Re: reliable get position of leftmost bit in large integers

Should your code not be:

perl -e 'for my $i (1..63) {if(int(log(2**($i-1))/log(2)) != $i-1) {pr
+int "$i\n"}}'
[download]

which gives no output for me.

UPDATE: Apologies, I think I get it now, the leading one of 2^i - 1 should be in the same place as the leading one for 2^(i-1) which is what you are testing.

My next thought was to use length(sprintf("%b",$x)) but that does not work either:

for my $i (1..63) {
    if(length(sprintf("%b",2**$i-1)) != length(sprintf("%b",2**($i-1))
+)) {
    print"$i\n";
    printf("%b\n",2**($i-1));
    printf("%b\n",(2**$i)-1);
    }
}
[download]

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Re^2: reliable get position of leftmost bit in large integers by wollmers (Scribe) on Mar 12, 2015 at 11:09 UTC
No, because the problem triggers on vectors filled with on-bits. But the exponentiation also seems to have a problem (thanks your remark): `$ perl -e 'print sprintf("%b",253-1),"\n"' 11111111111111111111111111111111111111111111111111111 $ perl -e 'print sprintf("%b",254-1),"\n"' 1000000000000000000000000000000000000000000000000000000 $ perl -e 'print sprintf("%b",2(53-1)),"\n"' 10000000000000000000000000000000000000000000000000000` [download]	[reply] [d/l]
Re^3: reliable get position of leftmost bit in large integers by Eily (Monsignor) on Mar 12, 2015 at 13:09 UTC
The problem is that ** returns a double (floating point), which explains the lack of precision. You should use 1<<54 like BrowserUk did in his answer.	[reply]
Re^4: reliable get position of leftmost bit in large integers by BrowserUk (Patriarch) on Mar 12, 2015 at 14:35 UTC
You should use 1<<54 like BrowserUk did in his answer. Even old dogs can learn :) With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday' Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". I'm with torvalds on this In the absence of evidence, opinion is indistinguishable from prejudice. Agile (and TDD) debunked	[reply]