How can I safely and quickly extract digits from an int?

Question

We currently have some code to extract digits from an int, but I need to convert this to a platform without snprintf, and I am afraid of a buffer overrun. I have started to write my own portable (and optimized) snprintf but I was told to ask here in case someone had a better idea.

int extract_op(int instruction)
{ 
    char buffer[OP_LEN+1];
    snprintf(buffer, sizeof(buffer), "%0*u", OP_LEN, instruction);
    return (buffer[1] - 48) * 10 + buffer[0] - 48;
}

We are using C strings because Speed is very important.

Answer 1

You don't need to form instruction into a character array for this purpose; you just need to keep "the two top digits", as follows:

int extract_op(unsigned int instruction)
{
    int first = 0;
    int second = 0;
    while(instruction) {
        second = first;
        first = instruction % 10;
        instruction /= 10;
    }
    return first + 10 * second;
}

I think the expression in the return is wrong, but it does mimic what you're doing: ten times the second digit, plus the first one.

I suspect that speed might be even better than what you're getting now, but that's up to you to measure on your specific platform and compiler, of course.

Answer 2

Using sprintf should be fine. sizeof type * 3 * CHAR_BIT / 8 + 2 is a sufficiently large buffer for printing an integer of type type. You can simplify this expression if you assume CHAR_BIT is 8 or if you only care about unsigned formats. The basic idea behind it is that each byte contributes at most 3 digits in decimal (or octal), and you need space for a sign and null termination.

Answer 3

So far there's one answer that swaps the last two digits and one that swaps the first two… it looks to me like "%0*u", OP_LEN is forcing the output to a particular width, and the significance of the extracted digits is predetermined by OP_LEN.

Assuming OP_LEN is a macro, we can get 10^(OP_LEN-2) with

#define DIVISOR ( (int) ( 1.e ## OP_LEN * 0.01 ) )

Then, similar to @zneak's answer,

int extract_op( int instruction )
{
    instruction /= DIVISOR;
    int tens = (instruction / 10) % 10;
    int units = instruction % 10;
    return units * 10 + tens;
}

#undef DIVISOR

Answer 4

U can store the digit u are getting into array. THIS ONE WAS THE CODE EXPLAINED BY ALEX. here i am adding some variables.

int a[5];

int extract_op(unsigned int instruction)
{
int i=0;    
int first = 0;
    int second = 0;
    while(instruction) {
        second = first;
        first = instruction % 10;
        instruction /= 10;
    }
    a[i]=first;
}

This is something will work for all integers will have max 5 digits. But still if u want to take dynamic array then u can use link list

Answer 5

should work also for 0 and <0.

int extract_op( int instruction )
{
  int numd = 1;
  while( instruction /= 10 )
    ++numd;
  return numd;
}