indices

问题 I had a generic question about JavaScript arrays. Are array indices in JavaScript internally handled as strings? I read somewhere that because arrays are objects in JavaScript, the index is actually a string. I am a bit confused about this, and would be glad for any explanation. 回答1: That is correct so: > var a = ['a','b','c'] undefined > a [ 'a', 'b', 'c' ] > a[0] 'a' > a['0'] 'a' > a['4'] = 'e' 'e' > a[3] = 'd' 'd' > a [ 'a', 'b', 'c', 'd', 'e' ] 回答2: Formally, all property names are

问题 Suppose I\'m representing a matrix foo of values using std::vector : int rows = 5; int cols = 10; auto foo = vector<vector<double>>(rows, vector<double>(cols)); Is there a cleverly simple way for me to get a vector<int> of size rows that contains the first \"column\" of foo: {foo[0][0], foo[0][1], foo[0][2], foo[0][3], foo[0][4] } Put another way, can I \"transpose\" foo so that the following three things are true: foo_transpose.size() == cols foo_transpose[0].size() == rows foo_transpose[0]

问题 I have a 2 dimensional NumPy array. I know how to get the maximum values over axes: >>> a = array([[1,2,3],[4,3,1]]) >>> amax(a,axis=0) array([4, 3, 3]) How can I get the indices of the maximum elements? So I would like as output array([1,1,0]) 回答1: >>> a.argmax(axis=0) array([1, 1, 0]) 回答2: >>> import numpy as np >>> a = np.array([[1,2,3],[4,3,1]]) >>> i,j = np.unravel_index(a.argmax(), a.shape) >>> a[i,j] 4 回答3: argmax() will only return the first occurrence for each row. http://docs.scipy

问题 I have a tbl_df where I want to group_by(u, v) for each distinct integer combination observed with (u, v) . EDIT: this was resolved by adding group_indices() back in dplyr 0.4.0 a) I then want to assign each distinct group some arbitrary distinct number label=1,2,3... e.g. the combination (u,v)==(2,3) could get label 1, (1,3) could get 2, and so on. How to do this with one mutate() , without a three-step summarize-and-self-join? dplyr has a neat function n() , but that gives the number of

问题 Is there a way to get the list index name in my lapply() function? n = names(mylist) lapply(mylist, function(list.elem) { cat(\"What is the name of this list element?\\n\" }) I asked before if it\'s possible to preserve the index names in the lapply() returned list, but I still don\'t know if there is an easy way to fetch each element name inside the custom function. I would like to avoid to call lapply on the names themselves, I\'d rather get the name in the function parameters. 回答1: