Implementation of excel's ROUND function

Question

How would you implement a ROUND function:

ROUND(value, number of digits)
pi=3.14159265358979323
so, for example, ROUND(pi, 3) = 3.142

if you had these functions at your disposal:
AINT - truncates a value to a whole number
ANINT - calculates the nearest whole number
NINT - returns the nearest integer to the argument

or never minding the above functions, how is floating ROUND done at all ?

Answer 1

I would assume, excuse my pseudo-code

function Round(value, num){
    numsToSave = POWER(10, num);
    value *= numsToSave ; //Get the numbers we don't want rounded on the left side of the floating point
    value = AINT( ANINT(value) );
    value /= numsToSave;

    return value;
}

or

function Round(value, num){
    numsToSave = POWER(10, num);
    value *= numsToSave ; //Get the numbers we don't want rounded on the left side of the floating point
    value = NINT(value);
    value /= numsToSave;

    return value;
}

Answer 2

If you don't need to worry about overflow, here's how:

ROUND(value, nod) = NINT(value * POWER(10, nod)) / POWER(10, nod)

Otherwise you need to take care of the integer part and the float part separately.