Member Avatar for k_en

hi, i am a newbie and i just started to learn C++.
My question is , if i put a number like 45678 in char x[6] then how do i separate each number and put it into int num[6].

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  • Latest Post Latest Post by Drowzee

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For a fixed C-style string like the one posted by the goal is to turn each character digit into its numeric value (so "45678" becomes 4,5,6,7,8). 's attempt to call atoi on the address of a single char is unsafe; as explained, atoi expects a null-terminated string and will read past a single byte, which is undefined behavior. Use a safe loop that validates each character and converts it explicitly.

A simple, modern and robust pattern uses std::string (or the original char array), std::vector<int> for storage, and std::isdigit for validation. Always call std::isdigit with static_cast<unsigned char>(ch) to avoid UB on platforms where char is signed. For arbitrary-width numbers (multi-digit tokens, signs) prefer parsing functions (std::from_chars, std::stoi or std::istringstream) instead of per-character subtraction.

Example conversion (safe, compact):

#include <string>
#include <vector>
#include <cctype>

std::string s = "45678";
std::vector<int> nums;
nums.reserve(s.size());

for (char ch : s) {
    unsigned char uch = static_cast<unsigned char>(ch);
    if (!std::isdigit(uch)) continue; // or handle error
    nums.push_back(static_cast<int>(uch - '0'));
}

Notes and troubleshooting:

  • Use std::vector rather than fixed C arrays unless you need raw arrays.
  • Avoid system("pause") and other nonportable calls.
  • If the input can contain signed or multi-digit numbers, parse tokens with std::from_chars/std::istringstream so you get proper error handling.
  • The core lesson from and is correct: validate characters and convert explicitly; don’t rely on atoi with non-null-terminated memory.

Recommended Answers

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Member Avatar for zyruz

somthing like this:

#include <iostream>

int main ()
{
    char c[6] = "45678";
    int num[5];
    for (int i = 0; i < 5; i++)
    {
        char temp = c[i];
        num[i] = atoi(&temp);
        std::cout << num[i] << std::endl;
    }
    system("pause");
}
Member Avatar for k_en

Thank you , why do i have to do this??? can anyone pls explain??

num = atoi(&temp);

Member Avatar for Rashakil Fol

somthing like this:

#include <iostream>

int main ()
{
    char c[6] = "45678";
    int num[5];
    for (int i = 0; i < 5; i++)
    {
        char temp = c[i];
        num[i] = atoi(&temp);
        std::cout << num[i] << std::endl;
    }
    system("pause");
}

This code is downright dangerous. Suppose I add one line that seems not to affect the outcome:

#include <iostream>

int main ()
{
    char c[6] = "45678";
    int num[5];
    for (int i = 0; i < 5; i++)
    {
        char temp = c[i];
        char khan = '9';
        num[i] = atoi(&temp);
        std::cout << num[i] << std::endl;
    }
}

On my system, I get 49 59 69 79 89 with this code. Why?

This happens because temp is just a single character. You don't know what goes after it. When you pass a pointer of temp to atoi, atoi will expect a pointer to an _array_ of characters. It will read numbers until it gets a nondigit -- a character that is not in the range '0' .. '9'.

When you ran your code, you got lucky. The position in memory next to wherever temp was stored did not have an ASCII digit.

With my code example, you might get different output, since the result is affected by where your compiler puts everything.

Since khan may get stored right next to temp in memory, atoi might think it's the next character in the string, and so it reads 49, 59, and so on.

If you want to convert a single ASCII digit to its corresponding integer, you just need to subtract '0'. For example:

char digit = '7';
int num = digit - '0'; /* num is now 7 */

Edit:

Also, on my system, with the compiler I'm using, this code will print "Hello, world!". (But the results may be different for different compilers.)

#include <cstdio>

int main()
{
	char a = '\0', b = '!', c = 'd', d = 'l', e = 'r', f = 'o',
		g = 'w', h = ' ', i = ',', j = 'o', k = 'l', l = 'l', 
		m = 'e', n = 'H';

	puts(& n);


}
Member Avatar for Drowzee

Rashakil Fol's last variation, using the subtraction of an ASCII '0', is the one I'd use.

And I've no clue what the last bit of code was, RF. That's stwange.

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