variable-assignment | 易学教程

Why is extended slice assignment less flexible than regular slice assignment?

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According to the Python documentation on extended slices : If you have a mutable sequence such as a list or an array you can assign to or delete an extended slice, but there are some differences between assignment to extended and regular slices. Assignment to a regular slice can be used to change the length of the sequence: >>> a = range(3) >>> a [0, 1, 2] >>> a[1:3] = [4, 5, 6] >>> a [0, 4, 5, 6] Extended slices aren't this flexible. When assigning to an extended slice, the list on the right hand side of the statement must contain the same number of items as the slice it is replacing: >>> a =

How can I evaluate variable to another variable before assigning? [closed]

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问题 Closed . This question needs to be more focused. It is not currently accepting answers. Want to improve this question? Update the question so it focuses on one problem only by editing this post. Closed 4 years ago . This question broken into subquestions : Pointers in Python suggested by one reply to look at, more here "why not to modify locals?" -question here. Original Question #!/usr/bin/python # # Description: trying to evaluate array -value to variable before assignment # but it

The efficient way to write move copy and move assignment constructors

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Are the following assignment and copy move constructors the most efficient? if anybody have other way please tell me? I mean what bout std::swap? and calling assignment through copy constructor is safe in the code below? #include <iostream> #include <functional> #include <algorithm> #include <utility> using std::cout; using std::cin; using std::endl; using std::bind; class Widget { public: Widget(int length) :length_(length), data_(new int[length]) { cout<<__FUNCTION__<<"("<<length<<")"<<endl; } ~Widget() { cout<<endl<<__FUNCTION__<<"()"<<endl; if (data_) { cout<<"deleting source"<<endl; }

Removing non-integers from a string in C++

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问题 There was a passing comment in a book of mine about people inputting commas into integers and messing up your program, but it didn't elaborate. That got me thinking, so I tried writing a little algorithm to take an std::string and remove all non-integer characters. This code compiles but skips over output. Why isn't anything being assigned to newstring? Does if(isdigit(fstring[i])) evaluate to true of the address it's pointing to hold a digit? //little algorithm to take the non-integers out

Python list extension and variable assignment

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问题 I tried to extend a list and was puzzled by having the result return with the value None. What I tried was this: >>> a = [1,2] >>> b = [3,4] >>> a = a.extend(b) >>> print a None I finally realized that the problem was the redundant assignment to 'a' at the end. So this works: >>> a = [1,2] >>> b = [3,4] >>> a.extend(b) >>> print a [1,2,3,4] What I don't understand is why the first version didn't work. The assignment to 'a' was redundant, but why did it break the operation? 回答1: Because, as

Yielding until all needed values are yielded, is there way to make slice to become lazy

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Is there way to stop yielding when generator did not finish values and all needed results have been read? I mean that generator is giving values without ever doing StopIteration. For example, this never stops: (REVISED) from random import randint def devtrue(): while True: yield True answers=[False for _ in range(randint(100,100000))] answers[::randint(3,19)]=devtrue() print answers I found this code, but do not yet understand, how to apply it in this case: http://code.activestate.com/recipes/576585-lazy-recursive-generator-function/ You can call close() on the generator object. This way, a

Invalid assignment left-hand side, javascript

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问题 I am sure I am doing something silly here: var addhtml = '<div id="leftbio" class="left-float">' += '<div id="bioname">e["screen_name]</div>' += '<div id="biophoto"><img src="e["profile_image_url"]"/></div>' += '<div id="biodetails">e["description"]</div>' += '</div>'; // invalid assignment left-hand side console.log(addhtml); And Netbeans is telling me that invalid assignment left-hand side error. Whats wrong ? 回答1: You don't need += to concatenate, you just need + This is ok var addhtml = '

Yielding until all needed values are yielded, is there way to make slice to become lazy

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问题 Is there way to stop yielding when generator did not finish values and all needed results have been read? I mean that generator is giving values without ever doing StopIteration. For example, this never stops: (REVISED) from random import randint def devtrue(): while True: yield True answers=[False for _ in range(randint(100,100000))] answers[::randint(3,19)]=devtrue() print answers I found this code, but do not yet understand, how to apply it in this case: http://code.activestate.com/recipes

Best way to name objects programmatically using R?

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I'm running various modeling algorithms on a data set. I've had best results by modeling my input variables to my responses one at a time, e.g.: model <- train(y ~ x1 + x2 + ... + xn, ...) Once I train my models, I'd like to not re-run them each time, so I've been trying to save them as .rda files. Here's an example loop for a random forest model (feel free to suggest a better way than a loop!): # data_resp contains my measured responses, one per column # data_pred contains my predictors, one per column for (i in 1:ncol(data_resp)) { model <- train(data_pred_scale[!is.na(data_resp[, i]), ],

Simultaneous variable assignment in Go different from individual variable assignment

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I was under the impression that despite the differences in syntax, function a and function b below were logically equivalent. However, they are not and I do not understand the difference between them. It seems to me that they are both assigning: the value of x to the variable z, the value of y to the variable x, and the value of x+y to the variable y. Could anyone help clear up my misunderstanding regarding the multiple variable assignment and the logical difference between function a and function b? package main import "fmt" func a() (int, int, int) { x:=1 y:=2 z:=3 z = x x = y y = x+y return