Find N letter words in file - c

Question

I have a problem, my program in C have to find words with N letters, count them and sort them in lexicographical order and save them to an another file. I've done the first 2 things, but sorting them and save to the file doesn't work. It saves only the last word to the second file... do you have any idea why?
this is my code:
#include <stdio.h>
#include <conio.h>
#include <ctype.h>
#include <stddef.h>
#include <string.h>
int main()
{
FILE *r, *fp;
char ch[100];
int n,i,j,x=0;
r=fopen("text.txt","r");
fgets(ch, 100, r);
char *s = ch;
char *p = s;
printf("Give the length of word: ");
scanf("%d",&n);
printf("\n\nWords with %d letters: \n\n",n);
while (*p) {
char *start;
int len;
while (*p && isspace(*p))
++p;
start = p;
while (*p && !isspace(*p))
++p;
len = p - start;
fp=fopen("text2.txt","w");
if (len == n) {
printf("%.*s\n", len, start);
x++;
fprintf(fp,"%.*s",len, start);
}
}
printf("\nNumber of words: %d ",x);
fclose(fp);
getch();
fclose(r);
}
my input file:
hi my name is Zsolt this program if for displaying words with N letters count them and sort them alphabeticaly a save them to an another file

Answer 1

It is because you open text2.txt in every iteration of your while loop. And what is more, you open it with mode "w" which if you look at the documentation states:
write: Create an empty file for output operations. If a file with the same name already exists, its contents are discarded and the file is treated as a new empty file.
So what is happening is, at every iteration, you open the file, discarding whatever was there before, (which after the first iteration would be a file with a single word in it).
Instead you should open it before you enter the while loop.
Additionally, you stated that you wanted to sort the words you found in lexicographical order before you wrote them to the new file. If your code had written the words as you had intended, then they would be in the order they appeared in the original file, not lexicographic order. You are better off saving the pointers to the n-length words in an array, sorting that array, and then writing it all in one go to your output file.
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#define MAX_WORDS 100
int
qstrcmp(const void *a, const void *b)
{
const char
*s = *(const char **)a,
*t = *(const char **)b;
return strcmp(s, t);
}
int
main()
{
FILE *input, *output;
input = fopen("text.txt", "r");
// Get length to filter by
unsigned long n;
scanf("%lu", &n);
char *words[MAX_WORDS];
int i = 0;
// Find words of correct length
char buf[100];
while (fscanf(input, "%99s", buf) != EOF) {
// Protect from buffer overflow
if (i >= MAX_WORDS) {
printf("Too many words!");
break;
}
if (strlen(buf) == n) {
words[i++] = strncpy(malloc(n+1), buf, n+1);
}
}
fclose(input);
// Sort in lexicographical order.
qsort(words, i, sizeof(char *), qstrcmp);
// Write to output
output = fopen("text2.txt", "w");
for (int j = 0; j < i; ++j) {
fprintf(output, "%s\n", words[j]);
}
fclose(output);
// Print number found.
printf("Found %d word%s of length %lu.\n", i, i == 1 ? "": "s", n);
return 0;
}
Implementation Notes
Sorting is achieved with qsort from "stdlib.h".
Pay attention to buffer overflows! In this case I just bail, but alternatively, you could re-allocate the memory for the words array.
Remember to copy the null-byte over when saving the word.
qsort passes references to the array elements it's sorting to its comparator function, so it will pass values of type const char **, this is why we need to use the qstrcmp wrapper function.