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23

What is the preferred way to remove spaces from a string in C++? I could loop through all the characters and build a new string, but is there a better way?

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10 Answers

up vote 84 down vote accepted

The best thing to do is to use the algorithm remove_if and isspace:

remove_if(str.begin(), str.end(), isspace);

Now the algorithm itself can't change the container(only modify the values), so it actually shuffles the values around and returns a pointer to where the end now should be. So we have to call string::erase to actually modify the length of the container:

str.erase(remove_if(str.begin(), str.end(), isspace), str.end());

We should also note that remove_if will make at most one copy of the data. Here is a sample implementation:

template<typename T, typename P>
T remove_if(T beg, T end, P pred)
{
    T dest = beg;
    for (T itr = beg;itr != end; ++itr)
        if (!pred(*itr))
            *(dest++) = *itr;
    return dest;
}
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16  
Because 'isspace' has overloads, you will probably need to qualify the generic code to use ::isspace (the C implementation that doesn't take a locale) or be greeted with cryptic template instantiation errors. – Bklyn Jan 5 '09 at 15:10
1  
All - be wary of the above method (The two single lines, not the templated version, although it may have the same issue). I used it in a project without realizing that it isn't always correct. For example, if you pass it the string "1 + 1" it returns "1+11". I switched to @rupello 's method below and it worked fine for this case. Happy coding! – JoeB Apr 19 '12 at 18:43
@Joe The answer explicitly mentions that you need to call erase afterwards. That will return the correct result. – Konrad Rudolph Sep 15 '12 at 7:17
4  
-1 this use of isspace is UB for all character sets except original 7-bit ASCII. C99 §7.4/1. it does not surprise me that it's been upvoted to the tune of 71 votes by now, in spite of being Very Bad Advice. – Cheers and hth. - Alf Oct 25 '12 at 13:48
up vote 1 down vote

You can use this solution for removing a char:

#include < algorithm >

str.erase(remove(str.begin(), str.end(), char_to_remove), str.end());
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#include < string.h > using namespace std; – sgmart May 21 at 23:50
up vote 18 down vote

From gamedev

string.erase(std::remove_if(string.begin(), string.end(), std::isspace), string.end());
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5  
This will not compile on standards-conforming implementations because of the locale-taking overloads of std::isspace. You'll need to use ::isspace or perform some unreadable machinations with std::bind2nd. Isn't generic code beautiful? – Bklyn Jan 5 '09 at 15:23
up vote 2 down vote 
string replaceinString(std::string str, std::string tofind, std::string toreplace)
{
        size_t position = 0;
        for ( position = str.find(tofind); position != std::string::npos; position = str.find(tofind,position) )
        {
                str.replace(position ,1, toreplace);
        }
        return(str);
}

use it:

string replace = replaceinString(thisstring, " ", "%20");
string replace2 = replaceinString(thisstring, " ", "-");
string replace3 = replaceinString(thisstring, " ", "+");
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up vote 2 down vote

Hi, you can do something like that. This function deletes all spaces.

string delSpaces(string &str) 
{
   str.erase(std::remove(str.begin(), str.end(), ' '), str.end());
   return str;
}

I made another function, that deletes all unnecessary spaces.

string delUnnecessary(string &str)
{
    int size = str.length();
    for(int j = 0; j<=size; j++)
    {
        for(int i = 0; i <=j; i++)
        {
            if(str[i] == ' ' && str[i+1] == ' ')
            {
                str.erase(str.begin() + i);
            }
            else if(str[0]== ' ')
            {
                str.erase(str.begin());
            }
            else if(str[i] == '\0' && str[i-1]== ' ')
            {
                str.erase(str.end() - 1);
            }
        }
    }
    return str;
}
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1  
slim and right to the point, thanks. – Felipe Gringo Apr 10 at 20:14
up vote 8 down vote

For trimming, use boost string algorithms:

#include <boost/algorithm/string.hpp>

using namespace std;
using namespace boost;

// ...

string str1(" hello world! ");
trim(str1);      // str1 == "hello world!"
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1  
-1: This trims; it does not remove all whitespace. – Thomas Eding Aug 7 '12 at 18:08
up vote -4 down vote 
#include <stringthing>

using namespace stringpower;

...

string str1(" hello world! ");
str1.spacechop(str1);      // str1 == "helloworld!"
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17  
What the hell is stringthing/stringpower/spacechop? – Matteo Italia Mar 13 '10 at 22:37
up vote 23 down vote

Can you use Boost String Algo? http://www.boost.org/doc/libs/1_35_0/doc/html/string_algo/usage.html#id1290573

erase_all(str, " ");
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It is slower than the remove_if(str.begin(), str.end(), isspace); that Matt Price mentioned. I don't know why. Actually, all the boost stuff, that have STL alternatives, are slower than the corresponding gcc ones (All the ones I tested). Some of them are immensely slower! (up to 5 times in unordered_map inserts) Maybe it is because of the CPU cache of the shared environment or something like it. – Tolga Aug 14 '12 at 20:23
up vote -1 down vote

I'm afraid it's the best solution that I can think of. But you can use reserve() to pre-allocate the minimum required memory in advance to speed up things a bit. You'll end up with a new string that will probably be shorter but that takes up the same amount of memory, but you'll avoid reallocations.

EDIT: Depending on your situation, this may incur less overhead than jumbling characters around.

You should try different approaches and see what is best for you: you might not have any performance issues at all.

Dave

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remove_if makes at most one copy of each value. So there really isn't that much overhead relative to what needs to be done. – Matt Price Sep 17 '08 at 14:04
up vote 17 down vote 
std::string::iterator end_pos = std::remove(str.begin(), str.end(), ' ');
str.erase(end_pos, str.end());
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6  
My up-vote for the canonical erase/remove idiom. Can be made into a one liner: str.erase (std::remove (str.begin(), str.end(), ' '), str.end()); – Bklyn Jan 5 '09 at 15:08
Worked perfectly, thanks. – user462608 Jan 10 at 10:12
Great method on how todo this, works perfectly! – Arno May 5 at 9:56

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