pformat brings Python-inspired string interpolation and formatting to R
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README.md

pformat

pformat provides powerful string interpolation and formatting capabilities. It is a R language implementation of Python's new style string formatting API, coupled with some original features.

Features

  • Supports numeric (integer, double, complex), date (Date, IDate, POSIXt) and character types out-of-the-box.
  • Supports expressions inside fields, avoiding intermediate variables.
  • named fields can be evaluated inside lists or data frames, saving typing.
  • Supports evaluation on the current environment. You can feed pformat() only the format string, and it will look for corresponding data on the environment.
  • It is vectorized, allowing formatting of extensive amounts of data with a single call.
  • It is possible to preparse format strings, which may avoid unnecessary calls and reduce computing time inside loops.
  • It is extensible: you can write custom formatters for your classes.

Examples

Basic usage

A pair of braces and everything inside them form a placeholder. The simplest use case is that of positional formatting: arguments are assigned to placeholders according to their position.

pformat("{} {}", "one", "two")
#> [1] "one two"

You can give placeholders an explicit positional index. This allows for re-arranging the order of display without changing the arguments. Note: contrary to Python, in pformat indices start from 1 and not from 0.

pformat("{2} {1}", "one", "two")
#> [1] "two one"

Vectorization

pformat is vectorized, so you can generate many formatted strings using a single call

pformat("Name: {}; Age: {}", c("Abby", "Bob", "Carl"), 22:24)
#> [1] "Name: Abby; Age: 22" "Name: Bob; Age: 23"  "Name: Carl; Age: 24"

Usual recycling rules apply:

pformat("{}-{}{}", "expr", c("a", "b", "c"), 1:2)
#> [1] "expr-a1" "expr-b2" "expr-c1"

Named placeholders

pformat provides three ways of using named placeholders:

  • keyword arguments.
  • passing a list, data frame, or environment as the first parameter. This allows pformat to be chained using the pipe operator %>% provided by the magrittr package.
  • evaluation on the current environment.

That's also the order which pformat uses when looking for corresponding names. The following example illustrates the three methods, where all calls produce same output.

# keyword arguments
pformat("Name: {name}; Age: {age}", name = c("Abby", "Bob", "Carl"), age = 22:24)
#> [1] "Name: Abby; Age: 22" "Name: Bob; Age: 23"  "Name: Carl; Age: 24"

# passing a data frame as the first parameter
people = data.frame(name = c("Abby", "Bob", "Carl"), age = 22:24)
pformat(people, "Name: {name}; Age: {age}")
#> [1] "Name: Abby; Age: 22" "Name: Bob; Age: 23"  "Name: Carl; Age: 24"

# the same as above but using pipe
library(magrittr)
people %>% pformat("Name: {name}; Age: {age}")
#> [1] "Name: Abby; Age: 22" "Name: Bob; Age: 23"  "Name: Carl; Age: 24"

# evaluation on the environment
name = c("Abby", "Bob", "Carl")
age = 22:24
pformat("Name: {name}; Age: {age}")
#> [1] "Name: Abby; Age: 22" "Name: Bob; Age: 23"  "Name: Carl; Age: 24"

Expressions

Placeholders can hold not only identifiers but any R expression, provided that its result type is supported by pformat.

n = 9
pformat("{n} x {i} = {n * i}", i = 1:10)
#>  [1] "9 x 1 = 9"   "9 x 2 = 18"  "9 x 3 = 27"  "9 x 4 = 36"  "9 x 5 = 45" 
#>  [6] "9 x 6 = 54"  "9 x 7 = 63"  "9 x 8 = 72"  "9 x 9 = 81"  "9 x 10 = 90"
df = data.frame(name = c("Walter", "Frederick", "Lindsey"), 
                surname = c("Unzueta", "Winstead", "Chambers"))
df %>% pformat("{substr(name, 1, 1)}. {surname}")
#> [1] "W. Unzueta"  "F. Winstead" "L. Chambers"

Formatting

Integers, floating point numbers, and strings

pformat uses (will use) the same format string syntax as Python. See Python docs. Development in progress.

Dates

Date formatting uses the traditional strftime() conversion specification, just like Python. See strftime's help page for more details.

pformat("Mother's day: {:%d/%m/%Y}", as.Date("2016-05-08"))
#> [1] "Mother's day: 08/05/2016"