Why is it not printing the array? - c

Question

I want to take the contents from a file and print them out in a random order. Right now I am just trying to take the contents from the file and printing it back out. for example right now the file has:
0 Abdelmeged Zane
1 Attri Sonal
And it prints out the second line not the first.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, char* argv[]){
FILE *file;
char string[50];
char array[100];
int i = 0;
char *p;
file = fopen(argv[1], "r");
if(file == NULL) {
perror("Error opening file");
return(-1);
}
printf("\nOriginal Array: \n");
while(fgets(string, 100, file)!= NULL) {
printf(string);
p = &array[i];
strcpy(p, string);
i++;
}
i = 0;
printf("\nShuffled Array: \n");
while(i < 20){
printf("%c", array[i]);
i++;
}
fclose(file);
return(0);
}

Answer 1

The problem with your code is you declare char array[100]; and then essentially overwrite the string you store in it with strcpy(p, string); with each iteration of your while loop. There is no need to use a pointer p in this case. What you need to do is allocate multiple pointers to hold multiple strings. The normal way is to allocate some number of pointers (10 below -- see MAXLINES), and then allocate space for each string as you use each pointer. The following is a quick example (note there are many different ways to do this). Also, since you are allocating memory, you are responsible for freeing it as well (also shown below). Drop a comment if you have questions.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAXLINES 10
int main(int argc, char* argv[]){
FILE *file;
char string[50];
char **array = NULL;
int i = 0;
if (argc < 2) {
fprintf (stderr, "Error: insufficient input. Usage: %s <filename>\n", argv[0]);
return 1;
}
file = fopen(argv[1], "r");
if(file == NULL) {
perror("Error opening file");
return 1;
}
array = calloc (MAXLINES, sizeof (*array)); /* MAXLINES char pointers */
if (!array) {
fprintf (stderr, "Error: calloc pointer allocation failed\n");
return 1;
}
printf("\nOriginal Array: \n");
while(fgets(string, 100, file)!= NULL) {
printf(" %s", string);
array[i] = strdup (string); /* strdup allocates space for string */
i++;
}
fclose(file); /* close it here, you are done with it */
i = 0; /* print array stored in memory */
printf("\nShuffled Array: \n");
while (array[i])
{
printf(" %s", array[i]);
i++;
}
i = 0;
while (array[i]) /* free memory dynamically allocated: */
{
free (array[i]); /* free array/string memory */
i++;
}
if (array) free (array); /* free pointer allocation */
return 0;
}
Note: you should also save the number of strings read after your loop with fgets (e.g. int numlines = i;). Do this before you reset i = 0;. That way you have a convenient reference to the number of strings to use in your shuffle. (you can also always use the while (array[i]) loop to count them again, but why?
input file:
$ cat dat/abde.txt
0 Abdelmeged Zane
1 Attri Sonal
output:
$ ./bin/shufs dat/abde.txt
Original Array:
0 Abdelmeged Zane
1 Attri Sonal
Shuffled Array:
0 Abdelmeged Zane
1 Attri Sonal

Answer 2

You are copying the string you read into the second char array, offsetting the start point by one each time. So at the end, assuming you don't have a buffer overflow, then your array will hold the first character of each line up to the last, which it will hold in its entirety.
Depending where you are in your study, either create a static array of arrays of chars to hold each line, or an array of pointers and malloc the copy you save.