Creating a comparison function to sort C-style strings

Question

I'd like to sort an array of C-style strings but I don't know how to create the comparison function properly.

//the array
char foo[2][4] = { "abc", "cde" };

//the comparison function
bool cmp(char * a, char * b) //<- actually passing parameters causes trouble
{
    for (int i = 0; i < 4; i++)
        if (a[i] < b[i])
            return true;
        else if (a[i] > b[i])
            return false;

    return true;
}

//sorting
std::sort(foo, foo + 2, &cmp);

I'd like this piece of code to be as fast as possible so I don't want to use vectors or structures and pass them as a reference.

Any help will be appreciated.

//edit I apologize for being imprecise. I don't want to re-implement lexicographical sorting because there is an STL function that does it appropriately. The comparison part doesn't really matter. I just wanted to have access to the elements of the strings (in this case 'letters'), do whatever I want to, and influence the std::sort function by returning true or false.

Take a look at the highest rated response: c++ sort with structs I wanted to do the same with an array of C-strings (not structures) - the comparison itself doesn't matter.

I don't understand, it will return true if they are equal and if their first character are not equal. — Djon
– Djon, Commented Jun 5, 2013 at 14:34
How is passing parameters to your cmp function causing trouble? — jonhopkins
– jonhopkins, Commented Jun 5, 2013 at 14:34
The comparison function is not your only problem. You can't sort foo because its elements are not assignable. — interjay
– interjay, Commented Jun 5, 2013 at 14:36

interjay · Accepted Answer · 2013-06-05 15:20:54Z

3

You can't sort the foo array, because it is a 2-dimensional array. std::sort requires that the elements of the array be assignable. But foo's elements are 1-dimensional arrays, and arrays cannot be assigned.

If you make foo an array of pointers instead, you will be able to sort it:

const char *foo[2] = { "abc", "cde" };

You will also need to make cmp take const char * pointers instead of char *:

bool cmp(const char * a, const char * b)

Other options are to make foo's elements std::array<char, 4>, or a struct containing a char[4] member. Using std::string is the most recommended option unless you have a good reason to avoid it.

Another thing to fix: Your cmp function returns true when passed two equal elements. It should return false instead.

Note that instead of your implementation of cmp, you can take advantage of the strcmp function:

bool cmp(const char * a, const char * b) {
    return std::strcmp(a, b) < 0;
}

edited Jun 5, 2013 at 15:20

answered Jun 5, 2013 at 15:13

interjay

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1 Comment

w0jt Over a year ago

Yes, that absolutely makes sense. I've just tried it and you're right. Thank you very much.

Shamim Hafiz - MSFT · Accepted Answer · 2013-06-05 14:38:56Z

-1

bool cmp(char * a, char * b) //<- actually passing parameters causes trouble
{
int lena = strlen(a);
int lenb = strlen(b) ;   
for (int i = 0; i < lena && i < lenb; i++)
        if (a[i] != b[i])
            return a[i]<b[i];
if ( lena<lenb ) return true;
return false;
}

You basically need to reimplement the strcmp function. Alternatively, you could use it directly, taking care to properly return the boolean values.

answered Jun 5, 2013 at 14:38

Shamim Hafiz - MSFT

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3 Comments

awesoon Over a year ago

Actually, your function will compare only first characters of the strings.

Balog Pal Over a year ago

and why one wants to incorrectly reimplement a library funcition?

Vincent Over a year ago

It's more clear to write return lena < lenb;. You can also pass the arguments as const.

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