slice

问题 According to this Go Data Structures article, under the Strings section it states that taking a slice of a string will keep the original string in memory. "(As an aside, there is a well-known gotcha in Java and other languages that when you slice a string to save a small piece, the reference to the original keeps the entire original string in memory even though only a small amount is still needed. Go has this gotcha too. The alternative, which we tried and rejected, is to make string slicing

问题 Suppose I want the first element, the 3rd through 200th elements, and the 201st element through the last element by step-size 3, from a list in Python. One way to do it is with distinct indexing and concatenation: new_list = old_list[0:1] + old_list[3:201] + old_list[201::3] Is there a way to do this with just one index on old_list ? I would like something like the following (I know this doesn't syntactically work since list indices cannot be lists and since Python unfortunately doesn't have

问题 In Python (2 and 3). Whenever we use list slicing it returns a new object, e.g.: l1 = [1,2,3,4] print(id(l1)) l2 = l1[:] print(id(l2)) Output >>> 140344378384464 >>> 140344378387272 If the same thing is repeated with tuple, the same object is returned, e.g.: t1 = (1,2,3,4) t2 = t1[:] print(id(t1)) print(id(t2)) Output >>> 140344379214896 >>> 140344379214896 It would be great if someone can shed some light on why this is happening, throughout my Python experience I was under the impression

问题 In Go, if I have a string variable s: var s string = "a,b,c,d,e" How can I convert or split or explode it into a slice or an array of strings so that it will become: arr[0] = "a" ... arr[4] = "e" 回答1: You should use the strings package for that. stringSlice := strings.Split(s, ",") http://play.golang.org/p/UKZbcuJUPP 来源： https://stackoverflow.com/questions/14263733/how-to-slice-a-string-using-a-delimiter

问题 I have this simple code: import "fmt" type Foo struct { val int } func main() { var a = make([]*Foo, 1) a[0] = &Foo{0} var b = [3]Foo{Foo{1}, Foo{2}, Foo{3}} for _, e := range b { a = append(a, &e) } for _, e := range a { fmt.Printf("%v ", *e) } } I was expecting it to print {0} {1} {2} {3} , however it prints {0} {3} {3} {3} . What happened here? 回答1: This is because in the for loop you operate with a copy and not with the slice/array element itself. The for ... range makes a copy of the

问题 I am a newbie to Python and have come across the following example in my book that is not explained very well. Here is my print out from the interpreter: >>> s = 'spam' >>> s[:-1] 'spa' Why does slicing with no beginning bound and a '-1' return every element except the last one? Is calling s[0:-1] logically the same as calling s[:-1] ? They both return the same result. But I'm not sure what python is doing exactly. Any help would be greatly appreciated. 回答1: Yes, calling s[0:-1] is exactly

问题 What's the best way to move a window over a numpy array so that each individual block does not overlap with the previous one and there is a 1 element gap between the blocks? I guess I should use np.hstack, but I can't figure out the slicing pattern. Basically what I am looking for is this: a = np.array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]) result = np.array([[0, 1, 2, 3], [5, 6, 7, 8]) Thanks! 回答1: What you want to to achieve in your short example can be done by reshaping the array, then removing

问题 Why when I slice a pandas dataframe containing only 1 row, the slice becomes a pandas series? How can I keep it a dataframe? df=pd.DataFrame(data=[[1,2,3]],columns=['a','b','c']) df Out[37]: a b c 0 1 2 3 a=df.iloc[0] a Out[39]: a 1 b 2 c 3 Name: 0, dtype: int64 回答1: To avoid the intermediate step of re-converting back to a DataFrame, use double brackets when indexing: a = df.iloc[[0]] print(a) a b c 0 1 2 3 Speed: %timeit df.iloc[[0]] 192 µs per loop %timeit df.loc[0].to_frame().T 468 µs per

问题 I need to generate a file filled with three "random" values per line (10 lines), but those values sum must equal 15. The structure is: "INDEX A B C". Example: 1 15 0 0 2 0 15 0 3 0 0 15 4 1 14 0 5 2 13 0 6 3 12 0 7 4 11 0 8 5 10 0 9 6 9 0 10 7 8 0 回答1: If the numbers can by any just use combinations: from itertools import combinations with open("rand.txt","w") as f: combs = [x for x in combinations(range(16),3) if sum(x ) == 15 ][:10] for a,b,c in combs: f.write("{} {} {}\n".format(a,b,c))

问题 Say I have a slice like x[p:-q:n] or x[::n] I want to use this to generate the index to be passed into numpy.ufunc.reduceat(x, [p, p + n, p + 2 * n, ...]) or numpy.ufunc.reduceat(x, [0, n, 2 * n, ...]) . What is the easiest and efficient way to get it done? 回答1: Building on the comments: In [351]: x=np.arange(100) In [352]: np.r_[0:100:10] Out[352]: array([ 0, 10, 20, 30, 40, 50, 60, 70, 80, 90]) In [353]: np.add.reduceat(x,np.r_[0:100:10]) Out[353]: array([ 45, 145, 245, 345, 445, 545, 645,