strncmp C Exercise

Question

I'm trying to do exercise 5-4 in the K&R C book. I have written the methods for strncpy and strncat, but I'm having some trouble understanding exactly what to return for the strncmp part of the exercise.

The definition of strncmp (from Appendix B in K&R book) is:

compare at most n characters of string s to string t; return <0 if s<t, 0 if s==t, or >0 if s>t

Lets say I have 3 strings:

char s[128] = "abc"
char t[128] = "abcdefghijk"
char u[128] = "hello"

And I want to compare them using the strncmp function I have to write. I know that

strncmp(s, t, 3)

will return 0 ,because abc == abc. Where I'm confused is the other comparisons. For example

strncmp(s, t, 5) and
strncmp(s, u, 4)

The first matches up the 3th position and then after that they no longer match and the second example doesn't match at all.

I really just want know what those 2 other comparisons return and why so that I can write my version of strncmp and finish the exercise.

Answer 1

Both return a negative number (it just compares using character order). I just did a quick test and on my machine it's returning the difference of the last-compared characters. So:

strncmp(s, t, 5) = -100  // '\0' - 'd'
strncmp(s, u, 4) = -7    // 'a' - 'h'

Is that what you're looking for?

Answer 2

it is lexicographic order, strings are compared in alphabetical order from left to right.

So abc < abcdefghijk < hello

strncmp(s, t, 5) = -1 strncmp(s, t, 5) = -1

Answer 3

The characters in the first non-matching positions are cast to unsigned char and then compared numerically - if that character in s1 is less than the corresponding character in s2, then a negative number is returned; if it's greater, a positive number is returned.