convert char to string - c++

Question

Hello? I want to know "how to convert char to string"
This is my C code
string firSen;
int comma1=0;
cout<<"Please write your sentence"<<endl;
getline(cin,first);
int a=firSen.first("string");
for(i=a;firSen[i] != ',';i++)
comma1=i;
cout<<firSen[comma1-3]<<firSen[comma1-2]<<firSen[comma1-1]<<endl;
I will write "The string is 100s, Thank you"
I know firSen[comma1-3]=1, firSen[comma1-2]=0, firSen[comma1-1]=0 for type of char.
And I want to put these char into string
(Like 1,0,0 into string of 100) because I want to use atoi function....
Do you know how to convert char into string?

Answer 1

You can use std::istringstream instead of atoi.
Something like this:
std::istringstream ss(firSen.substr(comma1-3));
int val;
ss >> val;

Answer 2

In this case, if you know the location and length that you want, you can just extract a substring:
std::string number(firSen, comma1-3, 3);
and convert that to an integer type using the C++11 conversion functions:
int n = std::stoi(number);
or, historically, a string stream:
int n;
std::stringstream ss(number);
ss >> n;
or, if you want to be really old-school, the C library
int n = std::atoi(number.c_str());
There are other ways of building strings. You can initialise it from a list of characters:
std::string number {char1, char2, char3};
You can append characters and other strings:
std::string hello = "Hello";
hello += ',';
hello += ' ';
hello += "world!";
or use a string stream, which can also format numbers and other types:
std::stringstream sentence;
sentence << "The string is " << 100 << ", thank you.";