CS140 Lecture notes -- Some more on Malloc and Fields

A simple malloc() example

In other words, suppose n is 3 and the numbers are 1, 2, and 3. Then the average is 2. The squares of the differences are 1, 0 and 1, so the variance is 2/3.

Suppose n is 3 and the numbers are 10, 5 and 3. The average is 18/3 = 6. The squares of the differences are (10-6)(10-6) = 16, (5-6)(5-6) = 1, and (3-6)(3-6) = 9. Thus, the variance is (16+1+9)/3 = 26/3 = 8.6667.

Ok, now take a few minutes and try to write variance.

Here's the strategy for writing variance

• First you need to get n from the command line arguments.
• First you need to malloc() space for n doubles.
• Next, you read them in using scanf().
• Next, you compute their average.
• Now, you compute the sum of the squares of the differences.
• Finally, you compute the variance and print them both out.

And here's the code (also in variance.c):

#include <stdio.h>

int main(int argc, char **argv)
{
  int n, i;
  double *values;
  double avg;
  double variance;

  
  /*  First you need to get n from the command line arguments. */
  
  if (argc != 2) {
    fprintf(stderr, "usage: variance n\n");
    exit(1);
  }
  n = atoi(argv[1]);
  if (n <= 0) exit(1);

  /*  Next, you need to malloc() space for n doubles.   */

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

  /*  Next, you read them in using scanf(). */

  for (i = 0; i < n; i++) {
    if (scanf("%lf", &(values[i])) != 1) exit(1);
  }

  /*  Next, you compute their average. */

  avg = 0;
  for (i = 0; i < n; i++) {
    avg += values[i];
  }
  avg /= n;

  /*  Now, you compute the sum of the squares of the differences. */

  variance = 0;
  for (i = 0; i < n; i++) {
    variance += ((values[i]-avg)*(values[i]-avg));
  }

  /*  Finally, you compute the variance and print them both out. */

  variance /= n;

  printf("Average:  %lf\n", avg);
  printf("Variance: %lf\n", variance);
  return 0;
}

It works quite nicely:

UNIX> variance 
usage: variance n
UNIX> variance 3
1 2 3
Average:  2.000000
Variance: 0.666667
UNIX> variance 3
1
       2                    3
Average:  2.000000
Variance: 0.666667
UNIX> variance 3
10


5
         3
Average:  6.000000
Variance: 8.666667
UNIX> 

Different input

We'll solve this by using the fields library instead. We'll read in lines using get_line() and if a line begins with '#', we'll ignore it. If it doesn't, then we'll use sscanf() to turn its fields into double's. The code is in variance2.c. Here is the only change from variance.c:

#include "fields.h"

main(int argc, char **argv)
{
  int n, i, j;
  IS is;
   
  ...
  
  /*  Next, you read them in using the fields library. */

  is = new_inputstruct(NULL);
  i = 0;
  while (i < n && get_line(is) >= 0) {
    if (is->text1[0] != '#') {
      for (j = 0; j < is->NF; j++) {
        if (i < n) {
          if (sscanf(is->fields[j], "%lf", &(values[i])) != 1) exit(1);
          i++;
        }
      }
    }
  }
  if (i < n) exit(1);

Note, I've covered for some common input errors such as not entering a double, and not entering enough numbers. Go over this code until you understand exactly what it is doing. I will expect you to be able to write code like this without problems.

When we run it, we see it works:

UNIX> make variance2
gcc -g -I/home/cs140/include -c variance2.c
gcc -g -I/home/cs140/include -o variance2 variance2.o /home/cs140/objs/fields.o
UNIX> variance2 3
1 2 3
Average:  2.000000
Variance: 0.666667
UNIX> variance2 3
1 2 Frog
UNIX> variance2 3
10
# Hi!!!!
5
3
Average:  6.000000
Variance: 8.666667
UNIX>