[C++] Beginner problem

Easier way to do that would be to use the integer remainder and divide operators. ( the % and / )

If you've got an arbitrary (non-negative) integer input you can add the digits together like this:



int input, sum = 0;

// get input from user here

while (input != 0)

{

sum += input % 10;

input = input / 10;

}



Another option is to neither read the input as a string or a number. Instead, read it character by character, and sum these directly.

Note that a program that uses your substring() method, or reads character by character, could handle 477,218,588 digits*. Reading the value directly into an integer like Milcho shows would only handle 10 digits*.

* [size="1"]Assuming you are using a 32 bit unsigned integers

How do you read the input digit by digit ?


EDIT:
Something like:

[font=verdana, geneva, lucida,]char number[MAX_DIGITS];
cin >> number;[/font]


?

That code will read a string into a fixed size buffer - almost certainly causing buffer overflow errors for the number of digits I was talking about, unless you were very generous with MAX_DIGITS.

I meant using a single character at a time:

#include <iostream>
#include <cctype>

int main()
{
char c;
while(std::cin >> c)
{
if(std::isdigit(c))
{
std::cout << "Got a digit: " << c << '\n';
}
else
{
std::cout << "Not a digit: " << c << '\n';
}
}
}

Converting a character to a number can be done in many ways. A simple one* is to subtract the character value '0' from the integer representation of the character. E.g.

int n = c - '0';

Remember that the textual character 0 is actually represented by a different number than the bit pattern 0*. In ASCII and 8 bit Unicode, this happens to be the bit pattern for 48.

Notice:

• '0' - '0' = 0
  
• '1' - '0' = 1
  
• ...
  
• '9' - '0' = 9
  
  
  * [size="1"]I'm ignoring text encoding here, to simplify. I'm also assuming character encodings for numbers begin with 0 and are contiguous.