The poissonian probability for N events with average A.

[Statistics] - Poisson.c

// Coded by ScratchyCode
// Compile in gcc with option -lm
#include <stdio.h>
#include <stdlib.h>
#include <math.h>

double factorial(long long int N);

int main(void){
    long long int N;
    double A, P;
    
    do{
        printf("\nEnter the number of events N: ");
        scanf("%lld",&N);
    }while(N < 0);
    if(N >= 20){
        printf("\nNumber too large!\n");
        exit(1);
    }
    
    do{
        printf("\nEnter the average A for the N events: ");
        scanf("%lf",&A);
    }while(A<0);
    
    P = (double)(pow(A,N) * exp(-A)) / (double)factorial(N);
    
    printf("\nThe poissonian probability for %lld events with average %lf is: %.20lf\n\n",N,A,P);
    
	return 0;
}

double factorial(long long int N){
    double i, F=1;
    
    if(N==0){
    	return 1;
    }
    
    for(i=1; i<=N; i++){
    	F = F * i;
    }
    
    return F;
}