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Date: Wed, 12 Jun 2024 17:53:35 -0400
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Subject: Re: "undefined behavior"?
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On 6/12/2024 5:30 PM, Barry Schwarz wrote:
> On Wed, 12 Jun 2024 16:47:23 -0400, DFS <nospam@dfs.com> wrote:
> 
>> Wrote a C program to mimic the stats shown on:
>>
>> https://www.calculatorsoup.com/calculators/statistics/descriptivestatistics.php
>>
>> My code compiles and works fine - every stat matches - except for one
>> anomaly: when using a dataset of consecutive numbers 1 to N, all values
>>> 40 are flagged as outliers.  Up to 40, no problem.  Random numbers
>> dataset of any size: no problem.
>>
>> And values 41+ definitely don't meet the conditions for outliers (using
>> the IQR * 1.5 rule).
>>
>> Very strange.
>>
>> Edit: I just noticed I didn't initialize a char:
>> before: char outliers[100];
>> after : char outliers[100] = "";
>>
>> And the problem went away.  Reset it to before and problem came back.
>>
>> Makes no sense.  What could cause the program to go FUBAR at data point
>> 41+ only when the dataset is consecutive numbers?
>>
>> Also, why doesn't gcc just do you a solid and initialize to "" for you?
> 
> Makes perfect sense.  The first rule of undefined behavior is
> "Whatever happens is exactly correct."  You are not entitled to any
> expectations and none of the behavior (or perhaps all of the behavior)
> can be called unexpected.

I HATE bogus answers like this.

Aren't you embarrassed to say things like that?



> Since we cannot see your code, I will guess that you use a non-zero
> value in outliers[i] to indicate that the corresponding value has been
> identified as an outlier.


No.

I compare the data point to the lower and upper bounds of a stat rule 
commonly called the "IQR Rule":

lo = Q1 - (1.5 * IQR)
hi = Q3 + (1.5 * IQR)

If it falls outside the range of lo-hi I strcat the value to a char.

The outlier routine starts line 170.

If you change

char outliers[200]="", temp[10]="";
to
char outliers[200], temp[10];

you might see what happens when you run the program for consecutive values:

$ ./prog 100 -c


=========================================================================

//this code is hereby released to the public domain

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

/*
  this program computes the descriptive statistics of a randomly 
generated set of N integers

  1.0 release Dec 2020
  2.0 release Jun 2024

  used the population skewness and Kurtosis formulas from:
 
https://www.calculatorsoup.com/calculators/statistics/descriptivestatistics.php
  also test the results of this code against that site

  compile: gcc -Wall prog.c -o prog -lm
  usage  : ./prog N -option (where N is 2 or higher, and option is -r or 
-c or -o)
            -r generates N random numbers
		   -c generates consecutive numbers 1 to N
		   -o generates random numbers with outliers
*/


//random ints
int randNbr(int low, int high) {
	return (low + rand() / (RAND_MAX / (high - low + 1) + 1));
}

//comparator function used with qsort
int compareint (const void * a, const void * b)
{
   if (*(int*)a > *(int*)b) return 1;
   else if (*(int*)a < *(int*)b) return -1;
   else return 0;
}


int main(int argc, char *argv[])
{
	if(argc < 3) {
		printf("Missing argument:\n");
		printf(" * enter a number greater than 2\n");
		printf(" * enter an option -r -c or -o\n");
		exit(0);
	}	
	
	
	//vars
	int i=0, lastmode=0;
	int N = atoi(argv[1]);
	int nums[N];
	//int *nums = malloc(N * sizeof(int));
	
	double sumN=0.0, median=0.0, Q1=0.0, Q2=0.0, Q3=0.0, IQR=0.0;
	double stddev = 0.0, kurtosis = 0.0;
	double sqrdiffmean = 0.0, cubediffmean = 0.0, quaddiffmean = 0.0;
	double meanabsdev = 0.0, rootmeansqr = 0.0;
	char mode[100], tmp[12];
	
	//generate random dataset
	if(strcmp(argv[2],"-r") == 0) {
		srand(time(NULL));
		for(i=0;i<N;i++) { nums[i] = randNbr(1,N*3); }
		
		printf("%d Randoms:\n", N);
		printf("No commas  : ");   for(i=0;i<N;i++) { printf("%d ", nums[i]); }
		printf("\nWith commas: "); for(i=0;i<N;i++) { printf("%d,", nums[i]); }
		qsort(nums,N,sizeof(int),compareint);
		printf("\nSorted     : "); for(i=0;i<N;i++) { printf("%d ", nums[i]); }
		printf("\nSorted     : "); for(i=0;i<N;i++) { printf("%d,", nums[i]); }
	}	
	
	//generate random dataset with outliers
	if(strcmp(argv[2],"-o") == 0) {
		srand(time(NULL));
		nums[0] = 1; nums[1] = 3;
		for(i=2;i<N-2;i++) { nums[i] = randNbr(100,N*30); }	
		nums[N-2] = 1000; nums[N-1] = 2000;
		
		printf("%d Randoms with outliers:\n", N);
		printf("No commas  : ");   for(i=0;i<N;i++) { printf("%d ", nums[i]); }
		printf("\nWith commas: "); for(i=0;i<N;i++) { printf("%d,", nums[i]); }
		qsort(nums,N,sizeof(int),compareint);
		printf("\nSorted     : "); for(i=0;i<N;i++) { printf("%d ", nums[i]); }
		printf("\nSorted     : "); for(i=0;i<N;i++) { printf("%d,", nums[i]); }
	}	
	
	
	//generate consecutive numbers 1 to N
	if(strcmp(argv[2],"-c") == 0) {
		for(i=0;i<N;i++) { nums[i] = i + 1; }
		
		printf("%d Consecutive:\n", N);
		printf("No commas     : ");   for(i=0;i<N;i++) { printf("%d ", nums[i]); }
		printf("\nWith commas   : "); for(i=0;i<N;i++) { printf("%d,", nums[i]); }
	}
	
	//various
	for(i=0;i<N;i++) {sumN += nums[i];}
	double min = nums[0], max = nums[N-1];

	
	//calc descriptive stats
	double mean = sumN / (double)N;
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