Best way to read a parse data from text file in C?

Question

I am working on an assignment that deals with reading data from a text file, and parsing that data to various arrays. For example, a portion of my text file looks as follows:

arbrick  pts/26       141.219.210.189  Thu Mar 29 11:23 - 11:24  (00:00)    
rjmcnama pts/27       141.219.205.107  Thu Mar 29 11:02   still logged in   
ajhoekst pts/26       99.156.215.40    Thu Mar 29 10:59 - 11:08  (00:08)    
eacarter pts/31       141.219.162.145  Thu Mar 29 10:50 - 10:51  (00:00)    
kmcolema pts/31       141.219.214.128  Thu Mar 29 09:44 - 09:47  (00:03) 

I need to parse the data into the following arrays: user id, terminal, ip address, and event times. How can I do this considering that there isn't a consistant amount of white space between the columns?

EDIT: I tried using the suggestion that Thiruvalluvar provided, but I just could not get it to work. However, I did switch to sscanf and that is working quite well almost ...

while(!feof(myfile)) {
        fgets(buffer, 256, myfile);
        sscanf(buffer, "%s %s %s %s", user_id[i], terminal_id[i], ip_addr[i], events[i]);
    } /*End while not EOF*/

What is working, is the user_id, terminal_id, and ip_addr arrays. However, the events array isn't working perfectly as of yet. Since the events array is a string that contains white space, how can I use sscanf to add the remainder of the buffer to the events array?

Answer 1

I think, the real part of the question is how to strore them in only 4 arrays. Eg:

arbrick  pts/26       141.219.210.189  Thu Mar 29 11:23 - 11:24  (00:00)    

Tokenizing this line with whitespace is goin to give many strings. But we are only interested in splitting the entire line into only 4 lines, not more than that.

Solution:

1. Read the line using fgets() .

2. Tokenize it using strtok() or strtok_r() (for thread-safe) with whitespace as delimiter.

3. Read the 1st 3 strings into the arrays: user_id, terminal_id and ip_address

4. Store ( and append) the rest of strings into the array events .

    int i = 0; int line_index = 0; char *p; while(...) //loop to read the file { fgets(line); p = strtok(line, " "); i=0; while(p!=NULL) { if(i==0) strcpy(user_id[line_index], p); if(i==1) strcpy(terminal_id[line_index], p); if(i==2) strcpy(ip_addr[line_index], p); else strcat(events[line_index], p); //anything else goes into array events i++; } line_index++; } //end of file-reading loop. 

Answer 2

Use fgets to read one line at a time. Operate on the line using sscanf calls to store the information, since the data is not in a consistent form (eg, "still logged in"). sscanf will read and discard any whitespace between the format specifiers.

Answer 3

Try this:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char** split (char* string, const char* delim) {
char* p;
int i = 0;
char** array;

array = malloc(strlen(string) * sizeof (char*));
p = strtok (string, delim);
while (p != NULL) {
    array[i] = malloc(sizeof(char) );
    array[i++] = p;
    p = strtok(NULL, delim);
}
return array;
}

void parseLine(char *line, char *user, char term[], char ip[], char event[]) {
char *copy = line;
char **array = split(copy, " ");

strcpy(user, *array++);
strcpy(term, *array++);
strcpy(ip, *array++);
array++;array++;array++;
strcpy(event, *array++);
if (strcmp(*array, "-")) {
    strcat(event, " still logged in");
} else {
    array++;
    strcat(event, " - ");
    strcat(event, *array++);
}
}

int main(void) {

char line[2048];
char user[64], term[64], ip[64], event[64];

while (fgets(line, 2048, stdin) != NULL) {
    parseLine(line, user, term, ip, event);
    printf("[%s][%s][%s][%s]\n", user, term, ip, event);
    /* use an array to save them ... */
}
return 0;
}

and then: ./a.out < file.txt

Answer 4

For what it is worth, here is my suggestion. Roll your own string-tokeniser as follows:

static char *string_tok(char **stringp, const char *delim)
{
    char *tok = *stringp + strspn(*stringp, delim);
    char *end = tok + strcspn(tok, delim);

    if (*end) {
        *end++ = '\0';
        end += strspn(end, delim);
    }
    *stringp = end;
    return tok;
}

Then just call it in sequence for each token. After the third call to string_tok the buffer buf holds a pointer to the start of the remainder of the string (the events). Note that buf must be writeable.

static void parse(char * buf)
{
    char * user_id = string_tok(&buf, " \t");
    char * term    = string_tok(&buf, " \t");
    char * ip      = string_tok(&buf, " \t");
    printf("user_id:  %s\n", user_id);
    printf("terminal: %s\n", term);
    printf("ip addr:  %s\n", ip);
    printf("events:   %s\n\n", buf);
}