Is number comparison faster than string comparison?

Question

I heard that hashing (ie converting a string or object to a number) is used for strings and such because it is easier to compare numbers than strings. If true, what is the reason for this ?

Answer 1

This is not necessarily the case, but probably the case most of the time.

Consider the following situation:

I want to compare the string "apples" vs. "oranges". If I only want to determine "apples" == "oranges", I need only compare the first character of each string: 'a' != 'o' => "apples" != "oranges". If I hash the string and then do the comparison, it is significantly slower as I have to parse both strings and feed them into a hashing algorithm before comparing the resultant integers.

If, however, I need to do this comparison many times, and perhaps I'm comparing "oranges" to "orangutans" a lot, then if I hash all the strings once and do the comparisons of integers many times, it will work out faster. This is the principle that a hash map is based on.

Note, however, that hashing a string is useful for direct equals comparisons, it cannot determine if strings are lexigraphically greater or less than each other, and so ordering strings is not possible via the hashing method. (This is why HashMap in Java is unordered).

Answer 2

Comparing two numbers is magnitudes faster than comparing two strings (representing the same numbers). Comparing two numbers simply require comparing individual bits and could be done super fast using any of AND, XOR, 2's complements, etc.

Comparing two strings is very slow and expensive. Most algorithms require iterating through entire string and matching each character.

For example let's say we want to compare 9 with 12 (false). For numeric comparison, let's assume the algorithm compares individual bit. 9 = 1001 12 = 1100

Here, the worst case algorithm will compare 4 bits.

Now if we represent "9" and "12" as strings, they will be stored in memory as 16 bits each (Recall: Java uses UTF-16 to represent strings in memory) and have to be passed to a String comparison algorithm. In fact, Java's actual String comparison function is below:

public boolean equals(Object anObject) {
    if (this == anObject) {
        return true;
    }
    if (anObject instanceof String) {
        String anotherString = (String)anObject;
        int n = count;
        if (n == anotherString.count) {
            char v1[] = value;
            char v2[] = anotherString.value;
            int i = offset;
            int j = anotherString.offset;
            while (n-- != 0) {
                if (v1[i++] != v2[j++])
                    return false;
            }
            return true;
        }
    }
    return false;
}

As you can see, there's a lot more going around for String comparison.