将字符串分割成固定大小的块

This seems like a really simple task, but I can't find a good solution in base R. I have a character string with 2N characters. How do I split this into a character vector of length N, with each element being a 2-character string?

这似乎是一个非常简单的任务，但是我在base r中找不到一个好的解决方案，我有一个包含2N个字符的字符串。如何将它分割成长度为N的字符向量，每个元素都是一个2字符的字符串?

I could use something like substr with Vectorize:

我可以用substr和Vectorize

vss <- Vectorize(substr, c("start", "stop"))
ch <- paste(rep("a", 1e6), collapse="")
vss(ch, seq(1, nchar(ch), by=2), seq(2, nchar(ch), by=2))

but this is really slow for long strings (O(N^2) I believe).

但这是很慢长字符串(O(N ^ 2)我相信)。

1 个解决方案

#1

If you want speed, Rcpp is always a good choice:

如果你想要速度，Rcpp总是一个不错的选择:

library(Rcpp);
cppFunction('
    List strsplitN(std::vector<std::string> v, int N ) {
        if (N < 1) throw std::invalid_argument("N must be >= 1.");
        List res(v.size());
        for (int i = 0; i < v.size(); ++i) {
            int num = v[i].size()/N + (v[i].size()%N == 0 ? 0 : 1);
            std::vector<std::string> resCur(num,std::string(N,0));
            for (int j = 0; j < num; ++j) resCur[j].assign(v[i].substr(j*N,N));
            res[i] = resCur;
        }
        return res;
    }
');

ch <- paste(rep('a',1e6),collapse='');
system.time({ res <- strsplitN(ch,2L); });
##    user  system elapsed
##   0.109   0.015   0.121
head(res[[1L]]); tail(res[[1L]]);
## [1] "aa" "aa" "aa" "aa" "aa" "aa"
## [1] "aa" "aa" "aa" "aa" "aa" "aa"
length(res[[1L]]);
## [1] 500000

Useful reference: http://gallery.rcpp.org/articles/strings_with_rcpp/.

参考:http://gallery.rcpp.org/articles/strings_with_rcpp/。

More demos:

更多的演示:

strsplitN(c('abcd','efgh'),2L);
## [[1]]
## [1] "ab" "cd"
##
## [[2]]
## [1] "ef" "gh"
##
strsplitN(c('abcd','efgh'),3L);
## [[1]]
## [1] "abc" "d"
##
## [[2]]
## [1] "efg" "h"
##
strsplitN(c('abcd','efgh'),1L);
## [[1]]
## [1] "a" "b" "c" "d"
##
## [[2]]
## [1] "e" "f" "g" "h"
##
strsplitN(c('abcd','efgh'),5L);
## [[1]]
## [1] "abcd"
##
## [[2]]
## [1] "efgh"
##
strsplitN(character(),5L);
## list()
strsplitN(c('abcd','efgh'),0L);
## Error: N must be >= 1.

There are two important caveats with the above implementation:

上述实施有两个重要的注意事项:

1: It doesn't handle NAs correctly. Rcpp seems to stringify to 'NA' when it's forced to come up with a std::string. You can easily solve this in Rland with a wrapper that replaces the offending list components with a true NA.

1:它不能正确地处理NAs。当Rcpp*设计出std::string时，它似乎会被绑定到NA。您可以用包装器在Rland中轻松地解决这个问题，包装器用一个真正的NA替换有问题的列表组件。

x <- c('a',NA); strsplitN(x,1L);
## [[1]]
## [1] "a"
##
## [[2]]
## [1] "N" "A"
##
x <- c('a',NA); ifelse(is.na(x),NA,strsplitN(x,1L));
## [[1]]
## [1] "a"
##
## [[2]]
## [1] NA
##

2: It doesn't handle multibyte characters correctly. This is a tougher problem, and would require a rewrite of the core function implementation to use a Unicode-aware traversal. Fixing this problem would also incur a significant performance penalty, since you wouldn't be able to preallocate each vector in one shot prior to the assignment loop.

2:它不能正确地处理多字节字符。这是一个比较困难的问题，需要重写核心函数实现，以使用具有单点感知的遍历。修复这个问题也会导致严重的性能损失，因为在赋值循环之前，您无法一次性分配每个向量。

strsplitN('aΩ',1L);
## [[1]]
## [1] "a"    "\xce" "\xa9"
##
strsplit('aΩ','');
## [[1]]
## [1] "a" "Ω"
##

#1