{"id":11141,"date":"2022-09-26T07:19:00","date_gmt":"2022-09-26T01:49:00","guid":{"rendered":"https:\/\/jassweb.com\/solved\/solved-does-an-array-of-vectors-form-a-2d-vector\/"},"modified":"2022-09-26T07:19:00","modified_gmt":"2022-09-26T01:49:00","slug":"solved-does-an-array-of-vectors-form-a-2d-vector","status":"publish","type":"post","link":"https:\/\/jassweb.com\/solved\/solved-does-an-array-of-vectors-form-a-2d-vector\/","title":{"rendered":"[Solved] Does an array of vectors form a 2D vector?"},"content":{"rendered":"<p> [ad_1]<br \/>\n<\/p>\n<div id=\"answer-45988637\" class=\"answer js-answer accepted-answer js-accepted-answer\" data-answerid=\"45988637\" data-parentid=\"45988450\" data-score=\"2\" data-position-on-page=\"1\" data-highest-scored=\"1\" data-question-has-accepted-highest-score=\"1\" itemprop=\"acceptedAnswer\" itemscope itemtype=\"https:\/\/schema.org\/Answer\">\n<div class=\"post-layout\">\n<div class=\"votecell post-layout--left\"><\/div>\n<div class=\"answercell post-layout--right\">\n<div class=\"s-prose js-post-body\" itemprop=\"text\">\n<p>No, but it gives similar behaviour. So what you have created is a pointer to an array of vectors. So you will end up with this:<\/p>\n<pre><code>adj[0] = []\nadj[1] = []\n...\nadj[number_nodes] = []\n<\/code><\/pre>\n<p>and doing a <code>push_back<\/code> is a legitimate way to add to the vectors:<\/p>\n<pre><code>adj[0].push_back(some_num) --&gt;  adj[0] = [some_num]\n<\/code><\/pre>\n<p><strong>But this is a terrible way to approach this! Why?<\/strong><\/p>\n<ul>\n<li>You are using raw memory that you will have to manage and make sure you delete.<\/li>\n<li>You cant use any of the awesome <code>std::vector<\/code> functionality on the first dimension of the matrix.<\/li>\n<li>There is no good way to figure out the size of the vector unless you know about the variable number_of_nodes.<\/li>\n<li>&#8230; A long list, but you get it.<\/li>\n<\/ul>\n<p>You can already see that a <code>std::vector<\/code> can be used as a 1D matrix. So use 2 vectors:<\/p>\n<pre><code>std::vector&lt;std::vector&lt;int&gt;&gt; Matrix2D;\n<\/code><\/pre>\n<p>If you need an initial size in some dimension then you can do this:<\/p>\n<pre><code>std::vector&lt;std::vector&lt;int&gt;&gt; Matrix2D(number_of_nodes, std::vector&lt;int&gt;());\n<\/code><\/pre>\n<p>Or this:<\/p>\n<pre><code>Matrix2D.resize(number_of_nodes);\n<\/code><\/pre>\n<p>Or if that size is fixed at compile time then you could even do this:<\/p>\n<pre><code>const int number_of_nodes = 10;\nstd::array&lt;std::vector&lt;int&gt;, number_of_nodes&gt; Matrix2D;\n<\/code><\/pre>\n<p>Or go extra big and get a library for Matrix use like <a rel=\"nofollow noopener\" target=\"_blank\" href=\"http:\/\/eigen.tuxfamily.org\/index.php?title=Main_Page\">Eigen.<\/a><\/p>\n<\/p><\/div>\n<div class=\"mt24\"><\/div>\n<\/div>\n<p>            <span class=\"d-none\" itemprop=\"commentCount\">8<\/span> <\/p><\/div>\n<\/div>\n<p>[ad_2]<\/p>\n<p>solved Does an array of vectors form a 2D vector? <\/p>\n","protected":false},"excerpt":{"rendered":"<p>[ad_1] No, but it gives similar behaviour. So what you have created is a pointer to an array of vectors. So you will end up with this: adj[0] = [] adj[1] = [] &#8230; adj[number_nodes] = [] and doing a push_back is a legitimate way to add to the vectors: adj[0].push_back(some_num) &#8211;&gt; adj[0] = [some_num] &#8230; <a title=\"[Solved] Does an array of vectors form a 2D vector?\" class=\"read-more\" href=\"https:\/\/jassweb.com\/solved\/solved-does-an-array-of-vectors-form-a-2d-vector\/\" aria-label=\"More on [Solved] Does an array of vectors form a 2D vector?\">Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[320],"tags":[324,771,712,806],"class_list":["post-11141","post","type-post","status-publish","format-standard","hentry","category-solved","tag-c","tag-graph","tag-pointers","tag-vector"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v26.5 - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>[Solved] Does an array of vectors form a 2D vector? - JassWeb<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/jassweb.com\/solved\/solved-does-an-array-of-vectors-form-a-2d-vector\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"[Solved] Does an array of vectors form a 2D vector? - JassWeb\" \/>\n<meta property=\"og:description\" content=\"[ad_1] No, but it gives similar behaviour. So what you have created is a pointer to an array of vectors. So you will end up with this: adj[0] = [] adj[1] = [] ... adj[number_nodes] = [] and doing a push_back is a legitimate way to add to the vectors: adj[0].push_back(some_num) --&gt; adj[0] = [some_num] ... 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So what you have created is a pointer to an array of vectors. So you will end up with this: adj[0] = [] adj[1] = [] ... adj[number_nodes] = [] and doing a push_back is a legitimate way to add to the vectors: adj[0].push_back(some_num) --&gt; adj[0] = [some_num] ... 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