In R, there are multiple ways to check if a string is a number. Here are three common ways to achieve this:
1. Using the is.numeric() function: This function checks if the argument passed to it is a numeric value or not. If the input is a string that can be converted to a numeric value, then it will return TRUE.
> is.numeric("23.5")
[1] TRUE
> is.numeric("hello")
[1] FALSE
2. Using the as.numeric() function: This function tries to convert the argument passed to it into a numeric value. If the input is a string that can be converted to a numeric value, then it will return the numeric value. If the input is not a numeric string, then it will return NA.
> as.numeric("23.5")
[1] 23.5
> as.numeric("hello")
[1] NA
3. Using regular expressions: Regular expressions can be used to match patterns in strings. In R, we can use the grepl() function with regular expressions to check if a string is numeric. The following regular expression will match any string that contains only digits and optionally a decimal point:
> grepl("^\\d+\\.?\\d*$", "23.5")
[1] TRUE
> grepl("^\\d+\\.?\\d*$", "hello")
[1] FALSE
In the above examples, ^ indicates the start of a string, $ indicates the end of a string, \\d indicates a digit, + indicates one or more of the preceding pattern, \\.? indicates a decimal point (optional), and * indicates zero or more of the preceding pattern.