deleting some characters from string and copying it - c++

Question

I am trying to delete characters that aren't numbers nor letters from a string by such a stupid method since I failed in getting other methods ( I am a real beginner "just trying ")
I know that this way isn't the right one but my question is what the problem with it what is the error since it doesn't work :S
string g = "9-=p98u;iu8y76";
string y;
int t = 0;
for (int i = 0; i < g.length(); i++)
{
if (isdigit(g[i]) || isalpha(g[i]))
{
y[t++] = g[i];
}
else
continue;
}
g = y;
cout << g;

Answer 1

The problem is that the size of y is 0, it's empty. Accessing its elements (using y[t++]) therefore reaches "after" the string—it's buffer overflow, and Undefined Behaviour.
You need to extend y. To do this with minimal changes to your code, you'd do this:
string g = "9-=p98u;iu8y76";
string y;
for (int i = 0; i < g.length(); i++)
{
if (isdigit(g[i]) || isalpha(g[i]))
{
y.push_back(g[i]);
}
else
continue;
}
g = y;
cout << g;
Of course, there are other ways to do that. Using standard algorithms and the erase-remove idiom would be more idiomatic C++. The entire code can be replaced with this:
auto shouldBeRemoved = [](char c) { !(isdigit(c) || isalpha(c)) };
g.erase(std::remove_if(g.begin(), g.end(), shouldBeRemoved), g.end());
cout << g;
std::remove_if works by reorganising the range so that all elements which match the predicate (i.e. those which should be removed) are moved after all elements which are to remain. The member function erase then erases all of those which were moved to the back.

Answer 2

This is reasonably expressed by the standard library. Something like
auto digit_or_alpha = [](char c){ return isdigit(c) || isalpha(c); };
std::copy_if(g.begin(), g.end(), std::back_inserter(y), digit_or_alpha );
Should work. back_inserter is in <iterator>. Angew provides the reason why yours doesn't work.

Answer 3

The problem is the way you are trying to extend the string y. Indexing can be applied only in the domain of the string (i.e. you cannot index beyond the length of the string)
change y[t++] = g[i] to y += g[i]
Additionally, i would like to mention that you don't need the else branch. When execution reaches the end of loop scope, it will "automatically" continue, it isn't needed to be expressed explicitly.
PS: It's classic C++, not C++11, I would accept Captain Giraffe's answer

Answer 4

The general approach for such tasks is to use member function erase along with standard algorithm std::remove_if declared in header <algorithm>
For example
#include <iostream>
#include <string>
#include <algorithm>
#include <cctype>
int main()
{
std::string s = "9-=p98u;iu8y76";
std::cout << s << std::endl;
s.erase( std::remove_if( s.begin(), s.end(),
[]( char c )
{
return !std::isalnum( ( unsigned char )c );
} ),
s.end() );
std::cout << s << std::endl;
return 0;
}
The program output is
9-=p98u;iu8y76
9p98uiu8y76
As for your code then you are trying to use the subscript operator
y[t++] = g[i];
for an empty string
string y;
Take into acoount that instead of two functions std::isalpha and std::isdigit you can use one function std::isalnum as shown in my demonstrative program.
If you want to write loops yourself then the program can look like
#include <iostream>
#include <string>
#include <cctype>
int main()
{
std::string s = "9-=p98u;iu8y76";
std::cout << s << std::endl;
std::string::size_type i = 0;
while ( i < s.length() && std::isalnum( ( unsigned char )s[i] ) ) ++i;
std::string::size_type j = i;
while ( i++ < s.length() )
{
if ( std::isalnum( ( unsigned char )s[i] ) ) s[j++] = s[i];
}
s.erase( j );
std::cout << s << std::endl;
return 0;
}
The program output is the same as above
9-=p98u;iu8y76
9p98uiu8y76
There is no need to use an additional string to do this operation.