convert 2d array to pointer? - c

Question

I'm trying to teach myself C, so I wrote a program to keep records for a gradebook! In my effort to learn pointers, I'm trying to convert one of my beginning projects of arrays to pointers. I want to convert my 2d array to pointers. Below is my original program to with the 2d array, and below that is my attempt to convert it to 2d pointer array.
ORIGINAL PROGRAM
int numberPeople, choice, i, j;
char people[15][3][100];
printf("Please indicate number of records you want to enter (min %d, max %d): ", 5, 15);
scanf("%d", &numberPeople);
while ((numberPeople < 5) || (numberPeople > 15)) {
printf("\nNumber not in specified range, try again.\n");
printf("Please indicate number of records you want to enter (min %d, max %d): ", 5, 15);
scanf("%d", &numberPeople);
}
printf("\n");
while ((getchar()) != '\n'); // flush the return (and anything else) after the number input above
printf("Enter the first name, last name, and grade (put a space in between each): \n");
for (i = 0; i < numberPeople; i++) {
char tempArr[MAXIMUM_LINE_LENGTH];
fgets(tempArr, MAXIMUM_LINE_LENGTH, stdin);
char *token = strtok(tempArr, " ");
for (j = 0; j < DATA_FIELDS && token != NULL; j++) {
strncpy(people[i][j], token, MAXIMUM_DATA_LENGTH);
token = strtok(NULL, " \r\n");
}
}
ATTEMPT AT 2D ARRAY -> POINTER
int numberPeople, choice, i, j;
char* people;
printf("Please indicate number of records you want to enter (min %d, max %d): ", 5, 15);
scanf("%d", &numberPeople);
people = (char*)(malloc(numberPeople*DATA_FIELDS*sizeof(char)));
while ((numberPeople < 5) || (numberPeople > 15)) {
printf("\nNumber not in specified range, try again.\n");
printf("Please indicate number of records you want to enter (min %d, max %d): ", 5, 15);
scanf("%d", &numberPeople);
}
printf("\n");
while ((getchar()) != '\n'); // flush the return (and anything else) after the number input above
printf("Enter the first name, last name, and grade (put a space in between each): \n");
for (i = 0; i < numberPeople; i++) {
char* tempArr;
fgets(tempArr, 100, stdin); // Thread 1: EXC_BAD_ACCESS code=1 address=0x0
char *token = strtok(tempArr, " ");
for (j = 0; j < 3 && token != NULL; j++) {
strncpy(people, token, 50);
token = strtok(NULL, " \r\n");
}
}
During the person input step is when it breaks. It works for the first entry, but then it hits a breakpoint (I'm using Xcode), and it reads "EXC_BAD_ACCESS", and I'm not too sure what this means, any tips would help, thanks!

Answer 1

Yes, you can convert to all pointers but I wouldn't recommend it. Here's an example:
char ***people = malloc(sizeof(*people) * 15);
for (int i = 0; i < 15; i++) {
people[i] = malloc(sizeof(*people[i]) * 3);
for (int j = 0; j < 3; j++) {
people[i][j] = malloc(sizeof(*people[i][j]) * 100);
}
}
This would re-create the array char people[15][3][100];. As you can see, it's messy. Plus, you have to free all that mem when you are done.
I cleaner way would be to use a struct:
struct Student {
char firstName[100];
char lastName[100];
char grade[100];
};
struct Student people[15];
This will set aside as much memory as the array. You could also dynamically allocate as many students as needed:
struct Student *people = malloc(sizeof(*people) * 20);
That will get 20 students. You can access them using array notation:
printf("%s %s %s", people[0].firstName, people[0].lastName, people[0].grade);
And you only need to call free once.

Answer 2

this pair of lines is not correct:
char* tempArr;
fgets(tempArr, 100, stdin);
the declaration of the pointer: tempArr is not allocating any actual memory to contain the line read by the the call to fgets() Suggest:
#define MAX_INPUT_LEN (100)
...
char tempArr[MAX_INPUT_LEN];
fgets( tempArr, MAX_INPUT_LEN, stdin );
Also, the returned value from the call to fgets() should be checked (!=NULL) to assure the operation was successful.