R:计算数据框中的行数，匹配字符位于字符串的指定位置

Question

我有一个包含字符列的数据框:

  strings
1 a;b;c;d
2 g;h;i;j
3     k;m
4       o

我想计算在字符串中特定位置具有特定指定字符的字符串(行)的数量。

例如。

Get count of number of strings with 3rd character as one of the characters in this set: {a,b,m}.

The output should be 2 in this case, since only the 1st and 3rd row have any characters in {a,b,m} as their 3rd character within the string.

我只能使用这段代码来查找任何包含“b”的字符串:

sum(grepl("b",df))

但是，这对于上述任务来说还不够好。请指教。

Answer 1

你可以试试grepl:

x = c('a;b;c;d','g;h;i;j','k;m','o')

sum(grepl('^.{2}[abm]', x))
#[1] 2