C++ - Building my own String class for school, can't overload < operator correctly

Question

This is what I have right now (str is a dynamic, null-terminating char array):

bool String::operator<(const String& rhs) const {
    if (str == rhs.str)
        return false;
    int i = 0;
    while (str[i] != '\0' && rhs[i] != '\0') {
        if (str[i] > rhs[i])
            return false;
        ++i;
    }
    return true;
}

This passes most tests, but it fails on:

String s1("abc");
String s2("abcde");

assert(!(s2 < s1));

No matter how I alter the function it always seem to fail one test or another. How would YOU overload this operator? Basically I just need to compare two null-terminating char arrays and see which one is the lesser (without any libraries).

Hint: You need an extra test before returning `true`. (Just because one of the strings at the end doesn't mean `*this` is less than `rhs`) — user786653, Oct 15 '16 at 19:37
Checkout the written answer in [Operator overloading](http://stackoverflow.com/questions/4421706/operator-overloading?rq=1s) — sudo_coffee, Oct 15 '16 at 19:38

score 2 · Answer 1 · answered Oct 15 '16 at 19:39

2

You can take advantage of null-terminated strings to simplify the basic algorithm to:

While the nth character of both strings are the same (incrementing n starting with 0):
If the nth character of both strings is '\0' the strings are obviously the same, otherwise:
Otherwise, compare the nth characters as unsigned values, to determine the result of the comparison.

answered Oct 15 '16 at 19:39

Sam Varshavchik

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Are those first two steps doing something different than what I do with `if (str == rhs.str) return false;` ? – conjenks Oct 15 '16 at 19:45
Yes, they are. Does `while (str[i] != '\0' && rhs[i] != '\0') ` mean "while the nth character of both strings are the same" to you? – Sam Varshavchik Oct 15 '16 at 19:46

Slava · Accepted Answer · 2016-10-15T19:59:36.367

1

If you change your loop to:

int i = 0;
while (true) {
    char l = str[i];
    char r = rhs.str[i++];
    if( l < r ) return true;
    if( l == 0 || l > r ) return false;
}

it should work. Note if you need to handle national alphabets properly, that usually has values > 127, you need to change l and r type to unsigned char

But easier solution would be:

return strcmp( str, rhs.str ) < 0;

edited Oct 15 '16 at 19:59

answered Oct 15 '16 at 19:46

Slava

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I just ran the tests and that does indeed work perfectly, but my class has not gotten to the `unsigned` keyword yet and I don't think the professor would approve of that solution (unfortunately). But I appreciate it! Is there a way to change that to not use `unsigned`? Because honestly I don't even know what that is in the first place – conjenks Oct 15 '16 at 19:51
if you do not use ASCII > 127 for your strings you may change them to `char`. But algo will break if you put symbols with negative value for signed char. – Slava Oct 15 '16 at 19:52
I take it back, it should work with signed `char` as well, changed answer – Slava Oct 15 '16 at 19:58
I changed them to just 'char' and it still passes all tests, so I will roll with it for now. Thank you. – conjenks Oct 15 '16 at 19:58

C++ - Building my own String class for school, can't overload < operator correctly

2 Answers2