Converting string to number in C++

Question

I intend to convert a string into an array of numbers. For instance the below code works well:

// A program to demonstrate the use of stringstream 
#include <iostream> 
#include <sstream> 
using namespace std; 

int main() 
{ 
    string s = "12345"; 

    // object from the class stringstream 
    stringstream geek(s); 

    // The object has the value 12345 and stream 
    // it to the integer x 
    int x = 0; 
    geek >> x; 

    // Now the variable x holds the value 12345 
    cout << "Value of x : " << x; 

    return 0; 
}

How do I do it for a very big string. For example, string s = "77980989656B0F59468581875D719A5C5D66D0A9AB0DFDDF647414FD5F33DBCBE"

I need to store this into an array of chars, arr[32]. arr[0] should have 0x77, arr[1] should have 0x98 and so on. Considering string s is of 64 bytes. my array would be 32 bytes long.

Can someone help with this?

So you're essentially asking how to convert a string which represents a decimal number into a string which represents a hexadecimal number. Might be worth noting that as the input and output of your problem. — goodvibration, May 27 '20 at 11:19
Does this answer your question? [Convert strings between hex format and binary format](https://stackoverflow.com/questions/18310952/convert-strings-between-hex-format-and-binary-format) — Abhishek Bhagate, May 27 '20 at 11:39
https://stackoverflow.com/questions/1070497/c-convert-hex-string-to-signed-integer - If you want to convert to decimal integer — Abhishek Bhagate, May 27 '20 at 11:46
@goovibration i want to store it into an array containing numbers. for ex: arr[0] should contain 0x77, arr[1] should contain 0x98 — Embedded Enthusiast, May 27 '20 at 11:54
@Abhishek the links you provided work for int and long int. My question is to store the exact string as hex numbers in an array — Embedded Enthusiast, May 27 '20 at 11:55
Use `substr` to extract two characters at a time, then, as with the current code, use `std::istringstream`, but also with `std::hex`, to parse a hexadecimal value. Mission accomplished. — Sam Varshavchik, May 27 '20 at 11:56
@EmbeddedEnthusiast you could store them into array directly as they are in string. Just interpret them as hex number whenever you read from array — Abhishek Bhagate, May 27 '20 at 11:57
@churill yes. Thats correct. 2 bytes are converted into one number that has to be stored in the array — Embedded Enthusiast, May 27 '20 at 13:08

Jeune Prime Origines · Answer 1 · 2020-05-27T14:07:36.097

You can try to split the input string into substrings where each substring has 2 characters length. Then convert the hexadecimal substring using std::stoi() function into integer and store each converted result to an std::vector container:

#include <vector>
#include <iostream>


std::vector<int> convert(const std::string& hex_str) {

    std::vector<int> output;
    std::string::const_iterator last = hex_str.end();
    std::string::const_iterator itr = hex_str.cbegin();

    if (hex_str.size() % 2) {
        last--;
    }

    while(itr != last) {

        std::string sub_hex_str;
        std::copy(itr,itr+2,std::back_inserter(sub_hex_str));
        try {
            output.push_back(std::stoi(sub_hex_str,0,16));
        }catch(const std::exception& e) {
            std::cerr << sub_hex_str << " : " << e.what() << std::endl;
        }

        itr += 2;       
    }

    return output;
}

int main()
{
    std::string a = "77980989656B0F59468581875D719A5C5D66D0A9AB0DFDDF647414FD5F33DBCBE";

    const auto output = convert(a);

    for(const auto& a: output) {
        std::cout << a << std::endl;
    }
}

Converting string to number in C++

1 Answers1