Question

I am just learning C and in my class this is a part of our first program.

The full description of the function I am trying to implement:

if that’s tl;dr, a key point is that I am not allowed to use functions from other libraries (so something like srtol is ruled out).

int parseHexString(char *hexString, int *integerRead); The first
parameter is a null terminated C string, that represents a hexadecimal
integer. This function parses this string, accumulating the integer
value it represents. This integer value is placed at the location
pointed to by the second parameter, integerRead. If a bad hexadecimal
character, thus an invalid hex value, is encountered, this function
stops looking at further characters within the string and returns -1.
If a good hex value is parsed, it returns 0.

The correct way to implement this function looks at the first
character within the string first and does not use a stack to
accomplish the parsing. Your first assembly language program will need
to implement what this function accomplishes, so you will save
yourself time by implementing this function the correct way.

For this function, do not call any functions from any libraries; as an
exception, for debugging purposes only, you may use printf(). It will
help our grading if you remove your debugging code before you submit
your assignment.

I am NOT just looking for a full implementation of this function, just some tips or hints to get me started.

I feel as though there is some intuitive way of doing this, but right now I am blanking. I’m concerned with how I am supposed to start at the first character of the string and then go forward from there to convert it to decimal.

Answer 1

How about:

• Get the length of the string
• Walk the string from left to right
• For each character:
  • Check if it’s a valid hex character
  • Add its decimal value multiplied by 16^x, where x is the number of characters left on the right