C: Reverse individual words in a string

Question

I have been practicing C and I wanted to reverse each word in a string. I wrote this code and although it works, I am not happy with my solution. It requires twice the length of the original string to store the result and buffer respectively and there are too many if statements which makes the code hacky. I am wondering how can I do better?

#include <stdio.h>
#include <string.h>
int main()
{
 char* str = "123 456 789";
 int i, j = 0, k, l = 0;
 int str_len = (int)strlen(str);
 char buffer[str_len];
 int buffer_size = 0;
 char result[str_len + 1];
 for (i = 0; i < str_len; i++)
 {
 char c = str[i];
 if (c == ' ' || (i + 1) == str_len)
 {
 short is_end_of_string = 0;
 if ((i + 1) == str_len)
 {
 buffer_size++;
 buffer[l] = c;
 is_end_of_string = 1;
 }
 for (k = 0; k < buffer_size / 2; k++)
 {
 char temp = buffer[k];
 buffer[k] = buffer[buffer_size - k - 1];
 buffer[buffer_size - k - 1] = temp;
 }
 for (k = 0; k < buffer_size; k++)
 {
 result[j++] = buffer[k];
 }
 if (!is_end_of_string)
 {
 result[j++] = c;
 buffer_size = 0;
 l = 0;
 }
 }
 else
 {
 buffer_size++;
 buffer[l++] = c;
 }
 }
 result[j] = 0;
 printf("%s\n", result);
 return 0;
}

Try it online

Answer 1

Clarify "word"

OP's "word" seems to be characters separated by a space ' '. A more common definition would use any white-space (tabs, end-of-line, etc.) , not just ' ' as a separator.

Research isspace() in <ctype.h>.

Alternative

It requires twice the length of the original string

No need to find the string or word length anywhere. O(1) extra memory needed, unlike OP's O(length) for an in-place reversal.

Algorithm:

p = string start
forever 
 while is white-space: p++
 if p[0] then
 note beginning address
 while p[0] and is non-white-space: p++
 note one-past-end-of-word address
 while one-past-end-of-word > beginning
 swap (beginning[0], one-past-end-of-word[-1])
 beginning++;
 one-past-end-of-word--;
 else we are done

For general use: String length may exceed INT_MAX

Use size_t to handle all possible string lengths. Cast not needed. Note that size_t is an unsigned type, so other parts of code made need changing too.

// int str_len = (int)strlen(str);
size_t str_len = strlen(str);

For boolean objects, use _Bool or bool

#include <stdbool.h>
// short is_end_of_string = 0;
bool is_end_of_string = false;
// is_end_of_string = 1;
is_end_of_string = true;

There are times to use maybe unsigned char for a boolean if we had a large array of booleans, yet this is not the case here.

Testing

Try more challenging strings too.

"123 456 789"
" 123 456 789"
"123 456 789 "
" 123 456 789 "
"123\t 456\n 789"
" "
""
"1 2 3\n"

Advanced

Access characters of a string via unsigned char * instead of char * for more proper use of is...() functions in <ctype.h> which can fail with negative values.

Pedantic: unsigned char * access also handles rare non-2's complement with signed char to properly distinguish between +0 and -0.

Print with sentinels

Easier to detect proper handling of leading/trailing white-space.

// printf("%s\n", result);
printf("<%s>\n", result);