I´m new and need help

[staticalloc]

Hi everybody!

Im an absolute beginner in c coding. I still have problems with dyn allocation and pointers.

could somebody help me with this? It calculates the skalar product of 2 vectors a and b with n dimensions.
can´t get this to work.

                             /* Skalarproduct dyn alloc. 14.2 */
#include <stdio.h>
#include <stdlib.h>
void main()
{
   int n, j, i;          /* n=dimension*/
   float *a, *b, skaprod=0.0;
   printf("Enter dimension of the vector?-->");
   scanf("%d", &n);
   a= (float*)malloc(n*sizeof(float));
   b= (float*)malloc(n*sizeof(float));
   for (i=0; i<n; i++)
   {
       printf("a[%d]=",i);
       scanf("%d",&a[i]);
   }
   for(j=0; j<n; j++)
   {
       printf("b[%d]=",j);
       scanf("%f",&b[j]);
   }
   for(i=0;i<n;i++)
      skaprod=skaprod+(a[i]*b[i]);
   printf("the result is-->%f\n",skaprod);
   free(a);
   free(b);
}

my compiler is lcc-win32 and I think its not easy to handle for beginners.

[jverkoey]

Welcome to the boards!

I believe "%d" needs to be "%f" in the first scanf function for it to read in correctly.

[Dave Evans]

I think jverkoey pointed out what needs to be fixed. Here's a suggestion for future reference:

When you are running a new program and it "almost works". Like, in this case the program runs to completion, but has the minor problem that it gives unexpected results (sometimes known as the wrong answer), put in printf() statements after your inputs.

For example,

   printf("\nYou entered the following values:\n");
   for (i = 0; i < n; i++) {
     printf("a[%d] = %f\n", i, a[i]);
   }
   printf("\n");
   for (i = 0; i < n; i++) {
     printf("b[%d] = %f\n", i, b[i]);
   }
   printf("\n");
   printf("the result is-->%f\n",skaprod);

You could see that wrong values being used for a[], and you would know where to look.


Remember, printf() is your friend.

This is a good-looking effort if you really are a beginner.

Good luck!

Dave

[staticalloc]

thanks for the advice and help.

[itsme86]

void main() is bad and I'm surprised you weren't already scolded for it.

Also, you should check to make sure malloc() was successful (if(a == NULL) { ... }) and don't type cast the return value of malloc() in C.

But not a bad job for a beginner

[staticalloc]

1. void main() is bad? Am I suppose to use int main(){return 0;} ?
2. type cast is no no in C? I thought without type casting, malloc will return a pointer to void by default?

[jverkoey]

About #1, it's kind of a taboo here at the forums to dance around with your arms flailing around screaming at people who use void main.

If you do a search for "void main" you'll find a lot of posts about it, no use re-explaining it here.

[Thantos]

2. type cast is no no in C? I thought without type casting, malloc will return a pointer to void by default?

In C void * is automatically converted to whatever pointer type to are assigning it to. So if you are assigning the void * to a char * it converts it for you. Casting the return of malloc has a nasty problem of hiding the fact that you might have forgotten to #include <stdlib>

[chrismiceli]

Casting the return of malloc has a nasty problem of hiding the fact that you might have forgotten to #include <stdlib>

You mean #include <stdlib.h>

[Hammer]

Some things to read:

void main
casting malloc
descriptive subject lines
getting a number from the user

>>printf("Enter dimension of the vector?-->");
After doing this, use fflush(stdout); to force the message to the screen. If you don't, the message might not be displayed, leaving the user wondering what the program is waiting for.

And lastly, welcome.