Read numbers from file into a dynamically allocated array

user2831017 picture user2831017 · Sep 30, 2013 · Viewed 12.8k times · Source

I need a function that reads grades (integers) from from file and returns a dynamically allocated array in which they are stored.

This is what I have tried:

int *readGrades() {
int *grades;
int x;
scanf("%d", &x);
grades = malloc(x * sizeof(int));
return 0;
}

However I don't get anything when I run the code. The grades are stored in file called 1.in:

29
6 3 8 6 7 4 8 9 2 10 4 9 5 7 4 8 6 7 2 10 4 1 8 3 6 3 6 9 4

and I run my program using: ./a.out < 1.in

Can anyone tell me what I did wrong?

Answer

LihO picture LihO · Sep 30, 2013

Problem: The following code:

int *readGrades() {
    int *grades;
    int x;
    scanf("%d", &x);
    grades = malloc(x * sizeof(int));
    return 0;
}

reads 1 int from the standard input, THEN it allocates an array of ints and it returns 0 which zero-initializes caller's pointer when used like this:

int* grades = readGrades();

Solution: Apart from reading the count of grades, the function should read the grades as well. The array should be initialized BEFORE the reading and the actual reading of grades should be done in a loop, which would initialize array's elements. At the end, a pointer to the first element should be returned:

int *readGrades(int count) {
    int *grades = malloc(count * sizeof(int));
    for (i = 0; i < count; ++i) {
        scanf("%d", &grades[i]);
    }
    return grades;                // <-- equivalent to return &grades[0];
}
...
int count;
scanf("%d", &count);              // <-- so that caller knows the count of grades
int *grades = readGrades(count);