Deleting spaces from the beginning of strings [duplicate] - c++

Question

How to remove spaces from a string object in C++.
For example, how to remove leading and trailing spaces from the below string object.
//Original string: " This is a sample string "
//Desired string: "This is a sample string"
The string class, as far as I know, doesn't provide any methods to remove leading and trailing spaces.
To add to the problem, how to extend this formatting to process extra spaces between words of the string. For example,
// Original string: " This is a sample string "
// Desired string: "This is a sample string"
Using the string methods mentioned in the solution, I can think of doing these operations in two steps.
Remove leading and trailing spaces.
Use find_first_of, find_last_of, find_first_not_of, find_last_not_of and substr, repeatedly at word boundaries to get desired formatting.

Answer 1

This is called trimming. If you can use Boost, I'd recommend it.
Otherwise, use find_first_not_of to get the index of the first non-whitespace character, then find_last_not_of to get the index from the end that isn't whitespace. With these, use substr to get the sub-string with no surrounding whitespace.
In response to your edit, I don't know the term but I'd guess something along the lines of "reduce", so that's what I called it. :) (Note, I've changed the white-space to be a parameter, for flexibility)
#include <iostream>
#include <string>
std::string trim(const std::string& str,
const std::string& whitespace = " \t")
{
const auto strBegin = str.find_first_not_of(whitespace);
if (strBegin == std::string::npos)
return ""; // no content
const auto strEnd = str.find_last_not_of(whitespace);
const auto strRange = strEnd - strBegin + 1;
return str.substr(strBegin, strRange);
}
std::string reduce(const std::string& str,
const std::string& fill = " ",
const std::string& whitespace = " \t")
{
// trim first
auto result = trim(str, whitespace);
// replace sub ranges
auto beginSpace = result.find_first_of(whitespace);
while (beginSpace != std::string::npos)
{
const auto endSpace = result.find_first_not_of(whitespace, beginSpace);
const auto range = endSpace - beginSpace;
result.replace(beginSpace, range, fill);
const auto newStart = beginSpace + fill.length();
beginSpace = result.find_first_of(whitespace, newStart);
}
return result;
}
int main(void)
{
const std::string foo = " too much\t \tspace\t\t\t ";
const std::string bar = "one\ntwo";
std::cout << "[" << trim(foo) << "]" << std::endl;
std::cout << "[" << reduce(foo) << "]" << std::endl;
std::cout << "[" << reduce(foo, "-") << "]" << std::endl;
std::cout << "[" << trim(bar) << "]" << std::endl;
}
Result:
[too much space]
[too much space]
[too-much-space]
[one
two]

Answer 2

Easy removing leading, trailing and extra spaces from a std::string in one line
value = std::regex_replace(value, std::regex("^ +| +$|( ) +"), "$1");
removing only leading spaces
value.erase(value.begin(), std::find_if(value.begin(), value.end(), std::bind1st(std::not_equal_to<char>(), ' ')));
or
value = std::regex_replace(value, std::regex("^ +"), "");
removing only trailing spaces
value.erase(std::find_if(value.rbegin(), value.rend(), std::bind1st(std::not_equal_to<char>(), ' ')).base(), value.end());
or
value = std::regex_replace(value, std::regex(" +$"), "");
removing only extra spaces
value = regex_replace(value, std::regex(" +"), " ");

Answer 3

I am currently using these functions:
// trim from left
inline std::string& ltrim(std::string& s, const char* t = " \t\n\r\f\v")
{
s.erase(0, s.find_first_not_of(t));
return s;
}
// trim from right
inline std::string& rtrim(std::string& s, const char* t = " \t\n\r\f\v")
{
s.erase(s.find_last_not_of(t) + 1);
return s;
}
// trim from left & right
inline std::string& trim(std::string& s, const char* t = " \t\n\r\f\v")
{
return ltrim(rtrim(s, t), t);
}
// copying versions
inline std::string ltrim_copy(std::string s, const char* t = " \t\n\r\f\v")
{
return ltrim(s, t);
}
inline std::string rtrim_copy(std::string s, const char* t = " \t\n\r\f\v")
{
return rtrim(s, t);
}
inline std::string trim_copy(std::string s, const char* t = " \t\n\r\f\v")
{
return trim(s, t);
}

Answer 4

Boost string trim algorithm
#include <boost/algorithm/string/trim.hpp>
[...]
std::string msg = " some text with spaces ";
boost::algorithm::trim(msg);

Answer 5

This is my solution for stripping the leading and trailing spaces ...
std::string stripString = " Plamen ";
while(!stripString.empty() && std::isspace(*stripString.begin()))
stripString.erase(stripString.begin());
while(!stripString.empty() && std::isspace(*stripString.rbegin()))
stripString.erase(stripString.length()-1);
The result is "Plamen"

Answer 6

Here is how you can do it:
std::string & trim(std::string & str)
{
return ltrim(rtrim(str));
}
And the supportive functions are implemeted as:
std::string & ltrim(std::string & str)
{
auto it2 = std::find_if( str.begin() , str.end() , [](char ch){ return !std::isspace<char>(ch , std::locale::classic() ) ; } );
str.erase( str.begin() , it2);
return str;
}
std::string & rtrim(std::string & str)
{
auto it1 = std::find_if( str.rbegin() , str.rend() , [](char ch){ return !std::isspace<char>(ch , std::locale::classic() ) ; } );
str.erase( it1.base() , str.end() );
return str;
}
And once you've all these in place, you can write this as well:
std::string trim_copy(std::string const & str)
{
auto s = str;
return ltrim(rtrim(s));
}

Answer 7

C++17 introduced std::basic_string_view, a class template that refers to a constant contiguous sequence of char-like objects, i.e. a view of the string. Apart from having a very similar interface to std::basic_string, it has two additional functions: remove_prefix(), which shrinks the view by moving its start forward; and
remove_suffix(), which shrinks the view by moving its end backward. These can be used to trim leading and trailing space:
#include <string_view>
#include <string>
std::string_view ltrim(std::string_view str)
{
const auto pos(str.find_first_not_of(" \t\n\r\f\v"));
str.remove_prefix(std::min(pos, str.length()));
return str;
}
std::string_view rtrim(std::string_view str)
{
const auto pos(str.find_last_not_of(" \t\n\r\f\v"));
str.remove_suffix(std::min(str.length() - pos - 1, str.length()));
return str;
}
std::string_view trim(std::string_view str)
{
str = ltrim(str);
str = rtrim(str);
return str;
}
int main()
{
std::string str = " hello world ";
auto sv1{ ltrim(str) }; // "hello world "
auto sv2{ rtrim(str) }; // " hello world"
auto sv3{ trim(str) }; // "hello world"
//If you want, you can create std::string objects from std::string_view objects
std::string s1{ sv1 };
std::string s2{ sv2 };
std::string s3{ sv3 };
}
Note: the use of std::min to ensure pos is not greater than size(), which happens when all characters in the string are whitespace and find_first_not_of returns npos. Also, std::string_view is a non-owning reference, so it's only valid as long as the original string still exists. Trimming the string view has no effect on the string it is based on.

Answer 8

Example for trim leading and trailing spaces following jon-hanson's suggestion to use boost (only removes trailing and pending spaces):
#include <boost/algorithm/string/trim.hpp>
std::string str = " t e s t ";
boost::algorithm::trim ( str );
Results in "t e s t"
There is also
trim_left results in "t e s t "
trim_right results in " t e s t"

Answer 9

/// strip a string, remove leading and trailing spaces
void strip(const string& in, string& out)
{
string::const_iterator b = in.begin(), e = in.end();
// skipping leading spaces
while (isSpace(*b)){
++b;
}
if (b != e){
// skipping trailing spaces
while (isSpace(*(e-1))){
--e;
}
}
out.assign(b, e);
}
In the above code, the isSpace() function is a boolean function that tells whether a character is a white space, you can implement this function to reflect your needs, or just call the isspace() from "ctype.h" if you want.

Answer 10

Example for trimming leading and trailing spaces
std::string aString(" This is a string to be trimmed ");
auto start = aString.find_first_not_of(' ');
auto end = aString.find_last_not_of(' ');
std::string trimmedString;
trimmedString = aString.substr(start, (end - start) + 1);
OR
trimmedSring = aString.substr(aString.find_first_not_of(' '), (aString.find_last_not_of(' ') - aString.find_first_not_of(' ')) + 1);

Answer 11

Using the standard library has many benefits, but one must be aware of some special cases that cause exceptions. For example, none of the answers covered the case where a C++ string has some Unicode characters. In this case, if you use the function isspace, an exception will be thrown.
I have been using the following code for trimming the strings and some other operations that might come in handy. The major benefits of this code are: it is really fast (faster than any code I have ever tested), it only uses the standard library, and it never causes an exception:
#include <string>
#include <algorithm>
#include <functional>
#include <locale>
#include <iostream>
typedef unsigned char BYTE;
std::string strTrim(std::string s, char option = 0)
{
// convert all whitespace characters to a standard space
std::replace_if(s.begin(), s.end(), (std::function<int(BYTE)>)::isspace, ' ');
// remove leading and trailing spaces
size_t f = s.find_first_not_of(' ');
if (f == std::string::npos) return "";
s = s.substr(f, s.find_last_not_of(' ') - f + 1);
// remove consecutive spaces
s = std::string(s.begin(), std::unique(s.begin(), s.end(),
[](BYTE l, BYTE r){ return l == ' ' && r == ' '; }));
switch (option)
{
case 'l': // convert to lowercase
std::transform(s.begin(), s.end(), s.begin(), ::tolower);
return s;
case 'U': // convert to uppercase
std::transform(s.begin(), s.end(), s.begin(), ::toupper);
return s;
case 'n': // remove all spaces
s.erase(std::remove(s.begin(), s.end(), ' '), s.end());
return s;
default: // just trim
return s;
}
}

Answer 12

This might be the simplest of all.
You can use string::find and string::rfind to find whitespace from both sides and reduce the string.
void TrimWord(std::string& word)
{
if (word.empty()) return;
// Trim spaces from left side
while (word.find(" ") == 0)
{
word.erase(0, 1);
}
// Trim spaces from right side
size_t len = word.size();
while (word.rfind(" ") == --len)
{
word.erase(len, len + 1);
}
}

Answer 13

To add to the problem, how to extend this formatting to process extra spaces between words of the string.
Actually, this is a simpler case than accounting for multiple leading and trailing white-space characters. All you need to do is remove duplicate adjacent white-space characters from the entire string.
The predicate for adjacent white space would simply be:
auto by_space = [](unsigned char a, unsigned char b) {
return std::isspace(a) and std::isspace(b);
};
and then you can get rid of those duplicate adjacent white-space characters with std::unique, and the erase-remove idiom:
// s = " This is a sample string "
s.erase(std::unique(std::begin(s), std::end(s), by_space),
std::end(s));
// s = " This is a sample string "
This does potentially leave an extra white-space character at the front and/or the back. This can be removed quite easily:
if (std::size(s) && std::isspace(s.back()))
s.pop_back();
if (std::size(s) && std::isspace(s.front()))
s.erase(0, 1);
Here's a demo.

Answer 14

I've tested this, it all works. So this method processInput will just ask the user to type something in. it will return a string that has no extra spaces internally, nor extra spaces at the begining or the end. Hope this helps. (also put a heap of commenting in to make it simple to understand).
you can see how to implement it in the main() at the bottom
#include <string>
#include <iostream>
string processInput() {
char inputChar[256];
string output = "";
int outputLength = 0;
bool space = false;
// user inputs a string.. well a char array
cin.getline(inputChar,256);
output = inputChar;
string outputToLower = "";
// put characters to lower and reduce spaces
for(int i = 0; i < output.length(); i++){
// if it's caps put it to lowercase
output[i] = tolower(output[i]);
// make sure we do not include tabs or line returns or weird symbol for null entry array thingy
if (output[i] != '\t' && output[i] != '\n' && output[i] != 'Ì') {
if (space) {
// if the previous space was a space but this one is not, then space now is false and add char
if (output[i] != ' ') {
space = false;
// add the char
outputToLower+=output[i];
}
} else {
// if space is false, make it true if the char is a space
if (output[i] == ' ') {
space = true;
}
// add the char
outputToLower+=output[i];
}
}
}
// trim leading and tailing space
string trimmedOutput = "";
for(int i = 0; i < outputToLower.length(); i++){
// if it's the last character and it's not a space, then add it
// if it's the first character and it's not a space, then add it
// if it's not the first or the last then add it
if (i == outputToLower.length() - 1 && outputToLower[i] != ' ' ||
i == 0 && outputToLower[i] != ' ' ||
i > 0 && i < outputToLower.length() - 1) {
trimmedOutput += outputToLower[i];
}
}
// return
output = trimmedOutput;
return output;
}
int main() {
cout << "Username: ";
string userName = processInput();
cout << "\nModified Input = " << userName << endl;
}

Answer 15

Why complicate?
std::string removeSpaces(std::string x){
if(x[0] == ' ') { x.erase(0, 1); return removeSpaces(x); }
if(x[x.length() - 1] == ' ') { x.erase(x.length() - 1, x.length()); return removeSpaces(x); }
else return x;
}
This works even if boost was to fail, no regex, no weird stuff nor libraries.
EDIT:
Fix for M.M.'s comment.

Answer 16

No boost, no regex, just the string library. It's that simple.
string trim(const string& s) { // removes whitespace characters from beginnig and end of string s
const int l = (int)s.length();
int a=0, b=l-1;
char c;
while(a<l && ((c=s[a])==' '||c=='\t'||c=='\n'||c=='\v'||c=='\f'||c=='\r'||c=='\0')) a++;
while(b>a && ((c=s[b])==' '||c=='\t'||c=='\n'||c=='\v'||c=='\f'||c=='\r'||c=='\0')) b--;
return s.substr(a, 1+b-a);
}

Answer 17

The constant time and space complexity for removing leading and trailing spaces can be achieved by using pop_back() function in the string. Code looks as follows:
void trimTrailingSpaces(string& s) {
while (s.size() > 0 && s.back() == ' ') {
s.pop_back();
}
}
void trimSpaces(string& s) {
//trim trailing spaces.
trimTrailingSpaces(s);
//trim leading spaces
//To reduce complexity, reversing and removing trailing spaces
//and again reversing back
reverse(s.begin(), s.end());
trimTrailingSpaces(s);
reverse(s.begin(), s.end());
}

Answer 18

char *str = (char*) malloc(50 * sizeof(char));
strcpy(str, " some random string (<50 chars) ");
while(*str == ' ' || *str == '\t' || *str == '\n')
str++;
int len = strlen(str);
while(len >= 0 &&
(str[len - 1] == ' ' || str[len - 1] == '\t' || *str == '\n')
{
*(str + len - 1) = '\0';
len--;
}
printf(":%s:\n", str);

Answer 19

void removeSpaces(string& str)
{
/* remove multiple spaces */
int k=0;
for (int j=0; j<str.size(); ++j)
{
if ( (str[j] != ' ') || (str[j] == ' ' && str[j+1] != ' ' ))
{
str [k] = str [j];
++k;
}
}
str.resize(k);
/* remove space at the end */
if (str [k-1] == ' ')
str.erase(str.end()-1);
/* remove space at the begin */
if (str [0] == ' ')
str.erase(str.begin());
}

Answer 20

string trim(const string & sStr)
{
int nSize = sStr.size();
int nSPos = 0, nEPos = 1, i;
for(i = 0; i< nSize; ++i) {
if( !isspace( sStr[i] ) ) {
nSPos = i ;
break;
}
}
for(i = nSize -1 ; i >= 0 ; --i) {
if( !isspace( sStr[i] ) ) {
nEPos = i;
break;
}
}
return string(sStr, nSPos, nEPos - nSPos + 1);
}

Answer 21

For leading- and trailing spaces, how about:
string string_trim(const string& in) {
stringstream ss;
string out;
ss << in;
ss >> out;
return out;
}
Or for a sentence:
string trim_words(const string& sentence) {
stringstream ss;
ss << sentence;
string s;
string out;
while(ss >> s) {
out+=(s+' ');
}
return out.substr(0, out.length()-1);
}

Answer 22

neat and clean
void trimLeftTrailingSpaces(string &input) {
input.erase(input.begin(), find_if(input.begin(), input.end(), [](int ch) {
return !isspace(ch);
}));
}
void trimRightTrailingSpaces(string &input) {
input.erase(find_if(input.rbegin(), input.rend(), [](int ch) {
return !isspace(ch);
}).base(), input.end());
}

Answer 23

This was the most intuitive way for me to solve this problem:
/**
* #brief Reverses a string, a helper function to removeLeadingTrailingSpaces
*
* #param line
* #return std::string
*/
std::string reverseString (std::string line) {
std::string reverse_line = "";
for(int i = line.length() - 1; i > -1; i--) {
reverse_line += line[i];
}
return reverse_line;
}
/**
* #brief Removes leading and trailing whitespace
* as well as extra whitespace within the line
*
* #param line
* #return std::string
*/
std::string removeLeadingTrailingSpaces(std::string line) {
std::string filtered_line = "";
std::string curr_line = line;
for(int loop = 0; loop < 2; loop++) {
bool leading_spaces_exist = true;
filtered_line = "";
std::string prev_char = "";
for(int i = 0; i < line.length(); i++) {
// Ignores leading whitespace
if(leading_spaces_exist) {
if(curr_line[i] != ' ') {
leading_spaces_exist = false;
}
}
// Puts the rest of the line in a variable
// and ignore back-to-back whitespace
if(!leading_spaces_exist) {
if(!(curr_line[i] == ' ' && prev_char == " ")) {
filtered_line += curr_line[i];
}
prev_char = curr_line[i];
}
}
/*
Reverses the line so that after we remove the leading whitespace
the trailing whitespace becomes the leading whitespace.
After the second round, it needs to reverse the string back to
its regular order.
*/
curr_line = reverseString(filtered_line);
}
return curr_line;
}
Basically, I looped through the string and removed the leading whitespace, then flipped the string and repeated the same process, then flipped back to normal.
I also added the functionality of cleaning up the line if there were back-to-back spaces.

Answer 24

My Solution for this problem not using any STL methods but only C++ string's own methods is as following：
void processString(string &s) {
if ( s.empty() ) return;
//delete leading and trailing spaces of the input string
int notSpaceStartPos = 0, notSpaceEndPos = s.length() - 1;
while ( s[notSpaceStartPos] == ' ' ) ++notSpaceStartPos;
while ( s[notSpaceEndPos] == ' ' ) --notSpaceEndPos;
if ( notSpaceStartPos > notSpaceEndPos ) { s = ""; return; }
s = s.substr(notSpaceStartPos, notSpaceEndPos - notSpaceStartPos + 1);
//reduce multiple spaces between two words to a single space
string temp;
for ( int i = 0; i < s.length(); i++ ) {
if ( i > 0 && s[i] == ' ' && s[i-1] == ' ' ) continue;
temp.push_back(s[i]);
}
s = temp;
}
I have used this method to pass a LeetCode problem Reverse Words in a String

Answer 25

void TrimWhitespaces(std::wstring& str)
{
if (str.empty())
return;
const std::wstring& whitespace = L" \t";
std::wstring::size_type strBegin = str.find_first_not_of(whitespace);
std::wstring::size_type strEnd = str.find_last_not_of(whitespace);
if (strBegin != std::wstring::npos || strEnd != std::wstring::npos)
{
strBegin == std::wstring::npos ? 0 : strBegin;
strEnd == std::wstring::npos ? str.size() : 0;
const auto strRange = strEnd - strBegin + 1;
str.substr(strBegin, strRange).swap(str);
}
else if (str[0] == ' ' || str[0] == '\t') // handles non-empty spaces-only or tabs-only
{
str = L"";
}
}
void TrimWhitespacesTest()
{
std::wstring EmptyStr = L"";
std::wstring SpacesOnlyStr = L" ";
std::wstring TabsOnlyStr = L" ";
std::wstring RightSpacesStr = L"12345 ";
std::wstring LeftSpacesStr = L" 12345";
std::wstring NoSpacesStr = L"12345";
TrimWhitespaces(EmptyStr);
TrimWhitespaces(SpacesOnlyStr);
TrimWhitespaces(TabsOnlyStr);
TrimWhitespaces(RightSpacesStr);
TrimWhitespaces(LeftSpacesStr);
TrimWhitespaces(NoSpacesStr);
assert(EmptyStr == L"");
assert(SpacesOnlyStr == L"");
assert(TabsOnlyStr == L"");
assert(RightSpacesStr == L"12345");
assert(LeftSpacesStr == L"12345");
assert(NoSpacesStr == L"12345");
}

Answer 26

What about the erase-remove idiom?
std::string s("...");
s.erase( std::remove(s.begin(), s.end(), ' '), s.end() );
Sorry. I saw too late that you don't want to remove all whitespace.