Arithmetic for loop through arrays of pointers

Question

I have two arrays of pointers. I need to do simple arithmetic with them while going in loop

 float **x_points, **y_points;

x_points = malloc(sizeof(float*) * n);
y_points = malloc(sizeof(float*) * n);

for( i = 0; i < n; i++) {
    printf("x");
    printf("%i",i);
    printf(" : ");
    x_points[i] = malloc( n * sizeof ( float ) );
    scanf("%f",x_points[i]);

    printf("y");
    printf("%i",i);
    printf(" : ");
    y_points[i] = malloc( n * sizeof ( float ) );
    scanf("%f",y_points[i]);
}

x_points[n] = NULL;
y_points[n] = NULL;

And here I have problems:

int k;

for(k=0; k < i; k++) {
    R += *x_points[k] * *y_points[k+1] - *x_points[k+1] * *y_points[k];
}

Couldn't you tell me why does this code shows me a window says that the system got a signal and that's why stopped the program? THank you, I will appreciate it!

This code has multiple problems, not just with that loop (the placement of which remains a mystery because you haven't posted an SSCCE). — WhozCraig
– WhozCraig, Commented Mar 5, 2014 at 12:03

Moah · Accepted Answer · 2014-03-05 11:58:31Z

1

You're allocating n pointers:

x_points = malloc(sizeof(float*) * n);
y_points = malloc(sizeof(float*) * n);

but here you're accessing past of the memory you've allocated:

x_points[n] = NULL;
y_points[n] = NULL;

Since C is 0-based for indices, you can only go from 0 to n-1.

So you either need to allocate n+1, or to put your null at n-1

Edit: also what hmjd said. I didn't catch it.

edited Mar 5, 2014 at 11:58

answered Mar 5, 2014 at 11:56

Moah

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Comments

haccks · Accepted Answer · 2014-03-05 11:57:40Z

0

The reason could be that you are accessing un-allocated memory location.
Change

for(k=0; k < i; k++)

to

for(k=0; k < i - 1; k++)

answered Mar 5, 2014 at 11:57

haccks

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KARTHIK BHAT · Accepted Answer · 2014-03-05 12:01:12Z

0

x_points[n] = NULL;
y_points[n] = NULL;

is also invalid as your assigning the value outside the memory allocated to it.

change the malloc call a bit if wanna still achieve this

malloc( n+1 * sizeof ( float ) )

answered Mar 5, 2014 at 12:01

KARTHIK BHAT

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