Random Numbers (mean)

[olig1905]

Hi, this is my first thread on this board. (sure many more will follow)

Basically i have to create a random number generator that generates 1000 random numbers from 0-9 and then display the mean average of these numbers.

I have been told by my lecturer that it should be about 4.5 however i am returned with a value of 4.0

here is my code:

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


main()
{
int randnum = 0 ;
float mean;
int sum = 0;
int x;

 srand(time(0));
 

 for (x = 0; x <= 1000; x++)
   {
     
     randnum =((int)10.0*rand()/(RAND_MAX + 1.0));
     sum += randnum;
     
   }

 mean = sum / 1000;

 printf(" mean: %f\n", mean); 
  
}

[iMalc]

When you perform mathematical operations in a prrogramming language, the types of the constants or variables are important. Dividing two ints produces an int result, whereas dividing two floats gives a float result.
If the types differ then promotion rules apply that turn one of them into the same type as the other. In this case if we can change 1000 into 1000.0f (making it a float) and then we are attempting to divide an int by a float.
What the promption rules then do is that the int is also promoted to a float and it performs float/float division, which will give around 4.5f, instead of int/int which rounds down giving you an int of 4.
Note that this all happens before the result is assigned to the variable on the left hand side of the assignment statement.

[stahta01]

float a;
a = 450/1000;

The answer for a is NOT 4.5; because C uses integer division when no floating point is involved on the right side of the equation. Note: The rule is more complex then this; but you should already know it; you just forgot it.

I think you also have a fence post error in your for loop.

And, main should return an int; and be defined as such.

iMalc, must write faster than me.

Tim S.

[olig1905]

thanks

yes, that is quite obvious!
cant believe i didn't notice that

Also, why should main return an int

and the fence post error was me fiddling about in desperation

[ssharish2005]

>Also, why should main return an int
Read this

ssharish

[CommonTater]

float a;
a = 450/1000;

The answer for a is NOT 4.5; because

You're right... The answer is 0.45 which an integer result will make into a 0 for you.

[olig1905]

Ok you guys have been very helpful!

As im sure you can tell i am new to C and also not that experienced in any other language.

I have written another programme that is supposed to count up the frequency of each random number generated!

I am fairly sure there will be a stupid mistake in this code due to the compile errors i recieve from gcc.

Compile errors:

random2.c: In function ‘arrayswitch’:
random2.c:9: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘{’ token
random2.c:37: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘{’ token
random2.c:5: error: old-style parameter declarations in prototyped function definition
random2.c:5: error: parameter name omitted
random2.c:53: error: expected ‘{’ at end of input

Code:

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

void arrayswitch(int)
int frequency[9];

int main()
{
  int randnum ;
  float mean;
  int x;

  srand(time(0));

  for (x = 1; x <= 10,000; x++)
    {
     
      randnum =( 10.0*rand()/(RAND_MAX + 1.0));
      arrayswitch(randnum);
    }
  printf("Frequency of 0: %d\n", frequency[0]);
  printf("Frequency of 1: %d\n", frequency[1]);
  printf("Frequency of 2: %d\n", frequency[2]);
  printf("Frequency of 3: %d\n", frequency[3]);
  printf("Frequency of 4: %d\n", frequency[4]);
  printf("Frequency of 5: %d\n", frequency[5]);
  printf("Frequency of 6: %d\n", frequency[6]);
  printf("Frequency of 7: %d\n", frequency[7]);
  printf("Frequency of 8: %d\n", frequency[8]);
  printf("Frequency of 9: %d\n", frequency[9]);
  return 0;
  
}

void arrayswitch(int number)
{
   switch(number)
    {
      Case '0' :frequency[0] = frequency[0] + 1; break;
      Case '1' :frequency[1] = frequency[1] + 1; break;
      Case '2' :frequency[2] = frequency[2] + 1; break;
      Case '3' :frequency[3] = frequency[3] + 1; break;
      Case '4' :frequency[4] = frequency[4] + 1; break;
      Case '5' :frequency[5] = frequency[5] + 1; break;
      Case '6' :frequency[6] = frequency[6] + 1; break;
      Case '7' :frequency[7] = frequency[7] + 1; break;
      Case '8' :frequency[8] = frequency[8] + 1; break;
      Case '9' :frequency[9] = frequency[9] + 1; break;
      default  :printf("ERROR"); break;

    }
}

[olig1905]

when that works i will tidy it up to make the printf statements a loop

[Bayint Naung]

Btw, did you really want '0' '1' in your switch-case. or 0 , 1 ?
Anway this big switch can be shortened.
Your 'Case' should be 'case'.

frequency can hold 9 int. 0-9 inclusive is 10 elements...

  for (x = 1; x <= 10,000; x++)           // remove , from 10,000

Edit: need a semicolon after your function prototype.

[CommonTater]

Suggestions in red.

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

void arrayswitch(int);
int frequency[9];

int main()
{
  int randnum ;
  float mean;
  int x;

  srand(time(0)); 

  for (x = 1; x <= 10,000; x++)
    {
     
     // randnum =( 10.0*rand()/(RAND_MAX + 1.0));
    randnum = rand() % 10;

      arrayswitch(randnum);
    }
  printf("Frequency of 0: %d\n", frequency[0]);
  printf("Frequency of 1: %d\n", frequency[1]);
  printf("Frequency of 2: %d\n", frequency[2]);
  printf("Frequency of 3: %d\n", frequency[3]);
  printf("Frequency of 4: %d\n", frequency[4]);
  printf("Frequency of 5: %d\n", frequency[5]);
  printf("Frequency of 6: %d\n", frequency[6]);
  printf("Frequency of 7: %d\n", frequency[7]);
  printf("Frequency of 8: %d\n", frequency[8]);
  printf("Frequency of 9: %d\n", frequency[9]);
  return 0;
  
}

void arrayswitch(int number)
{

     ++frequency[number];

/*
   switch(number)
    {
      Case '0' :frequency[0] = frequency[0] + 1; break;
      Case '1' :frequency[1] = frequency[1] + 1; break;
      Case '2' :frequency[2] = frequency[2] + 1; break;
      Case '3' :frequency[3] = frequency[3] + 1; break;
      Case '4' :frequency[4] = frequency[4] + 1; break;
      Case '5' :frequency[5] = frequency[5] + 1; break;
      Case '6' :frequency[6] = frequency[6] + 1; break;
      Case '7' :frequency[7] = frequency[7] + 1; break;
      Case '8' :frequency[8] = frequency[8] + 1; break;
      Case '9' :frequency[9] = frequency[9] + 1; break;
      default  :printf("ERROR"); break;

    }
*/
}