Strings. Higher-level operations on strings.
Extra String Methods
These are convenient borrowings from Python, as described in 3.6.1 of the Python
reference, but note that indices in Lua always begin at one. stringx defines
functions like isalpha and isdigit, which return true
if s is only composed
of letters or digits respectively. startswith and endswith are convenient
ways to find substrings. (endswith works as in Python 2.5, so that `f:endswith
{'.bat','.exe','.cmd'}` will be true for any filename which ends with these
extensions.) There are justify methods and whitespace trimming functions like
strip.
> stringx.import() > ('bonzo.dog'):endswith {'.dog','.cat'} true > ('bonzo.txt'):endswith {'.dog','.cat'} false > ('bonzo.cat'):endswith {'.dog','.cat'} true > (' stuff'):ljust(20,'+') '++++++++++++++ stuff' > (' stuff '):lstrip() 'stuff ' > (' stuff '):rstrip() ' stuff' > (' stuff '):strip() 'stuff' > for s in ('one\ntwo\nthree\n'):lines() do print(s) end one two three
Most of these can be fairly easily implemented using the Lua string library,
which is more general and powerful. But they are convenient operations to have
easily at hand. Note that can be injected into the string table if you use
stringx.import, but a simple alias like local stringx = require 'pl.stringx'
is preferrable. This is the recommended practice when writing modules for
consumption by other people, since it is bad manners to change the global state
of the rest of the system. Magic may be used for convenience, but there is always
a price.
String Templates
Another borrowing from Python, string templates allow you to substitute values looked up in a table:
local Template = require ('pl.text').Template t = Template('${here} is the $answer') print(t:substitute {here = 'Lua', answer = 'best'}) ==> Lua is the best
'$ variables' can optionally have curly braces; this form is useful if you are
glueing text together to make variables, e.g ${prefix}_name_${postfix}
. The
substitute
method will throw an error if a $ variable is not found in the
table, and the safe_substitute
method will not.
The Lua implementation has an extra method, indent_substitute
which is very
useful for inserting blocks of text, because it adjusts indentation. Consider
this example:
-- testtemplate.lua local Template = require ('pl.text').Template t = Template [[ for i = 1,#$t do $body end ]] body = Template [[ local row = $t[i] for j = 1,#row do fun(row[j]) end ]] print(t:indent_substitute {body=body,t='tbl'})
And the output is:
for i = 1,#tbl do local row = tbl[i] for j = 1,#row do fun(row[j]) end end
indent_substitute
can substitute templates, and in which case they themselves
will be substituted using the given table. So in this case, $t
was substituted
twice.
pl.text also has a number of useful functions like dedent
, which strips all
the initial indentation from a multiline string. As in Python, this is useful for
preprocessing multiline strings if you like indenting them with your code. The
function wrap
is passed a long string (a paragraph) and returns a list of
lines that fit into a desired line width. As an extension, there is also indent
for indenting multiline strings.
New in Penlight with the 0.9 series is text.format_operator
. Calling this
enables Python-style string formating using the modulo operator %
:
> text.format_operator() > = '%s[%d]' % {'dog',1} dog[1]
So in its simplest form it saves the typing involved with string.format; it
will also expand $
variables using named fields:
> = '$animal[$num]' % {animal='dog',num=1} dog[1]
As with stringx.import you have to do this explicitly, since all strings share the same metatable. But in your own scripts you can feel free to do this.
Another Style of Template
A new module is template, which is a version of Rici Lake's Lua Preprocessor. This allows you to mix Lua code with your templates in a straightforward way. There are only two rules:
- Lines begining with
#
are Lua - Otherwise, anything inside
$()
is a Lua expression.
So a template generating an HTML list would look like this:
<ul> # for i,val in ipairs(T) do <li>$(i) = $(val:upper())</li> # end </ul>
Assume the text is inside tmpl
, then the template can be expanded using:
local template = require 'pl.template' local my_env = { ipairs = ipairs, T = {'one','two','three'} } res = template.substitute(tmpl, my_env)
and we get
<ul> <li>1 = ONE</li> <li>2 = TWO</li> <li>3 = THREE</li> </ul>
There is a single function, template.substitute which is passed a template
string and an environment table. This table may contain some special fields,
like \_parent
which can be set to a table representing a 'fallback' environment
in case a symbol was not found. \_brackets
is usually '()' and \_escape
is
usually '#' but it's sometimes necessary to redefine these if the defaults
interfere with the target language - for instance, $(V)
has another meaning in
Make, and #
means a preprocessor line in C/C++.
Finally, if something goes wrong, passing _debug
will cause the intermediate
Lua code to be dumped if there's a problem.
Here is a C code generation example; something that could easily be extended to be a minimal Lua extension skeleton generator.
local subst = require 'pl.template'.substitute local templ = [[ #include <lua.h> #include <lauxlib.h> #include <lualib.h> > for _,f in ipairs(mod) do static int l_$(f.name) (lua_State *L) { } > end static const luaL_reg $(mod.name)[] = { > for _,f in ipairs(mod) do {"$(f.name)",l_$(f.name)}, > end {NULL,NULL} }; int luaopen_$(mod.name) { luaL_register (L, "$(mod.name)", $(mod.name)); return 1; } ]] print(subst(templ,{ _escape = '>', ipairs = ipairs, mod = { name = 'baggins'; {name='frodo'}, {name='bilbo'} } }))
File-style I/O on Strings
pl.stringio provides just three functions; stringio.open is passed a string,
and returns a file-like object for reading. It supports a read
method, which
takes the same arguments as standard file objects:
> f = stringio.open 'first line\n10 20 30\n' > = f:read() first line > = f:read('*n','*n','*n') 10 20 30
lines
and seek
are also supported.
stringio.lines
is a useful short-cut for iterating over all the lines in a
string.
stringio.create creates a writeable file-like object. You then use write
to
this stream, and finally extract the builded string using value
. This 'string
builder' pattern is useful for efficiently creating large strings.