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What is an invariant?

The word seems to get used in a number of contexts. The best I can figure is that they mean a variable that can't change. Isn't that what constants/finals (darn you Java!) are for? ...
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x not in y” or “not x in y”

...tor, which could be applied to a in b just as easily as any other boolean expression, whereas not in was a separate operator for convenience and clarity. The disassembly above was revealing! It seems that while not obviously is a logical negation operator, the form not a in b is special cased so th...
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How to filter by IP address in Wireshark?

... Match destination: ip.dst == x.x.x.x Match source: ip.src == x.x.x.x Match either: ip.addr == x.x.x.x share | improve this answer | ...
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How can I create a correlation matrix in R?

... An example, d &lt- data.frame(x1=rnorm(10), x2=rnorm(10), x3=rnorm(10)) cor(d) # get correlations (returns matrix) ...
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undefined reference to `WinMain@16'

... Consider the following Windows API-level program: #define NOMINMAX #include <windows.h> int main() { MessageBox( 0, "Blah blah...", "My Windows app!", MB_SETFOREGROUND ); } Now let's build it using GNU toolchain (i.e. g++), no special options. Here gnuc is just a batch file th...
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Finding differences between elements of a list

...t izip as zip def differences(seq): iterable, copied = tee(seq) next(copied) for x, y in zip(iterable, copied): yield y - x Or using itertools.islice instead: from itertools import islice def differences(seq): nexts = islice(seq, 1, None) for x, y in zip(seq, nexts):...
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Using python map and other functional tools

... nest of bugs introduced by having global state. As maptest depends on an external definition of bars, this principle is broken. – image_doctor Jul 10 '13 at 13:22 3 ...
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Does a break statement break from a switch/select?

...o Programming Language Specification. A "break" statement terminates execution of the innermost "for", "switch" or "select" statement. BreakStmt = "break" [ Label ] . If there is a label, it must be that of an enclosing "for", "switch" or "select" statement, and that is the one whose...
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How do I specify the Linq OrderBy argument dynamically?

...typeof(Student).GetProperty(param); var orderByAddress = items.OrderBy(x => propertyInfo.GetValue(x, null)); share | improve this answer | follow | ...
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foreach with index [duplicate]

... there a C# equivalent of Python's enumerate() and Ruby's each_with_index ? 10 Answers ...