如何从字符串中提取带有“e”的数字部分

Question

我有一个类似“reflectance_scales=5.011129178e-05”的字符串。 我想以数字形式提取“5.011129178e-05”部分。

我试过了：

gsub("[^0-9.]", "",  "reflectance_scales=5.011129178e-05")

但它只提取 5.011129178 而不是 e-05。

Answer 1

我们可以使用sub提取数字并使用eval(parse将其转换为数字。

eval(parse(text = sub(".*?(\\d.*)", "\\1",  "reflectance_scales=5.011129178e-05")))
#[1] 0.000050111

如果您没有表示数字的科学格式，这也应该有效。

eval(parse(text = sub(".*?(\\d.*)", "\\1",  "reflectance_scales=5.0123")))
#[1] 5.0123

---

您也可以在不使用eval parse 的情况下使用相同的代码，也可以将其包装在as.numeric 中

as.numeric(sub(".*?(\\d.*)", "\\1",  "reflectance_scales=5.011129178e-05"))
#[1] 0.000050111

Answer 2

你可以试试

r <- gsub("\\w.*?=", "", "reflectance_scales=5.011129178e-05")

给了

> r
[1] "5.011129178e-05"

如果你想要它作为数字类型，那么使用as.numeric(r)

Answer 3

比@RonakShah 的回答更简单，并使用@llllllIIIIll 的想法，您可以分两步完成（无需正则表达式）：

word = "reflectance_scales=5.011129178e-05"
numb = as.numeric(unlist(strsplit(word,"=")))
numb = numb[!is.na(numb)]

然后得到：

> str(numb)
 num 5.01e-05

Answer 4

基础 R 解决方案：

# Solution 1, string split, unlist, coerce to numeric, subset out NAs, coerce to numeric, 
# format with scientific notation (type coercion to string): 

format(as.numeric(na.omit(as.numeric(unlist(strsplit(X, "="))))), scientific = TRUE)

# Solution 2, substitution of alphabetic characters coercion to numeric
# format with scientific notation (type coercion to string)

format(as.numeric(gsub("[^0-9]+", "", X)), scientific = TRUE)

# Solution 3, string split, digit extraction:

grep("\\d+", unlist(strsplit(X, "=")), value = TRUE)

数据：

X <- as.character("reflectance_scales=5.011129178e-05")

基准测试：

# Install pacakges if they are not already installed: 

necessary_packages <- c("bench")

# Create a vector containing the names of any packages needing installation: 

new_packages <- necessary_packages[!(necessary_packages %in% installed.packages()[,"Package"])]

# If the vector has more than 0 values, install the new pacakges
# (and their) associated dependencies: 

if(length(new_packages) > 0){

  install.packages(new_packages, dependencies = TRUE)

}

# Initialise the packages in the session: 

lapply(necessary_packages, require, character.only = TRUE)

# Benchmark the solutions: 

function_performance <- bench::mark(

  # Solution 1, string split, unlist, coerce to numeric, subset out NAs, coerce to numeric, 
  # format with scientific notation (type coercion to string): 

  format(as.numeric(na.omit(as.numeric(unlist(strsplit(X, "="))))), scientific = TRUE),

  # Solution 2, substitution of alphabetic characters coercion to numeric
  # format with scientific notation (type coercion to string)

  format(as.numeric(gsub("[^0-9]+", "", X)), scientific = TRUE),

  # Solution 3, string split, digit extraction:

  grep("\\d+", unlist(strsplit(X, "=")), value = TRUE), 

check = FALSE)

# Check the function performance: 

View(function_performance)

Answer 5

或者只是像列表定义一样解析它：

x <- "reflectance_scales=5.011129178e-05"
eval(str2lang(paste0("list(",x,")")))[[1]]
#> [1] 5.011129e-05

^{由reprex package (v0.3.0) 于 2019 年 12 月 4 日创建}

与 Ronak 的回答相比，这非常有效：

x <- "reflectance_scales=5.011129178e-05"
bench::mark(
mm = eval(str2lang(paste0("list(",x,")")))[[1]],
ronak = eval(parse(text = sub(".*?(\\d.*)", "\\1",  x)))
)
#> # A tibble: 2 x 6
#>   expression      min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr> <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 mm            5.9us    7.8us    72889.    2.08KB     7.29
#> 2 ronak        29.6us   31.4us    24103.      280B     9.65

^{由reprex package (v0.3.0) 于 2019 年 12 月 4 日创建}