大约有 16,000 项符合查询结果（耗时：0.0368秒） [XML]
How to swap keys and values in a hash
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Ruby has a helper method for Hash that lets you treat a Hash as if it was inverted (in essence, by letting you access keys through values):
{a: 1, b: 2, c: 3}.key(1)
=> :a
If you want to keep the inverted hash, then Hash#...
Split (explode) pandas dataframe string entry to separate rows
...ld and create a new row per entry (assume that CSV are clean and need only be split on ','). For example, a should become b :
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Is there XNOR (Logical biconditional) operator in C#?
I'm new to C# and could not find XNOR operator to provide this truth table:
4 Answers
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Undo scaffolding in Rails
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First, if you have already run the migrations generated by the scaffold command, you have to perform a rollback first.
rake db:rollback
You can create scaffolding using:
rails generate scaffold MyFoo
(or similar), and you can destroy/undo it using
rails...
Compare two data.frames to find the rows in data.frame 1 that are not present in data.frame 2
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This doesn't answer your question directly, but it will give you the elements that are in common. This can be done with Paul Murrell's package compare:
library(compare)
a1 <- data.frame(a = 1:5, b = letters[1:5])
a2 <- data.frame(a = 1:3, b = letters[1:3])
compa...
Are braces necessary in one-line statements in JavaScript?
I once heard that leaving the curly braces in one-line statements could be harmful in JavaScript. I don't remember the reasoning anymore and a Google search did not help much.
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Regex lookahead, lookbehind and atomic groups
I found these things in my regex body but I haven't got a clue what I can use them for.
Does somebody have examples so I can try to understand how they work?
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App Inventor 2 数学代码块 · App Inventor 2 中文网
... 最小值 (min)
最大值 (max)
平方根 (square root)
绝对值 (abs)
相反数 (neg)
对数 (log)
e^
四舍五入 (round)
上取整 (ceiling)
下取整 (floor)
求模 (modulo)
余数 (remainder)
商 (quotient)
正弦 (sin)
余弦 (cos)
正切 (tan)
反正弦 (asin...
Why is there no logical xor in JavaScript?
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JavaScript traces its ancestry back to C, and C does not have a logical XOR operator. Mainly because it's not useful. Bitwise XOR is extremely useful, but in all my years of programming I have never needed a logical XOR.
If you have two boolean variables ...
Drop unused factor levels in a subsetted data frame
I have a data frame containing a factor . When I create a subset of this dataframe using subset or another indexing function, a new data frame is created. However, the factor variable retains all of its original levels, even when/if they do not exist in the new dataframe.
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