How to add numerical values inside a C string?

Question

Good morning, I need to write a code that receives a number and returns the sum of the digits of that number.
For example, if the user type 123456789 the program must print 45 (1+2+3+4...).
I think the easiest way would be to store the number in a string and then add the elements in each position of the string, but I don’t know how to add the elements of the string.
Some websites suggest using the function atoi() but it takes the value of the whole string and not of a specific element.

Answer 1

The function atoi to convert an entire string to number and not each character. To convert a character to number just use the ascii table and subtract.

If you have the character '2' and want to keep the number 2 can subtract the value of the character '0' you’re gonna give him 2

Example:

• '2' => letter 50 of the ascii table
• '0' => letter 48 of the ascii table

'2'-'0' = 50 - 48 = 2 that was the value you wanted.

To use this in your code you can do the logic even without using the string library and using pointers, thus:

char numero[] = "123456789";

char *letra = numero; //ponteiro letra aponta para o primeiro caractere
int soma = 0;

while (*letra!='\0'){ //enquanto não apanhar o terminador da string
    soma += (*letra) - '0'; //conversão de letra para número e soma
    letra++; //aponta para a proxima letra
}

printf("%s dá como soma de numeros %d", numeros,soma);

Watch it work on Ideone

Answer 2

In C# you can use the Convert.Toint16:

string digitos = "123456";
string retorno = string.Empty;
int total = 0;
for(int i = 0; i < digitos.Length; i++){
   total += Convert.ToInt16(digitos[i]);

   if (!string.isNullOrEmpty(retorno))
      retorno += " +";
   retorno += digitos[i];
}
retorno = total + " (" + retorno + ")";
Console.WriteLine(retorno);

---

In C, as I recall, you can make the conversion that way; I believe printfs mount the expected return:

char digitos[6] = "123456";
int total = 0;
for(int i = 0; i < digitos.Length; i++){
   total += (digitos[i] - '0');
}
printf("%s (", total);
for(int i = 0; i < digitos.Length; i++){
   if (i == 0)
      printf("%c ", digitos[i]);
   else
      printf("+ %c", digitos[i]);
}
printf(")",);

Answer 3

How about:

#include <stdio.h>

int digsum( const char * s )
{
    int i = 0;
    int soma = 0;

    while( s[i] )
        soma += ( s[i++] - '0' );

    return soma;
}

int main( void )
{
    printf( "%d\n", digsum("123456789") );
    return 0;
}