I am trying to get the sum of 1 + 2 + ... + 1000000000
, but I'm getting funny results in PHP and Node.js.
PHP
$sum = 0;
for($i = 0; $i <= 1000000000 ; $i++) {
$sum += $i;
}
printf("%s", number_format($sum, 0, "", "")); // 500000000067108992
Node.js
var sum = 0;
for (i = 0; i <= 1000000000; i++) {
sum += i ;
}
console.log(sum); // 500000000067109000
The correct answer can be calculated using
1 + 2 + ... + n = n(n+1)/2
Correct answer = 500000000500000000, so I decided to try another language.
GO
var sum , i int64
for i = 0 ; i <= 1000000000; i++ {
sum += i
}
fmt.Println(sum) // 500000000500000000
But it works fine! So what is wrong with my PHP and Node.js code?
Perhaps this a problem of interpreted languages, and that's why it works in a compiled language like Go? If so, would other interpreted languages such as Python and Perl have the same problem?
Python works:
>>> sum(x for x in xrange(1000000000 + 1))
500000000500000000
Or:
>>> sum(xrange(1000000000+1))
500000000500000000
Python's int
auto promotes to a Python long
which supports arbitrary precision. It will produce the correct answer on 32 or 64 bit platforms.
This can be seen by raising 2 to a power far greater than the bit width of the platform:
>>> 2**99
633825300114114700748351602688L
You can demonstrate (with Python) that the erroneous values you are getting in PHP is because PHP is promoting to a float when the values are greater than 2**32-1:
>>> int(sum(float(x) for x in xrange(1000000000+1)))
500000000067108992