How to erase all non alphanumeric characters from a string?

[abdalimran]

I want to erase all non alphanumeric characters from a string. I've tried the following way using erase() function. But it erases all the characters from where it finds a non alphanumeric character.

If I input: it. is? awesome!
It Prints: it

But it should print: itisawesome

#include <bits/stdc++.h>

using namespace std;

int main()
{
    string s;
    getline(cin,s);

    for(int i=0; i<s.length(); i++)
    {
        if(!isalpha(s[i]))
            s.erase(i);
    }

    cout<<s<<endl;

    return 0;
}

[shadowCODE]

for(string::iterator i = s.begin(); i != s.end(); i++)
   {
   		if(!isalpha(s.at(i - s.begin())))
			s.erase(i);
   }

[abdalimran]

@shadowCODE
If I Input: it.. is?? awe!?some??..
It doesn't work.

It should print: itisawesome

[shadowCODE]

for(string::iterator i = s.begin(); i != s.end(); i++)
{
    if(!isalpha(s.at(i - s.begin())))
    {
        s.erase(i);
        i--;
    }
}

[shadowCODE]

Your post specifies alphanumeric. If this is really what you want, you should use isalnum instead of isalpha

[JLBorges]

#include <iostream>
#include <string>
#include <cctype>
#include <algorithm>

int main()
{
    const std::string str = "it. is? awesome!" ;

    {
        // simple
        std::string a ;
        for( char c : str ) if( std::isalnum(c) ) a += c ;
        std::cout << a << '\n' ;
    }

    {
        // complicated: after erasing, stay at the same position for the next char
        // but cater to the erase having reduced the size of the string
        std::string b = str ;
        for( std::size_t i = 0 ; i < b.size() ; ++i )
            while( !std::isalnum( b[i] ) && i < b.size() ) b.erase(i,1) ;
        std::cout << b << '\n' ;
    }

    {
        // erase(iterator) returns the iterator following the erased character.
        // http://en.cppreference.com/w/cpp/string/basic_string/erase
        std::string c = str ;
        for( std::string::iterator iter = c.begin() ; iter != c.end() ; )
            if( !std::isalnum(*iter) ) iter = c.erase(iter) ;
            else ++iter ; // not erased, increment iterator
        std::cout << c << '\n' ;
    }

    {
        // erase-remove idiom https://en.wikipedia.org/wiki/Erase-remove_idiom
        std::string d = str ;
        d.erase( std::remove_if( d.begin(), d.end(), []( char c ) { return !std::isalnum(c) ; } ), d.end() ) ;
        std::cout << d << '\n' ;
    }
}

http://coliru.stacked-crooked.com/a/4de5c2f5b8b0d0ec

[abdalimran]

Thank you all! :)