C++: Using ATOI for ASCII to Integer, and sprintf for Integer to String Conversions

There was someone who asked this question on a forum I read (students looking for help with their homework… what slackers, but I can relate since I was there once too):

I have a function X that needs to take in two parameters, and integer and a character. The function needs to replace the hundredths place in the integer with the value in the character. How can I do this?

The solution is actually pretty simple and requires a little math. I was convinced that there was some slick way to do this with bit masking, but I couldn’t figure it out. (Note: if someone has a solution for that, don’t hesitate to post it).

//Convert hundredths place to whatever single character digit
//is passed in
long hundredthsConversion(long n, char c)
{
//create an array of 2 characters, and initialize the first
//character to the value passed in to the function, and the
//second one to NULL
char temp[2] = {c, 0};

//integers to hold the converted ATOI result, and the value of
//N after stripping out the hundredths place
long digitConversion, n_strip_hundreds_digit;

//Convert this single digit numeral to a number, then multiply
//by 100. This will be added to n with the hundred digit striped
//out
digitConversion = (atoi(temp) * 100);

//Divide n by 100, than divide that result by 10, giving the number. Multiple that by 100 to give the value in the hundredths without
//values in the > 100 or the < 100 places
n_strip_hundreds_digit = (n - (( (n / 100) % 10) * 100));

//return the value without the hundredths and add in the single
//character for the hundredths place.
return n_strip_hundreds_digit + digitConversion;
}

I originally had the above as a single line equation, but separated it for clarity.

Alternatively, you could change the value in N to a string, and replace the 3rd to last value with the value in C, but for no good technical reason I don’t really like the idea off converting from one type back and forth. Also, Linux and *BSD do not have a native itoa function, so you would need to use sprintf to convert from an integer to a string. If I did use this method, however, it might look something like this (Note: comments striped for brevity, and function elongated for clarity of steps):

long hundredthsConversion(long n, char c)
{
char buffer[255];
int I;

for (I = 0; I < 255; I++)
buffer[I] = 0;
sprintf(buffer, “%d”, n);

I = strlen(buffer);

switch(i)
{
case 1:
buffer[2] = buffer[0];
buffer[0] = c;
buffer[1] = ‘0’;
break;
case 2:
buffer[2] = buffer[1];
buffer[1] = buffer[0];
buffer[0] = c;
break;
default:
buffer[I – 3] = c;
break;
}

return atol(buffer);
}


Both functions compiled under the GNU C++ Compiler using only iostream and the standard namespace. Under C, you would need to include the appropriate headers for atoi and sprintf.