How to Reverse Every Other Word in a String in C

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#include <stddef.h>
#include <stdbool.h>

void reverse_alternate(const char *string, char *result)
{
  bool is_word = false, is_second = false, not_first = false;
  for (const char *s = string; *s; ++s) {
    if (*s == ' ' && is_word) {
      is_word = false;
      is_second = !is_second;
    } else if (*s != ' ' && !is_word) {
      if (not_first) *result++ = ' ';
      is_word = not_first = true;
    }
    
    if (is_second && is_word) {
      size_t i = 0;
      for (const char *ss = s; *ss && *ss != ' '; ++ss, ++i)
        ;
      for (size_t j = 0; j < i; ++j)
        result[j] = s[i - j - 1];
      s += i - 1;
      result += i;
    } else if (*s != ' ')
      *result++ = *s;
  }
  *result = '\0';
}

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#include <string.h>
#include <stdlib.h>
#include <stdio.h>

extern char *strdup (const char *);

char *strrev (char *string)
{
  size_t length = strlen(string);

  for (size_t i = 0; i < length / 2; i++) {
    char tmp = string[i];
    string[i] = string[length - 1 - i];
    string[length - 1 - i] = tmp;
  }
  return string;
}

void reverse_alternate(const char *string, char *result)
{
  static const char *const whitespace = " \t\n\r\v\f";

  *result = '\0';
  char *copy = strdup(string);

  char *word = strtok(copy, whitespace);

  for (int i = 0; word != NULL; word = strtok(NULL, whitespace), i++) {
    if (i % 2)
      strrev(word);
    result += sprintf(result, "%s%s", (i == 0) ? "" : " " , word);
  }
  free(copy);
}

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#include <string.h>
#include <stdlib.h>

void reverse_alternate(const char *string, char *result) {
    result[0] = '\0';
    size_t length = 0, start = 0, stop = 0, parity = 1, index = 0;
    for(size_t i=0; i<=strlen(string); i++) {
        char curr = string[i];
        if((curr == ' ' || curr == '\0') && length) {
            if(parity) {
                for(size_t j = start; j < stop; j++) {
                    result[index++] = string[j];
                }
            }
            else {
                for(size_t j = stop - 1; j >= start; j--) {
                    result[index++] = string[j];
                }
            }
            result[index++] = ' ';
            length = 0;
            parity ^= 1;
        }      
        if(curr == ' ') {
            continue;
        }
        if(length == 0) {
            start = i;
            stop = i;
        }
        length++;
        stop++;
    }
    result[index - 1] = '\0';
}

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#include <criterion/criterion.h>
#include <stdlib.h>
#include <string.h>

void reverse_alternate(const char *string, char *result);
void tester(const char *string, char *expected);

Test(reverse_alternate, Sample_Tests) {
    tester("Did it work?", "Did ti work?");
    tester("I really hope it works this time...", "I yllaer hope ti works siht time...");
    tester("Reverse this string, please!", "Reverse siht string, !esaelp");
    tester("Have a beer", "Have a beer");
    tester("   ", "");
}

void tester(const char *string, char *expected) {
	size_t length = strlen(string);
    char submitted[length + 1];
	memset(submitted, '@', length + 1);
    reverse_alternate(string, submitted);
    cr_assert(                                !strcmp(submitted,         expected),
      "< Incorrect Result >\n \nstring  =  \"%s\"\n \nSubmitted: \"%s\"\nExpected:  \"%s\"\n \n",
                                string,               submitted,         expected
    );
}