Error when trying to reverse string except for special characters - c++

Question

I need to reverse only the parts of the string that are alphabetic while leaving the special characters in the string untouched. For example,
Input: str = "Ab,c,de!$"
Output: str = "ed,c,bA!$"
I have written the following C++ code to do the same:
#include<stdio.h>
#include<iostream>
#include<string>
#include<math.h>
#include<stdlib.h>
#include<ctype.h>
using namespace std;
int main() {
char s[100],t[100];
cin>>s;
int len;
for ( len = 0; ; len++ ) {
if( s[len] =='\0') {
break;
}
}
//cout<<len<<endl;
t[len] = '\0';
for ( int i = 0; i < len; i++ ) {
if ( isalpha(s[i])) {
t[len-i-1] = s[i];
// cout<<t[len-i-1]<<endl;
}
else {
t[i] = s[i];
//cout<<t[i]<<endl;
}
}
//cout<<s<<endl;
cout<<t;
return 0;
}
For the above mentioned sample input, I get SOH as the output instead. What is the mistake I am making?

Answer 1

It occurred to me that it would be natural to write this using boost::iterator::filter_iterator and std::reverse:
#include <string>
#include <algorithm>
#include <iostream>
#include <boost/iterator/filter_iterator.hpp>
using namespace std;
int main()
{
auto is_regular = [](char c){return c != ',';};
string s{"hello, cruel world"};
std::reverse(
boost::make_filter_iterator(is_regular, s.begin(), s.end()),
boost::make_filter_iterator(is_regular, s.end(), s.end()));
cout << s << endl;
}
This outputs:
dlrow, leurc olleh
Note that this only treats commas as special, but you get the idea.
Following is a solution that is more along the lines of the the code in your question. I think the problem is that, to do this correctly, you need to have two indices - one running from the left and one from the right - and run until they cross each other. Otherwise you can get all sorts of strange things like things being crossed twice.
#include<stdio.h>
#include<iostream>
#include<string>
#include<math.h>
#include<stdlib.h>
#include<ctype.h>
#include<cstring>
using namespace std;
int main()
{
char s[101]="hello, cruel world", t[100];
auto len = strlen(s);
t[len--] = '\0';
//cout<<len<<endl;
int i = 0;
while(i <= len)
if (!isalpha(s[i]))
{
t[i] = s[i];
++i;
continue;
}
else if (!isalpha(s[len]))
{
t[len] = s[len];
--len;
continue;
}
else
{
t[len] = s[i];
t[i] = s[len];
++i;
--len;
}
cout<<t<< endl;
return 0;
}

Answer 2

It is better and more safe to use standard class std::string instead of a character array.
The program can look the following way
#include <iostream>
#include <string>
#include <cctype>
int main()
{
std::string s;
std::getline( std::cin, s );
if ( !s.empty() )
{
for ( std::string::size_type first = 0, last = s.size(); first < --last; ++first )
{
while ( first != last && !std::isalpha( ( unsigned char )s[first] ) ) ++first;
while ( first != last && !std::isalpha( ( unsigned char )s[last] ) ) --last;
if ( first != last ) std::swap( s[first], s[last] );
}
std::cout << s << std::endl;
}
}
If to enter string
Ab,c,de!$
then the output will look like
Ab,c,de!$
ed,c,bA!$