Infinite loop using string.substr(post,len) - c++

Question

Could you figure out why it keeps looping infinitely in the console? The programmer's supposed to list out each character of a user-inserted string and next to each unique character, in brackets, it's supposed to display the number of times that character occurs in the string... no idea why.
#include <iostream>
#include <string>
#include <algorithm>
using namespace std;
int main () {
string input;
cout << "input string: " , cin >> input;
sort (input.begin() , input.end());
while (!input.empty()) {
int j{1}, i{0};
while (input.at(i) == input.at(i+1)) {
j++;
i++;
}
cout << input.at(i) << " (" << j << "), ";
input.substr(i);
}
return 0;
}

Answer 1

This statement
input.substr(i);
does not change the object input itself.
So, either you will have an infinite loop if for some index i input.at(i) is not equal to input.at(i+1), or you can have an exception out of range because i + 1 can be equal to input.size().
From the description of the member function at
Throws: out_of_range if pos >= size().
The program can be implemented in different ways. For example the following way
#include <iostream>
#include <string>
#include <algorithm>
int main()
{
std::cout << "input string: ";
std::string input;
std::cin >> input;
std::sort( input.begin() , input.end() );
for ( size_t i = 0; i < input.size(); )
{
size_t j = input.find_first_not_of( input[i], i );
if ( j == std::string::npos ) j = i + 1;
if ( i != 0 ) std::cout << ", ";
std::cout << input[i] << " (" << j - i << ")";
i = j;
}
std::cout << '\n';
return 0;
}
The program output is
input string: Hello
H (1), e (1), l (2), o (1)
Or you can use the standard container std::map or std::unordered_map as for example
#include <iostream>
#include <string>
#include <map>
int main()
{
std::cout << "input string: ";
std::string input;
std::cin >> input;
std::map<char, size_t> m;
for ( const auto &c : input )
{
++m[c];
}
bool first = true;
for ( const auto &p : m )
{
if ( !first ) std::cout << ", ";
std::cout << p.first << " (" << p.second << ")";
first = false;
}
std::cout << '\n';
return 0;
}
If you want that characters of the inputted string were output in the order in which they are present in the string then the program can look like
#include <iostream>
#include <string>
#include <map>
int main()
{
std::cout << "input string: ";
std::string input;
std::cin >> input;
auto less = [&input]( const auto &c1, const auto &c2 )
{
return input.find( c1 ) < input.find( c2 );
};
std::map<char, size_t, decltype( less )> m( less );
for ( const auto &c : input )
{
++m[c];
}
bool first = true;
for ( const auto &p : m )
{
if ( !first ) std::cout << ", ";
std::cout << p.first << " (" << p.second << ")";
first = false;
}
std::cout << '\n';
return 0;
}
Or without changing the original string and without using an additional container the program can look the following way.
#include <iostream>
#include <string>
int main()
{
std::cout << "input string: ";
std::string input;
std::cin >> input;
for ( size_t i = 0; i < input.size(); i++ )
{
size_t j = 0;
while ( j != i && input[j] != input[i] ) j++;
if ( j == i )
{
size_t count = 1;
while ( ++j < input.size() )
{
if ( input[j] == input[i] ) ++count;
}
if ( i != 0 ) std::cout << ", ";
std::cout << input[i] << " (" << count << ")";
}
}
std::cout << '\n';
return 0;
}
The program output might look like
input string: elephant
e (2), l (1), p (1), h (1), a (1), n (1), t (1)