{"id":43,"date":"2021-05-24T17:18:31","date_gmt":"2021-05-24T17:18:31","guid":{"rendered":"https:\/\/blog.wordgeeks.net\/?page_id=43"},"modified":"2026-01-19T02:39:49","modified_gmt":"2026-01-19T02:39:49","slug":"programming-traps","status":"publish","type":"page","link":"https:\/\/blog.wordgeeks.net\/?page_id=43","title":{"rendered":"Programming Traps"},"content":{"rendered":"\n

partial_sum<\/code> and overflow in C++<\/h3>\n\n\n\n

If we have a vector<int> a<\/code> and vector<int64_t> b<\/code>, doing partial_sum(a.begin(), a.end(), b.begin())<\/code> WILL cause potential overflow, because the implementation<\/a> is maintaining a running sum of type typename iterator_traits::value_type<\/code> (int<\/code> in this case). Similarly, be careful of the initial value in accumulate<\/code><\/a>.<\/p>\n\n\n\n

Initialization<\/h3>\n\n\n\n

I vaguely knew that you can use curly braces when invoking constructors in C++, but never dived into it.<\/p>\n\n\n\n

Things like vector<int> a{3, 2};<\/code> contradict my intuition. Instead of creating a vector of 3 elements with the value of 2, it creates a vector of 2 elements, with value 3 and 2. That’s called list initialization<\/a> (see constructor 10 for vector<\/a>). You can compare https:\/\/gcc.godbolt.org\/z\/j13hfocv8<\/a> and https:\/\/gcc.godbolt.org\/z\/1svqMGzcY<\/a>, and pay attention to the actual constructor invoked. The same applies to string<\/a> (https:\/\/gcc.godbolt.org\/z\/ed1sjKPMb<\/a>).<\/p>\n\n\n\n

Perhaps the most interesting part is, if a constructor with initializer list is missing, curly braces will match to other constructors<\/a>.<\/p>\n\n\n\n

Tuple Assignment in Python<\/h3>\n\n\n\n

When you write a, b, c = x, y, z<\/code> in Python, the assignment<\/strong> happens from left to right<\/strong>.<\/p>\n\n\n\n

Most of the time, this won’t be a problem. Until I was dealing with some Linked List problem, and tried to reverse the list as I go. What I wrote was prev, curr, curr.next = curr, curr.next, prev<\/code> (the curr<\/code> moves to the next node BEFORE we update curr.next<\/code>).<\/p>\n\n\n\n

endl in C++<\/h3>\n\n\n\n

From cppreference<\/a>:<\/p>\n\n\n\n

\n

Inserts a newline character into the output sequence os<\/code> and flushes it as if by calling os.put(os.widen('\\n'))<\/code> followed by os.flush()<\/code>.”<\/p>\n<\/blockquote>\n\n\n\n

When a problem requires large amounts of output, the repeated flush can lead to slow execution.<\/p>\n\n\n\n

multiset in C++<\/h3>\n\n\n\n

The erase<\/code> function removes all<\/strong> elements with the key equivalent to the given parameter if it’s of Key<\/code> (const reference or r-value reference)<\/a>.<\/p>\n\n\n\n

To remove a single occurrence, use an iterator<\/a>.<\/p>\n\n\n\n

Emplace with nested vector<\/h3>\n\n\n\n

What is wrong with this code:<\/p>\n\n\n\n

void f() {\n  int n;\n  cin >> n;\n  vector<vector<int>> vs(n);\n  for (int i = 0; i < n; ++i) {\n    int m;\n    cin >> m;\n    vs.reserve(m);\n    for (int j = 0; j < m; ++j) {\n      int k;\n      cin >> k;\n      vs.emplace_back(some_func_that_returns_int(k));\n    }\n  }\n}<\/pre><\/div>\n\n\n\n
<\/p>\n","protected":false},"excerpt":{"rendered":"
partial_sum and overflow in C++ If we have a vector<int> a and vector<int64_t> b, doing partial_sum(a.begin(), a.end(), b.begin()) WILL cause potential overflow, because the implementation is maintaining a running sum of type typename iterator_traits::value_type (int in this case). Similarly, be careful of the initial value in accumulate. Initialization I vaguely knew that you can use… Continue reading Programming Traps<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":1,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-43","page","type-page","status-publish","hentry","entry"],"_links":{"self":[{"href":"https:\/\/blog.wordgeeks.net\/index.php?rest_route=\/wp\/v2\/pages\/43","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/blog.wordgeeks.net\/index.php?rest_route=\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/blog.wordgeeks.net\/index.php?rest_route=\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/blog.wordgeeks.net\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/blog.wordgeeks.net\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=43"}],"version-history":[{"count":8,"href":"https:\/\/blog.wordgeeks.net\/index.php?rest_route=\/wp\/v2\/pages\/43\/revisions"}],"predecessor-version":[{"id":274,"href":"https:\/\/blog.wordgeeks.net\/index.php?rest_route=\/wp\/v2\/pages\/43\/revisions\/274"}],"wp:attachment":[{"href":"https:\/\/blog.wordgeeks.net\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=43"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}