String Interpolation

From Erlang Community

(Difference between revisions)

Revision as of 04:46, 31 October 2006

Problem

You want to create string using a mixture of literals and computed values. For example you want to display the current date is the string "Today's date is "

Solution

Make use of Erlang's generic display procedure and variable argument list. For an easy case (displaying date and time), we can make use of the httpd_util:rfc1123_date/0 function to get a string date, and the built-in concatenation (++) operator to construct the string we want:

1> BBBB = "Today's date and time is " ++ httpd_util:rfc1123_date() ++ ".\n".
"Today's date and time is Fri, 20 Aug 2004 22:03:13 GMT.\n"

Although if we only construct this string to then immediately pass it off to

a port, we can more efficiently:

2> BBBB = ["Today's date and time is ", httpd_util:rfc1123_date(), ".\n"].
["Today's date and time is ", "Fri, 20 Aug 2004 22:03:13 GMT", ".\n"]

-- see the Efficiency Guide for more information about this.

Other cases require that you use the generic io_lib formatting functions to create strings with the data you want. For example, we could achive the same

result as follows:

3> io:fwrite("Today's date is ~s, ~s ~w ~w.\n", ["Fri", "Aug", 20, 2004]).
Today's date is Fri, Aug 20 2004.
4> io_lib:fwrite("Today's date is ~s, ~s ~w ~w.\n", ["Fri", "Aug", 20, 
2004]). [84, 111, 100, 97, 121, 39, 115, 32, 100,    % abbreviated output.
97, 116, 101, 32, 105, 115, 32, "Fri", 44,
32, "Aug", 32, "20", 32, "2004", 46, 10]
5> lists:flatten(v(4)).
"Today's date is Fri, Aug 20 2004.\n"

Finally, this unoptimized module offers more traditional string

interpolation:

-module(template).
-export([subst/2]).

subst(Fmt,[]) -> lists:flatten(Fmt);
subst(Fmt,[{K,V}|T]) ->
    {ok,Fmt1,_} = regexp:gsub(Fmt,"\\$" ++ atom_to_list(K),[V]),
    subst(Fmt1,T).

6> Vars = [{patient,"Mary Becket"}, {diagnosis,"You are an octopus."}],
6> template:subst("E-bloodwork autodoctor: Dear $patient, today's checkup 
indicated a problem: $diagnosis.\n", Vars). "E-bloodwork autodiagnosis: Dear 
Mary Becket, today's checkup indicated a problem: You are an octopus.\n"

Retrieved from "http://www.trapexit.org/String_Interpolation"

Categories: CookBook | StringRecipes

String Interpolation

From Erlang Community

Revision as of 04:46, 31 October 2006

Problem

Solution

Search

Navigation

Erlang/OTP Projects

Documentation

Planet Trapexit

Bookmark

Personal tools

 == Problem ==
-You want to create string using a mixture of literals and computed values. For example you want to display the current date is the string "Today's date is "
+You want to create string using a mixture of literals and computed values.
+For example you want to display the current date is the string "Today's date
+is "
 == Solution ==
-Make use of Erlang's generic display procedure and variable argument list. For an easy case (displaying date and time), we can make use of the httpd_util:rfc1123_date/0 function to get a string date, and the built-in concatenation (++) operator to construct the string we want:
+Make use of Erlang's generic display procedure and variable argument list.
+For an easy case (displaying date and time), we can make use of the
+httpd_util:rfc1123_date/0 function to get a string date, and the built-in
+concatenation (++) operator to construct the string we want:
 <code>
 > BBBB = "Today's date and time is " ++ httpd_util:rfc1123_date() ++ ".\n".
 </code>
-Although if we only construct this string to then immediately pass it off to a port, we can more efficiently:
+Although if we only construct this string to then immediately pass it off to
-<code>
+a port, we can more efficiently: <code>
 > BBBB = ["Today's date and time is ", httpd_util:rfc1123_date(), ".\n"].
 ["Today's date and time is ", "Fri, 20 Aug 2004 22:03:13 GMT", ".\n"]
 -- see the Efficiency Guide for more information about this.
-Other cases require that you use the generic io_lib formatting functions to create strings with the data you want. For example, we could achive the same result as follows:
+Other cases require that you use the generic io_lib formatting functions to
-<code>
+create strings with the data you want. For example, we could achive the same
-> io:fwrite("Today's date is ~s, ~s ~w ~w.\n", ["Fri", "Aug", 20, 2004]).
+result as follows: <code>
+> io:fwrite("Today's date is ~s, ~s ~w ~w.\n", ["Fri", "Aug", 20, 2004]).
 Today's date is Fri, Aug 20 2004.
+> io_lib:fwrite("Today's date is ~s, ~s ~w ~w.\n", ["Fri", "Aug", 20,
+]). [84, 111, 100, 97, 121, 39, 115, 32, 100,    % abbreviated output.
+, 116, 101, 32, 105, 115, 32, "Fri", 44,
+, "Aug", 32, "20", 32, "2004", 46, 10]
+> lists:flatten(v(4)).
+"Today's date is Fri, Aug 20 2004.\n"
 </code>
-Finally, this unoptimized module offers more traditional string interpolation:
+Finally, this unoptimized module offers more traditional string
-<code>
+interpolation: <code>
 -module(template).
 -export([subst/2]).
 </code>
 <code>
 > Vars = [{patient,"Mary Becket"}, {diagnosis,"You are an octopus."}],
-> template:subst("E-bloodwork autodoctor: Dear $patient, today's checkup indicated a problem: $diagnosis.\n", Vars).
+> template:subst("E-bloodwork autodoctor: Dear $patient, today's checkup
-"E-bloodwork autodiagnosis: Dear Mary Becket, today's checkup indicated a problem: You are an octopus.\n"
+indicated a problem: $diagnosis.\n", Vars). "E-bloodwork autodiagnosis: Dear
+Mary Becket, today's checkup indicated a problem: You are an octopus.\n"
 </code>
 [[Category:CookBook]][[Category:StringRecipes]]