Reversing words in a string using three pointers? - c

Question

I am trying to reverse words in a string and I believe that I have written the right logic, but while debugging I came to know that the value that I am putting in new_array char pointer variable is getting lost? And I don't seem to have any idea why?
Can you tell me what is the mistake I am doing and what can be done to correct it?
#include <stdio.h>
void reverse_words(char *arr, int size) {
char *ptr = arr;
char *new_array = (char*)malloc(sizeof(char*) * size);
while (*ptr != '\0') {
ptr++;
}
ptr--;
for (int i = size - 1; i >= 0; i--) {
if (*ptr != ' ') {
ptr--;
} else {
char *temp = ptr;
temp++;
//
// Problem is in this block new_array value is lost when i increment it
while (*temp != ' ' && *temp != '\0') {
*new_array = *temp;
new_array++;
temp++;
}
if (i != 0) {
*new_array = *ptr;
ptr--;
}
}
}
*new_array = '\0';
strcpy(arr, new_array);
return;
}
int main() {
char arr[] = "My job is coding";
int size = sizeof(arr);
reverse_words(arr, size);
printf("%s", arr);
return 0;
}

Answer 1

Your code is too complicated and has several problems:
you allocate too much memory: (char*)malloc(sizeof(char*) * size); allocates size times the size of a pointer. Use malloc(size); to allocate size bytes.
there are errors in your pointer manipulations,
you forget to free the allocated memory, causing a memory leak.
you do not need to pass the size of the array, the length of the string is computed by scanning for the null terminator, just allocate one extra byte for the null terminator.
There is an alternative solution to reverse the words int the string in place without memory allocation:
for each word, reverse the word
final step: reverse the string
Here is the code using a utility function:
#include <stdio.h>
void reverse_mem(char *str, int size) {
for (int i = 0, j = size; i < --j; i++) {
char c = str[i];
str[i] = str[j];
str[j] = c;
}
}
void reverse_words(char *arr) {
for (int i = 0, j = 0;; i = j) {
for (; str[i] == ' '; i++)
continue;
if (str[i] == '\0')
break;
for (j = i; str[j] != '\0' && str[j] != ' '; j++)
continue;
reverse_mem(str + i, j - i);
}
reverse_mem(str, size);
}
int main(void) {
char arr[] = "My job is coding";
reverse_words(arr);
printf("%s\n", arr);
return 0;
}

Answer 2

you are not getting output as expected because of this statement ..
*new_array = *temp; //2nd time new_array is not pointing previous location so how will you retrieve data
new_array++;// you lost previous data bcz pointers no longer holds previous address
when main loop fail new_array pointer will point to last but it should holds starting address. Instead of incrementing new_array directly do like
* (new_array + j) = *temp
I modified your code as . .
void reverse_words(char *arr,int size)
{
char * ptr = arr;
char *new_array = (char*)malloc(sizeof(char*) * size);
printf("initial : %u \n",ptr);
while(*ptr != '\0')
{
ptr++;
}
int j=0;
ptr--;
printf("first : %c : %u \n",*ptr,ptr);
for(int i = size-1;i >=0;i--)
{
if(*ptr != ' ' ) //&& i!=0)
{
ptr--;//from last to back
}
else
{
char * temp = ptr;
temp++;
while(*temp != '\0' && *temp!=' ')
{
new_array[j++]=*temp;
temp++;
}
if(i!=0)
{
new_array[j++] = *ptr;
ptr--;
}
}
}
new_array[j] = '\0';
printf("new = %s \n",new_array);
strcpy(arr,new_array);
}
I hope you got where it's going wrong, Modify the condition for first words because once i becomes 0 you are not writing any logic for that.
My Solution according to your requirements
void reverse_words(char *arr,int size) {
char *new_array = malloc(size * sizeof(char));
int i,j,k=0;
for(i = size-1 ;i >= -1 ; i--) {
if(arr[i] ==' ' || i == -1)
{
for(j=i+1;arr[j]!=' ' && arr[j]!='\0'; j++)
new_array[k++] = arr[j];
new_array[k++] = ' ';
}
}
new_array[k] = '\0';
strcpy(arr,new_array);
}