How can I make this program find standard deviation?

[jsp418]

Hi. I am required to write a program that calculates the mean and standard deviation. So far, I wrote the program so that it finds the mean. Now how can I make it add standard deviation? I've only learned up to loops, so please do not go too advanced.

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


void
msg1(void)
{
     printf("\n\n\n\n\n\n\n\n\n\n\n\n\n\n\… );
     printf("\n EE 150 \n" );
     printf("\n This program calculates the mean of a given");
     printf("\n data set. \n\n\n\n\n\n\n\n" );

}

int
main(void)
{


double sum = 0, data, mean;
int n = 0;
char unit[20], ans='Y';
 
msg1();

printf("\n\nPlease enter the unit of the data set: >");
scanf("%s", &unit);

while (ans=='y'||ans=='Y')
{
      printf("\n\nEnter the data point (in %s): >", unit);
      scanf("%lf", &data);

      n = ++n;
      sum += data;

      printf("\n\nDo you want to add another data point? Y for yes. N for no.\n");
      scanf("%c", &ans);
      scanf("%c", &ans);

}

if (ans=='n'||ans=='N')
{
      mean = sum / n;
      printf("\n\nThe mean of the data set in %s is %g.", unit,mean);
}

return 0;
}

I've tried putting something like:
calculation += (data - mean)(data-mean)
stdev = sqrt (calculation/ (n-1))
but the answer always comes out wrong.

Thanks in advance!

[anduril462]

A few things:

1. C needs math to be explicit. It can't infer multiplication, you must use a * explicitly: calculation += (data - mean) * (data - mean). I don't know if that's a typo/omission or you actually tried coding it that way.
2. You won't know the mean until you've read in all the data points, summed them and divided by n.
3. You can't determine 'calculation' (I think the technical term for that is variance) without the mean. This implies you must process the data twice (once to calculate the mean, once to calculate the variance). You will need at least 2 loops.
4. The above implies you must store all the numbers the user enters. I suggest an array.

[AndiPersti]

I get one error and several warnings when I run your code:

printf("\n\n\n\n\n\n\n\n\n\n\n\n\n\n\… );

You are missing the " at the end of the string literal.

char unit[20], ans='Y';
... 
scanf("%s", &unit);

scanf() expects a pointer as its second argument. "unit" is already a pointer to the first element of the array, thus no need for the &-operator.
Furthermore I suggest using

scanf("%19s", unit);

to avoid buffer overflows (unit is capable of storing 20 characters, %19s reads at most 19 character, thus there is still room for the terminating "\0".)

n = ++n;

++n increments n "in place". No need to assign n to itself.

scanf("%c", &ans);
scanf("%c", &ans);

If you want to ignore any whitespace (including the newline-character) before reading the character IMHO it's simpler and more understandable to use

scanf(" %c", &ans);

(notice the space before %c!).

Bye, Andreas

[jsp418]

Hi! Thank you very much for the replies! I have not learned arrays yet, but I found out how to calculate stdev without arrays using a different stdev formula. I would appreciate if you could test it out and make sure everything is okay before I turn it in.

Disregard the ugly skeleton lol.

/***************************************************************************//*                                                                         */
/*                       LEEWARD COMMUNITY COLLEGE                         */
/*                        PHYSICS AND ENGINEERING                          */
/*                                                                         */
/*                                                                         */
/*  NAME:   J. Preza                                                    */
/*                                                                         */
/*  DATE:   09/14/12                                                       */
/*                                                                          */
/*  FILE:   PROG2_STDEV_PREZA.C                                            */
/*                                                                            */
/*  FUNCTION:  To compute the standard deviation or sample standard        */
/*             deviation of a data set                                     */
/*                                                                         */
/*  INPUTS:    unit of data, data                                             */
/*                                                                         */
/*  OUTPUTS:   Mean(in proper unit)                                          */
/*                                                                            */
/***************************************************************************/


/***********************  PREPROCESSOR DIRECTIVES  *************************/


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






/**************************  MESSAGE FUNCTION  *****************************/


void
msg1(void)
{
    printf("\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n" );
    printf("\n                      EE 150  \n"                 );
    printf("\n      This program calculates the standard       ");
    printf("\n         deviation of a given data set \n\n\n\n\n\n\n\n" );


}


            /* SOMETIMES YOU WILL PUT OTHER FUNCTIONS HERE */


/***************************  MAIN FUNCTION  *******************************/


int
main(void)
{


/*********************  DECLARATION OF VARIABLES  **************************/


double sum = 0, calculation = 0, data, square, stdev;
int n = 0;
char unit[20], ans='Y';


/*******************THE EXECUTABLE PART OF THE PROGRAM  ********************/


msg1();


    printf("\n\nPlease enter the unit of the data set: >");
    scanf("%s", &unit);
    
    while (ans=='y'||ans=='Y')
    {
          printf("\n\nEnter the data point (in %s): >", unit);
          scanf("%lf", &data);
          
          n = ++n;
          calculation += data*data;
          sum+=data;
          
          printf("\n\nDo you want to add another data point? Y for yes. N for no.\n");
          scanf("%c", &ans);
          scanf("%c", &ans);
          
    }


    if (ans=='n'||ans=='N')
    {
          square = sum * sum;
          stdev = sqrt(((n*calculation)-square)/(n*(n-1)));
          printf("\n\nThe Standard Deviation of the data set in %s is %g.", unit,stdev);
          }
    
 
    
    




return 0;
}


/***************************************************************************/

Thanks!!!!!!!!!!!!!!!

[grumpy]

You can't determine 'calculation' (I think the technical term for that is variance) without the mean. This implies you must process the data twice (once to calculate the mean, once to calculate the variance). You will need at least 2 loops.

That's not true. There are techniques to calculate variance using one pass. Admittedly those processes compute quantities that are related to the mean, but it is not necessary to use one pass to calculate the mean, and another to compute the variance. Those techniques also do not require the values the user inputs to be stored away either.

Mathematics is a wonderful discipline

[qny]

/* ... */
    scanf("%s", &unit);
/* ... */
          n = ++n;
/* ... */

NO!
You've been told before how to correct these two errors.
Read the thread again.