Format Strings

Syntax#

io:format(FormatString, Args) % write to standard output
io:format(standard_error, FormatString, Args) % write to standard error
io:format(F, FormatString, Args) % write to open file
io_lib:format(FormatString, Args) % return an iolist

Common control sequences in format strings

While there are many different control sequences for io:format and io_lib:format, most of the time you’ll use only three different ones: ~s, ~p and ~w.

~s

The ~s is for strings.

It prints strings, binaries and atoms. (Anything else will cause a badarg error.) It doesn’t quote or escape anything; it just prints the string itself:

%% Printing a string:
> io:format("~s\n", ["hello world"]).
hello world

%% Printing a binary:
> io:format("~s\n", [<<"hello world">>]).
hello world

%% Printing an atom:
> io:format("~s\n", ['hello world']).
hello world

~w

The ~w is for writing with standard syntax.

It can can print any Erlang term. The output can be parsed to return the original Erlang term, unless it contained terms that don’t have a parsable written representation, i.e. pids, ports and references. It doesn’t insert any newlines or indentation, and strings are always interpreted as lists:

> io:format("~w\n", ["abc"]).
[97,98,99]

~p

The ~p is for pretty-printing.

It can can print any Erlang term. The output differs from ~w in the following ways:

Newlines are inserted if the line would otherwise be too long.
When newlines are inserted, the next line is indented to line up with a previous term on the same level.
If a list of integers looks like a printable string, it is interpreted as one.

io:format(“~p\n”, [{this,is,a,tuple,with,many,elements,‘and’,a,list,‘of’,numbers,[97,98,99],that,‘end’,up,making,the,line,too,long}]).

{this,is,a,tuple,with,many,elements,‘and’,a,list,‘of’,numbers,“abc”,that, ‘end’,up,making,the,line,too,long}

If you don’t want lists of integers to be printed as strings, you can use the ~lp sequence (insert a lowercase letter L before p):

> io:format("~lp\n", [[97,98,99]]).
[97,98,99]

> io:format("~lp\n", ["abc"]).
[97,98,99]