storing input into malloc array (1d and 2d) and printing

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?

"it does not let me enter any numbers" So you do not get a chance to enter in n between printf("Enter number of entries: "); and printf("Enter %d reals: ", n);? — chux
– chux, Commented Jan 22, 2015 at 16:01

engineerC · Accepted Answer · 2012-10-23 04:12:42Z

3

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;
}

answered Oct 23, 2012 at 4:12

engineerC

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2 Comments

orpqK Over a year ago

hey CaptainMurphy, thanks very much. I just have a follow up to print id's: how do i put out the numbers like this: say i enter: 1.5 , it will display like: 1.50e+00

engineerC Over a year ago

%e or %g will work. Take a look at the printf format specifiers: cplusplus.com/reference/clibrary/cstdio/printf

halfelf · Accepted Answer · 2012-10-23 04:18:21Z

1

In the read_vector(int n) function's for loop:

for (i=0; i<n; i++) 
   vec[i] = n;       // this should be scanf("%lf",vec+i) to read input from stdin

and notice your { and } there. If there's only one line in the loop, { and } is not necessary, OR you have to use a pair of them. The return clause must be out of the loop.

Btw, add return 0 at the end of your main function.

edited Oct 23, 2012 at 4:18

answered Oct 23, 2012 at 3:58

halfelf

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2 Comments

orpqK Over a year ago

I just edited the changes you said in, but it gave an segmentation fault.

orpqK Over a year ago

the read_vector function is now : double *read_vector(int n){ double *vec = malloc(n * sizeof(double)); int i; for (i = 0; i < n; i++) scanf("%lf",vec[i]) return vec; }

Nano · Accepted Answer · 2015-01-22 14:57:22Z

0

simple..you forgot to write scanf..!

double *read_vector(int n)
{

    double *vec = malloc(n * sizeof(double));

    int i;
    for (i = 0; i < n; i++)
       scanf("%d",&vec[i]);

    return vec;
}

edited Jan 22, 2015 at 14:57

Nano

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answered Jan 22, 2015 at 14:19

Ankit Kathuria

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