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I had this interview question like a year ago and was asked to code, on a piece of paper, how to reverse each word of a string. Since I am used to Java, I proposed the obvious answer of using split + reverse, which are native commands in Java. I was then told I couldn't use those, so I floundered and ended up with a really terrible solution (even though it technically would've worked).

Anyway, it was bugging me lately, so I gave it a shot in straight C, which I am not very good at, so it took me a good while to actually get it working.

I was wondering:

  1. Is this a good solution?
  2. Have I forgotten anything obvious?
  3. Have I done anything non-kosher in the C world?

Again, I'm not very good at C, so even small points will probably help me out.

#include <stdio.h>

void reverseString(char* start, char* end){
   while (start < end){
      char temp = *start;
      *start = *end;
      *end = temp;

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

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

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

    return p-1;

void main(){
    char arr[] = "kevin is a good programmer";
    char* test = arr;

    while (test != '\0'){
        char* curWordStart = word_start_index(test);
        if (curWordStart == NULL)
        char* curWordEnd = word_end_index(curWordStart);
        reverseString(curWordStart, curWordEnd); 
        test = curWordEnd + 1;
    printf("%s \n", arr);

Also, would taking a different approach, like in higher level languages of breaking the string into an array of strings (so I guess a 2D array of chars), then stepping through and reversing each one, be a good approach as well? I thought about this first and was unable to hash it out.

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Another thing that you need to consider is that words are not always terminated by a space. Punctuation also counts!.:,;? And, being awkward, what about numbers? – Glenn Rogers Nov 5 '12 at 12:19
up vote 5 down vote accepted

I have a few issues with in your code:

  • Consistency in naming - use either camelCase or not_camel_case but don't mix

  • consistency in braces. The opening brace for a function goes in the first column.

  • word_start_index and word_end_index should take a const parameter

  • word_start_index is the same as strspn(string, " "); or if you are also looking for punctuation, strspn(string, " \t\n.,;:");

  • word_end_index - as for word_start_index but use strcspn (note the 'c')

  • word_end_index as a function (ie not in your context) fails for an empty string or a string starting with a space (it returns the char before the string starts).

  • variable test in main() is misnamed. I would prefer something that shows it is a string.

  • the test while (test != '\0') in main() is wrong - should be *test != '\0'. Your loop always exits from the break

  • no return or parameters in main()

Also, arguably the position of the stars in your pointers is wrong. I prefer char* p to be written char *p, which makes it clear that it is p that takes the star. Consider code such as char* a, b;. This is bad because it gives the impression that b is a pointer.

share|improve this answer
Thanks for the comments! Do you think taking a different approach like in higher level languages of breaking the string into an array of string (so I guess a 2-d array of chars), then stepping through and reversing each one would be good approach as well? I thought about this first and was unable to hash it out. – KDiTraglia Nov 5 '12 at 15:11
No, your approach is fine – William Morris Nov 5 '12 at 15:16
OK, thanks for the details, I was definitely unclear about how some of the pointers need to be declared and compared and had some trial and error there. The camel case issues were just a result of copy pasting a bit off the web, but again thanks for the help. – KDiTraglia Nov 5 '12 at 15:18
@WilliamMorris, hello, what is the benefit of changing the parameters to const? Thank you. – Jonathan Dec 31 '15 at 21:07

I like it; nice and logical and easy to follow.

The only change I would make is the test for space.

*p == ' '

I would replace this with

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I also met this question during a interview several years before, the string was null-terminated and separated by spaces. my idea was same as yours, just save some lines of code, comments in line.

// Reverse the characters between pointer p and q
void ReverseWord(char* p, char* q)
    while(p < q)
        char t = *p ;
        *p++   = *q ;
        *q--   = t ;

// Reverse all words in a sentence.
void ReverseSentence(char *s)
    char *p = s ;   // point to the start position of a word
    char *q = s ;   // point to the end position of a word(white space or '\0')

    while(*q != '\0') // While string not ends
        if (*q == ' ') // Get a word?
            ReverseWord(p, q - 1) ;
            q++ ; // move to next word
            p = q ;
            q++ ;

    ReverseWord(p, q - 1) ; // Reverse the last word
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Mine is also very similiar:

#include <stdio.h>
#include <ctype.h>

void reverse(char *p, char *q)
    while (p<q)
        char t = *p;
        *p++   = *q;
        *q--   =  t;

char *reverse_each_word(char* str)
    char *p, *q = str;
        p = q; while(*p && !isalnum(*p)) p++; // Skip non-word chars
        q = p; while(*q &&  isalnum(*q)) q++; // Skip     word chars
        if(*p) reverse(p, q-1);
    return str;

int main()
    char str[] = "An answer on";
    printf("%s\n", str);
    printf("%s\n", reverse_each_word(str));    
    return 0;
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