[{"id":1793,"href":"https:\/\/www.elbeno.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1081\/revisions\/1793"}],"wp:attachment":[{"href":"https:\/\/www.elbeno.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1081"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.elbeno.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1081"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.elbeno.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1081"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}