Count the number trailing blank space in a string - c++

Question

Can anyone help me in decreasing the complexity of the code below which counts the number of trailing blank spaces in a string.
void main()
{
string url = "abcd ";
int count = 1;
for (int i = 0; i < url.length(); i++)
{
if (url.at(i) == ' ')
{
for (int k = i + 1; k < url.length(); k++)
{
if (url.at(k) != ' ')
{
count = 1;
break;
}
else
{
count++;
i++;
}
}
}
}
cout<< count;
}

Answer 1

#include <iostream>
#include <string>
#include <algorithm>
#include <cstring>
using namespace std;
int main()
{
string url = "abcd "; // four spaces
string rev = url;
reverse(rev.begin(), rev.end());
cout << "There are " << strspn(rev.c_str(), " ") << " trailing spaces." << endl;
return 0;
}
We can do this without reversing the string, and without using a C function like strspn. For example, look up the string::find_last_not_of function. It will find the last character in the string which is not in the specified set, and return its position. If your set is the set " " (space) then it finds the last non-space character. The difference between that position and the string length is the count of trailing spaces.
#include <iostream>
#include <string>
#include <cstring>
using namespace std;
int main()
{
string url = "abcd "; // four spaces
size_t last_nonspace = url.find_last_not_of(" ");
cout << "There are " << url.length() - last_nonspace - 1 << " trailing spaces." << endl;
return 0;
}
Note that if there is no non-space character in the string (the string is either empty or contains only spaces), the find_last_not_of function returns string::npos which is just (size_t) -1, the largest value of size_t. When this is subtracted from the length, and then 1 is subtracted, the resulting value is just the length. The arithmetic works out in all cases.

Answer 2

start backward counting the white space
void main() {
string url = "abcd ";
int count = 0;
for (int i = url.length(); i > 0; i--) {
if (url.at(i) == ' ') {
count++;
} else {
break;
}
}
cout<< count;
}

Answer 3

You can do this by using the find_if() algorithm function:
#include <algorithm>
#include <string>
#include <cctype>
#include <iostream>
using namespace std;
int main()
{
string s = "This string has spaces at the end ";
string::iterator it =
find_if(s.rbegin(), s.rend(), [](char ch) { return !isspace(ch); }).base();
cout << "The number of trailing spaces is " << std::distance(it, s.end());
}
Basically we start from the end of the string, and let find_if() find the first non-space character (I used isspace instead of the hard-coded 32).
After that, the iterator that is returned will point to the non-space character, so it's a matter of just knowing the "distance" between that point and the end (that is what std::distance will do for us).