Reversing every word in a string

Question

Examples:

char test1[] = "               ";
char test2[] = "   hello  z";
char test3[] = "hello world   ";
char test4[] = "x y z ";

Results:

"               "
"   olleh  z"
"olleh dlrow   "
"x y z "

The problem:

Reverse every world in a string, ignore the spaces.

The following is my code. The basic idea is to scan the string, when finding a word, then reverse it. The complexity of the algorithm is O(n), where n is the length of the string.

Could anyone help me verify it? Is there any better solution?

void reverse_word(char* s, char* e)
{
    while(s < e)
    {
        char tmp = *s;
        *s = *e;
        *e = tmp;
        ++s;
        --e;
    }
}

char* word_start_index(char* p)
{
    while((*p != '\0') && (*p == ' '))
    {
        ++p;    
    }

    if(*p == '\0')
        return NULL;
    else
        return p;
}

char* word_end_index(char* p)
{
    while((*p != '\0') && (*p != ' '))
    {
        ++p;
    }

    return p-1;
} 

void reverse_string(char* s)
{
    char* s_w = NULL;
    char* e_w = NULL;
    char* runner = s;

    while(*runner != '\0')
    {
        char* cur_word_s = word_start_index(runner);
        if(cur_word_s == NULL)
            break;
        char* cur_word_e = word_end_index(cur_word_s);
        reverse_word(cur_word_s, cur_word_e);
        runner = cur_word_e+1;    
    }
}

Answer 1

If using C++ is an option for you (which your tags suggest), then you might consider using std::swap for swapping the characters. You might also want to use isspace to check for whitespace other than ' '.

You could also write it all in a single function:

void reverse_every_word(char* s)
{
    char* front;
    char* back;
    while(*s != '\0')
    {
        // handle whitespace
        while(*s != '\0' && isspace(*s))
            s++;

        // skip to the end of the current word
        front = s;
        while(*s != '\0' && !isspace(*s))
            s++;

        // reverse
        back = s-1;
        while (front < back)
            std::swap(*front++, *back--);
    }
}

Answer 2

Comments on code

In C++ code you do not want to be messing with C-Strings. All that memory management makes the code hard to maintain and is difficult to get correct. prefer to use std::string which does all the hard work and lets you concentrate on the algorithm.

void reverse_word(char* s, char* e)

There are already algorithms for reversing stuff std::reverse() and swapping the two values std::swap() so there is no need to implement either.

    while(s < e)
    {
        char tmp = *s;
        *s = *e;
        *e = tmp;
        ++s;
        --e;
    }

Space ' ' is not the only white space character. Rather than explicitly testing for a space you should use the standard function for testing if a character is white space std::is_space()

    while((*p != '\0') && (*p == ' '))
    

Comments on Algorithm

There is already a std::reverse() algorithm.
Using this would reduce the complexity of your code to just finding the beginning and end of each word. Since operator>> does this automatically it makes the code very trivial:

void reverse_string(std::string& paragraph)
{
    std::stringstream pStream(paragraph);
    std::string       word;

    paragraph.clear();
    while(pStream >> word)
    {
        std::reverse(word.begin(), word.end());
        paragraph.append(word);
        paragraph.append(" ");
    }
}

Answer 3

Having C++11, we could do,

template<class InputIterator, class UnaryPredicate>
void reverseSeparate(InputIterator first, InputIterator last, UnaryPredicate pred)
{
    first = std::find_if_not(first,last,pred);
    while(first!=last){
        InputIterator t = std::find_if(first,last,pred);
        std::reverse(first,t);
        first = std::find_if_not(t,last,pred);
    }
}

using this, we can (among other things) reverse every word in a string.

std::string s = "hi   hello world ";
reverseSeparate(s.begin(),s.end(), [](char c){return c==' ';});