Thread: reverse words

Hi all,

This C program reverses the first n words in a string, for example:
echo "hello world do not reverse" | rev 2 should output:
"world hello do not reverse"
Here's my code:

Code:

 
       1 #include <stdio.h>
      2 #include <stdlib.h>
      3 #include <ctype.h>
      4 #include <string.h>
      5
      6 void swap(char* str, int i, int j) {
      7    char t = str[i];
      8    str[i] = str[j];
      9    str[j] = t;
     10 }
     11
     12 void reverse_string(char* str, int length) {
     13    int i;
     14    for (i = 0; i < length / 2; i++)
     15       swap(str, i, length - i  - 1);
     16 }
     17
     18 int main(int argc, char *argv[]) {
     19    char str[1024];
     20    char chars[1024];
     21    int l;
     22    int p;
     23    int i;
     24     if (argc != 2) {
     25        printf("Invalid argument");
     26        return 1;
     27     }
     28     if (atoi(argv[1]) <= 0) {
     29        printf("Invalid argument");
     30        return 2;
     31     }
     32     str[0] = '\0';
     33     while (fgets(chars, 1024, stdin))
     34        strcat(str, chars);
     35     l = strlen(str);
     36     reverse_string(str, l);
     37     p = 0;
     38     for(i = 0; i < l; i++) {
     39        if(str[i] == ' ') {
     40           reverse_string(&str[p], i - p);
     41           p = i + 1;
     42        }
     43     }
     44     reverse_string(&str[p], l - p);
     45     printf("%s\n", str);
     46     return 0;
     47 }

The above code should reverse all the words in the string. How can I modify the code so it'll be for the first n words only?

Thanks in advance

Walk through the string until you find a space character then increment a variable counter until it equals n, then you can reverse those words. just an idea im sure there are many better ways to go about doing this.

How?

Originally Posted by ydan87

How?

I would use a 2 dimension char array[50][50], and set it to all '\0' (NULL char's).

Now put each word in the line of text, into it's own row of the array[0], is the first word, (a pointer to it), and array[1], is the address of the second word, etc.

Get all the words, so you can reconstruct the full line of text. Send the first N words to the reverse string function to have them reversed.

You have some good code there, unfortunately, you didn't write it, and now it has to be changed a bit. This is probably exactly what your teacher wanted to happen.

In programming, the details are important to the overall design of the program, and the code you have, is not exactly right.

The above is the simplest way I know of, to do this.

If you understand how and why the above program accomplishes reversing the string word-by-word and not just the reverse of the whole string, then you'll see how simple it is to reverse only the first n words of the string. It basically comes down to figuring out where the end of the nth word is in the string.

Recursion its another option:

Code:

void rev(char *o,char *s,int i)
{
  char t[100];
  int n;
  if( i-- && 1==sscanf(s,"%s%n",t,&n) )
  {
    rev(o,s+n,i);
    if(*o)
      strcat(o," ");
    if(o==s)
      memcpy(o+strlen(o),t,strlen(t));
    else
      strcat(o,t);
  }
  else
    *o=0;
}


int main()
{
  char s[]="three two one four five";
  rev(s,s,3);  /* reverve first 3 words */
  puts(s);
  return 0;
}