This should be really simple, but it's driving me nuts. The code below should calculate how many errors you would get per million iterations at a given rate of error. It does the calculation 5 times, just to demonstrate the distribution of errors. 
for ($x=10; $x<1000000; $x=$x*10){
    $error_rate = $x;
    print "Error rate = 1 every $x, errors = ";
    for ($y=0;$y<5;$y++){
        errortest();
    }
    print "\n";
}

sub errortest {
    $errors = 0;
    for ($n=0;$n<1000000;$n++){
        $random = int (rand($error_rate));
        if ($random == 1){$errors++}
    }
    print $errors, " ";
}

Output: 

Error rate = 1 every 10, errors = 99778 100371 99586 99912 99778
Error rate = 1 every 100, errors = 10007 9962 9803 10063 10132
Error rate = 1 every 1000, errors = 1004 1019 1049 1011 945
Error rate = 1 every 10000, errors = 90 84 87 79 83
Error rate = 1 every 100000, errors = 0 0 0 0 0

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My question: why aren't the values correct? You'd expect 100 errors at a 1 in 10000 error rate, not the 84 (average) above. At a 1 in 100000 rate you'd expect 10, not 0. What's going on here - I'm going crazy. This should be so simple ...

In reply to Math all gone wrong... by carlos fandango

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