How do I compare string pointers? - c

Question

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void sort(char *array[1000][3], int size){
char *temp;
temp = malloc(30);
int j, i;
for(i = 0; i < size-1; i ++){
for(j = j+1; j < size; j ++){
if(strcmp(array[i][0],array[j][0]) > 0){
strcpy(temp, array[i][0]);
strcpy(array[i][0], array[j][0]);
strcpy(array[j][0], temp);
}
}
}
}
int main(){
FILE * myfile;
myfile = fopen("/public/lab4/hurricanes.csv", "r");
char line[100];
char *token;
char *array[1000][3];
int counter = 0;
if(myfile == NULL){
perror("Could not open file");
return 1;
}
while(fgets(line, 100,myfile) != NULL){
token = (char*) malloc((strlen(line)+1) * sizeof(char));
strcpy(token,line);
token = strtok(token, ",");
for(int i = 0; token != NULL; i ++){
array[counter][i] = token;
token = strtok(NULL, ",");
}
counter ++;
}
printf("%s", array[0][0]);
sort(array, counter);
printf("%s", array[0][0]);
return 0;
}
The file gives info on hurricanes with each line looking similar to this
Easy,Category 4 hurricane,5-Sep,1950
Having trouble being able to compare some of the string pointers to sort them alphabetically. Not sure if I need to use malloc to allocate some memory or what I need to do. Right now the array is staying the exact same.

Answer 1

Well, your program has only a few drawbacks, all of them commented in the corrected version in comments. I have edited on top of your source code and tried to conserver as much of your original code as possible, so it is coded as you did in the first place. Please, read the code and don't hesitate to make any comment you want:
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
/* better use constants, so in case you have to change them, you
* only change them here. */
#define MAX 1000
#define NFIELDS 3
/* this is for pretty formatting error messages. */
#define F(_fmt) __FILE__":%d:%s: "_fmt, __LINE__, __func__
/* if you pass the field number used to sort, then you
* have a more flexible way to sort (any field can be used)
*/
void sort(char *array[MAX][NFIELDS], int size, int field)
{
/* space for NFIELDS pointers, no need to malloc it */
char *temp[NFIELDS];
int j, i;
for(i = 0; i < size - 1; i++) {
/* ---v--- oops!!! (there was a j here) :) */
for(j = i+1; j < size; j++) {
if(strcmp(array[i][field], array[j][field]) > 0){
/* exchange the arrays of NFIELD pointers */
memcpy(temp, array[i], sizeof temp);
memcpy(array[i], array[j], sizeof array[i]);
memcpy(array[j], temp, sizeof array[j]);
}
}
}
}
void print_array(char *array[MAX][NFIELDS], int size)
{
int i;
for (i = 0; i < size; i++) {
int j;
printf("#%d:", i);
for (j = 0; j < NFIELDS; j++) {
printf(" %s", array[i][j]);
}
printf("\n");
}
}
int main()
{
char *filename = "/public/lab4/hurricanes.csv";
/* better do the declaration and the initialization at the same time */
FILE *myfile = fopen(filename, "r");
char line[100];
char *array[MAX][NFIELDS];
int counter = 0;
if(myfile == NULL){
fprintf(stderr,
F("FOPEN: %s: %s(errno = %d)\n"),
filename, strerror(errno), errno);
return 1;
}
/* don't hesitate to use the sizeof operator below, it will save you a
* lot of nightmares */
while(fgets(line, sizeof line, myfile) != NULL) {
/* don't do anything if we don't have enough space */
if (counter >= MAX) {
fprintf(stderr, F("MAX NUMBER OF LINES EXHAUSTED (%d)\n)"), MAX);
exit(1);
}
/* NEVER, NEVER, NEVER... cast the result of malloc. See text
* below */
/* by the way, why don't use strdup(3) instead? (it does the
* allocation and the copy in one shot) */
char *token = strdup(line);
if (!token) {
fprintf(stderr, F("Not enough memory: %s (errno = %d)\n"),
strerror(errno), errno);
exit(1);
}
/* don't use strtok, because it considers a sequence of commas as
* a single delimiter, use strsep(3), that will consider ",," as
* three empty strings. By the way, be careful because in your
* sample data you have included commas in the dates. */
int i;
char *s;
/* Add also \n to the separator string, so you don't get the
* last \n included in the last field.
/* Here: ---vv--- */
for (i = 0;
(i < NFIELDS) && (s = strsep(&token, ",\n")) != NULL;
i++)
{
array[counter][i] = s;
}
counter++;
}
print_array(array, counter);
sort(array, counter, 2);
printf("\n");
print_array(array, counter);
return 0;
}
Note: Never cast the result of malloc(3) this is a legacy from ancient times, when there was no void type to allow for automatic pointer conversion. Casting malloc makes the compiler to silently comply if you forget to #include <stdlib.h> and this can make an error if pointer types are not the same size as integer types (you get an undefined behaviour on the compiler assuming by mistake that malloc() returns an int, but as you have stated so in the source, there will be no message from the compiler) Casting malloc(3) is a very bad habit, and makes you more difficult to search for errors.