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[since C++11] std::array的使用 - C/C++ - 清泛网 - 专注C/C++及内核技术
...类、没有虚函数.
因此不支持这样的构造方法：array<int, 3> a({1, 2, 4});
初始化array最常用的方法是使用赋值运算符和初始化列表：
array<int, 3> a = {1, 2, 3};
array<int, 100> b = {1, 2, 3}; // a[0] ~ a[2] = 1, 2, 3; a[3] ~ a[99] = 0, 0, 0 ... 0;
array<in...
How to get row from R data.frame
...
130
x[r,]
where r is the row you're interested in. Try this, for example:
#Add your data
x <...
Can dplyr package be used for conditional mutating?
... 1 & b == 4), 2,
ifelse(a == 0 | a == 1 | a == 4 | a == 3 | c == 4, 3, NA)))
Added - if_else: Note that in dplyr 0.5 there is an if_else function defined so an alternative would be to replace ifelse with if_else; however, note that since if_else is stricter than ifelse (both leg...
How to reorder data.table columns (without copying)
...
Use setcolorder():
library(data.table)
x <- data.table(a = 1:3, b = 3:1, c = runif(3))
x
# a b c
# [1,] 1 3 0.2880365
# [2,] 2 2 0.7785115
# [3,] 3 1 0.3297416
setcolorder(x, c("c", "b", "a"))
x
# c b a
# [1,] 0.2880365 3 1
# [2,] 0.7785115 2 2
# [3,] 0.329741...
What's the difference between “mod” and “remainder”?
... is a difference between modulus and remainder. For example:
-21 mod 4 is 3 because -21 + 4 x 6 is 3.
But -21 divided by 4 gives -5 with a remainder of -1.
For positive values, there is no difference.
share
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...
How do I make a matrix from a list of vectors in R?
...ne option is to use do.call():
> do.call(rbind, a)
[,1] [,2] [,3] [,4] [,5] [,6]
[1,] 1 1 2 3 4 5
[2,] 2 1 2 3 4 5
[3,] 3 1 2 3 4 5
[4,] 4 1 2 3 4 5
[5,] 5 1 2 3 4 5
[6,] 6 1 2 3 ...
Assign multiple columns using := in data.table, by group
...
This now works in v1.8.3 on R-Forge. Thanks for highlighting it!
x <- data.table(a = 1:3, b = 1:6)
f <- function(x) {list("hi", "hello")}
x[ , c("col1", "col2") := f(), by = a][]
# a b col1 col2
# 1: 1 1 hi hello
# 2: 2 2 hi hello
...
Select multiple columns in data.table by their numeric indices
... all just work:
library(data.table)
dt <- data.table(a = 1, b = 2, c = 3)
# select single column by index
dt[, 2]
# b
# 1: 2
# select multiple columns by index
dt[, 2:3]
# b c
# 1: 2 3
# select single column by name
dt[, "a"]
# a
# 1: 1
# select multiple columns by name
dt[, c("a", ...
Initializing a member array in constructor initializer
...assignments in the body. This is what boost::array does.
Does the C++03 standard say anything special about initializing aggregates (including arrays) in ctor initializers? Or the invalidness of the above code is a corollary of some other rules?
A mem-initializer uses direct initialization. ...
“ValueError: zero length field name in format” error in Python 3.0,3.1,3.2
I'm trying learn Python (3 to be more specific) and I'm getting this error:
3 Answers
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