String interpolation

str_interp() is superseded in favour of str_glue().

String interpolation is a useful way of specifying a character string which depends on values in a certain environment. It allows for string creation which is easier to read and write when compared to using e.g. paste() or sprintf(). The (template) string can include expression placeholders of the form ${expression} or $[format]{expression}, where expressions are valid R expressions that can be evaluated in the given environment, and format is a format specification valid for use with sprintf().

Usage

str_interp(string, env = parent.frame())

Arguments

string

A template character string. This function is not vectorised: a character vector will be collapsed into a single string.

env

The environment in which to evaluate the expressions.

Value

An interpolated character string.

See also

str_glue() and str_glue_data() for alternative approaches to the same problem.

Author

Stefan Milton Bache

Examples

# Using values from the environment, and some formats
user_name <- "smbache"
amount <- 6.656
account <- 1337
str_interp("User ${user_name} (account $[08d]{account}) has $$[.2f]{amount}.")
#> [1] "User smbache (account 00001337) has $6.66."

# Nested brace pairs work inside expressions too, and any braces can be
# placed outside the expressions.
str_interp("Works with } nested { braces too: $[.2f]{{{2 + 2}*{amount}}}")
#> [1] "Works with } nested { braces too: 26.62"

# Values can also come from a list
str_interp(
  "One value, ${value1}, and then another, ${value2*2}.",
  list(value1 = 10, value2 = 20)
)
#> [1] "One value, 10, and then another, 40."

# Or a data frame
str_interp(
  "Values are $[.2f]{max(Sepal.Width)} and $[.2f]{min(Sepal.Width)}.",
  iris
)
#> [1] "Values are 4.40 and 2.00."

# Use a vector when the string is long:
max_char <- 80
str_interp(c(
  "This particular line is so long that it is hard to write ",
  "without breaking the ${max_char}-char barrier!"
))
#> [1] "This particular line is so long that it is hard to write without breaking the 80-char barrier!"