Trimming internal whitespace in std::string

Question

I'm looking for an elegant way to transform an std::string from something like:

std::string text = " a\t very \t ugly \t\t\t\t string ";

To:

std::string text = "a very ugly string";

I've already trimmed the external whitespace with boost::trim(text);

[edit] Thus, multiple whitespaces, and tabs, are reduced to just one space [/edit]

Removing the external whitespace is trivial. But is there an elegant way of removing the internal whitespace that doesn't involve manual iteration and comparison of previous and next characters? Perhaps something in boost I have missed?

Just a note, I've not really used boost::split and boost::join, but the obvious way to write this in Python is ' '.join(text.split()), and something similar should be possible. It's not necessarily as efficient as something that copies the bytes straight to their final location, but it's concise and clear. — Steve Jessop
– Steve Jessop, Commented Feb 19, 2012 at 19:01
Yeah; split and join work great if you don't mind copying; if you are worried about efficiency (in this case), writing your own loop is probably best. — Marshall Clow
– Marshall Clow, Commented Feb 19, 2012 at 19:29
@Marshall: I'm working on the basis that the question says, "elegant", not "fast but ugly" ;-) — Steve Jessop
– Steve Jessop, Commented Feb 20, 2012 at 9:50

Seth Carnegie · Accepted Answer · 2012-02-19 16:34:17Z

8

You can use std::unique with std::remove along with ::isspace to compress multiple whitespace characters into single spaces:

std::remove(std::unique(std::begin(text), std::end(text), [](char c, char c2) { return ::isspace(c) && ::isspace(c2); }), std::end(text));

edited Feb 19, 2012 at 16:34

answered Feb 19, 2012 at 16:24

Seth Carnegie

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5 Comments

Sarfaraz Nawaz Over a year ago

It will not solve his problem. test also contains '\t' which is not equal to ' '.

Travis Gockel Over a year ago

Won't this also do things like "letting" -> "leting" and skip over ` \t` pairs?

Seth Carnegie Over a year ago

Whoops fixed it again, previously it wouldn't combine, for instance, a space and a tab next to each other, but now it does.

Steve Jessop Over a year ago

Doesn't this result in "a\tvery ugly string" for the sample input, which is wrong? You could add a pass of transform (or maybe a boost::transform_iterator?) to replace all whitespace with space characters, but sometimes it's OK to give up and write a loop ;-)

Fernando Silveira Over a year ago

Why std::remove? You need std::replace_if after std::unique to replace \t characters with ' ' and it still wouldn't remove the leading and trailing whitespaces. This answer doesn't do what the OP asked.

Sarfaraz Nawaz · Accepted Answer · 2012-02-19 16:41:40Z

7

std::istringstream iss(text); text = ""; std::string s; while(iss >> s){ if ( text != "" ) text += " " + s; else text = s; } //use text, extra whitespaces are removed from it

edited Feb 19, 2012 at 16:41

answered Feb 19, 2012 at 16:28

Sarfaraz Nawaz

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6 Comments

Seth Carnegie Over a year ago

Ah, interesting way of doing it, +1, though I've no idea which is more efficient between yours and mine (or that it matters for small strings or "cold" areas of code)

Sarfaraz Nawaz Over a year ago

I think, in the else-block text.append(" " + s); would be little bit faster.

Seth Carnegie Over a year ago

That wouldn't do the same thing would it? (Right now it overwrites what was there before with operator= but append would be like changing it to +=; I think it might be a typo in the original code)

Sarfaraz Nawaz Over a year ago

@SethCarnegie: But that is what we want. Sorry, it was supposed to be +=, rather than +. I don't know why people voted it when it was not entirely correct :P

Seth Carnegie Over a year ago

Also a pedantic note, it'd probably be better to do if (!text.empty()) than if (text != "")

|

caktux · Accepted Answer · 2014-04-08 07:48:24Z

5

#include <boost/algorithm/string/trim_all.hpp> string s; boost::algorithm::trim_all(s);

answered Apr 8, 2014 at 7:48

caktux

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Comments

Community · Accepted Answer · 2017-05-23 12:08:00Z

Most of what I'd do is similar to what @Nawaz already posted -- read strings from an istringstream to get the data without whitespace, and then insert a single space between each of those strings. However, I'd use an infix_ostream_iterator from a previous answer to get (IMO) slightly cleaner/clearer code.

std::istringstream buffer(input); std::copy(std::istream_iterator<std::string>(buffer), std::istream_iterator<std::string>(), infix_ostream_iterator<std::string>(result, " "));

Marshall Clow · Accepted Answer · 2012-02-19 19:28:03Z

If you check out https://svn.boost.org/trac/boost/ticket/1808, you'll see a request for (almost) this exact functionality, and a suggested implementation:

std::string trim_all ( const std::string &str ) { return boost::algorithm::find_format_all_copy( boost::trim_copy(str), boost::algorithm::token_finder (boost::is_space(),boost::algorithm::token_compress_on), boost::algorithm::const_formatter(" ")); }

Tried adding a code block but no luck.. adding an answer, but this is the right track I think.

Philipp · Accepted Answer · 2012-02-19 21:19:37Z

Here is a possible version using regular expressions. My GCC 4.6 doesn't have regex_replace yet, but Boost.Regex can serve as a drop-in replacement:

#include <string> #include <iostream> // #include <regex> #include <boost/regex.hpp> #include <boost/algorithm/string/trim.hpp> int main() { using namespace std; using namespace boost; string text = " a\t very \t ugly \t\t\t\t string "; trim(text); regex pattern{"[[:space:]]+", regex_constants::egrep}; string result = regex_replace(text, pattern, " "); cout << result << endl; }

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