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Covariance, Invariance and Contravariance explained in plain English?
Today, I read some articles about Covariance, Contravariance (and Invariance) in Java. I read the English and German Wikipedia article, and some other blog posts and articles from IBM.
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Shortcuts in Objective-C to concatenate NSStrings
Are there any shortcuts to ( stringByAppendingString: ) string concatenation in Objective-C, or shortcuts for working with NSString in general?
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Reference — What does this symbol mean in PHP?
This is a collection of questions that come up every now and then about syntax in PHP. This is also a Community Wiki, so everyone is invited to participate in maintaining this list.
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When is “i += x” different from “i = i + x” in Python?
...he standard notation of i = i + . Is there a case in which i += 1 would be different from i = i + 1 ?
3 Answers
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Why avoid increment (“++”) and decrement (“--”) operators in JavaScript?
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My view is to always use ++ and -- by themselves on a single line, as in:
i++;
array[i] = foo;
instead of
array[++i] = foo;
Anything beyond that can be confusing to some programmers and is just not worth it in my view. For loops are an exception, as the...
How to check if PHP array is associative or sequential?
PHP treats all arrays as associative, so there aren't any built in functions. Can anyone recommend a fairly efficient way to check if an array contains only numeric keys?
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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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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...