Question

I am tryin to take input from the user and store it into an array and print it out: i have 2 functions:

 /* Read a vector with n elements: allocate space, read elements,
    return pointer */
double *read_vector(int n){
    double *vec = malloc(n * sizeof(double));

    int i;
    for (i = 0; i < n; i++)
       vec[i] = n;

    return vec;
}

and the print function is:

void print_vector(int n, double *vec){
    int i;
    for (i = 0; i < n; i++) {
       printf("%d\n", vec[i]);
    }
}

the main function is:

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

double *read_vector(int n);
void print_vector(int n, double *vec);
void free_vector(double *vec);

int main(){
    int n;
    double *vector;
    /* Vector */
    printf("Vector\n");
    printf("Enter number of entries: ");
    scanf("%d", &n);
    printf("Enter %d reals: ", n);
    vector = read_vector(n);
    printf("Your Vector\n");
    print_vector(n,vector);
    free_vector(vector);
}

when i run this, it does not let me enter any numbers, it just skips it and prints out 0’s. How do i fix this?

Answer 1

Try the code below. You’re almost certainly either not compiling with warnings, or ignoring the warnings. All warnings mean something, and to a beginner they all matter. With gcc use the -Wall option, or even -pedantic.

As halfelf pointed out, you need a scanf in your read loop but it needs to be a pointer (&vec[i]). Always return something at the end of main. Also check the return value of malloc, it could fail and return a null pointer.

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

double *read_vector(int n)
{
    double *vec = malloc(n * sizeof(double));
    int i;

    for (i = 0; i < n; i++) {
        printf("Enter number %i of %i: ", i + 1, n);
        scanf("%lf", &vec[i]);
    }
    return vec;
}

void print_vector(int n, double *vec)
{
    int i;

    for (i = 0; i < n; i++) {
        printf("%f\n", vec[i]);
    }
}

void free_vector(double *vec)
{
    free(vec);
}


int main()
{
    int n;
    double *vector;

    printf("Vector\n");
    printf("Enter number of entries: ");
    scanf("%i", &n);
    vector = read_vector(n);
    printf("Your Vector\n");
    print_vector(n, vector);
    free_vector(vector);
    return 0;
}