understanding if conditions

rsiedl has asked for the wisdom of the Perl Monks concerning the following question:

Hi,

I've got a simple question as I am still just learning.
I am trying to understand the following block of code...

my $n = 4;
for my $i (1..2**$n-1) {
    for my $j (0..$n-1) {
        if ($i & 2**$j) {
            print $j, "\n";
        } # end-if
    } # end-for
} # end-for
[download]

The area I am having trouble understanding is the

if ($i & 2**$j) {
} # end-if
[download]

Is this saying

if ($i == 2**$j) {
} # end-if
[download]

Any help would be appreciated.

Cheers,
Reagen

Update: Title =~ s/for loops/if conditions/

Comment on understanding if conditions Select or Download Code

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Re: understanding for loops by BrowserUk (Patriarch) on Nov 21, 2004 at 11:10 UTC
'&' is bit-wise boolean and. The expression `($i & 2$j)`, is testing the binary value of `$i` and returns true if the `$j`th bit is set to one. This may be clarified by this slightly modified version of your example code: [11:04:18.32] P:\test>perl my $n = 4; for my $i (1..2$n-1) { printf "%2d (0b%04b) has bits [ ", $i, $i; for my $j (0..$n-1) { if ($i & 2**$j) { print $j, " "; } # end-if } # end-for print "] set\n"; } # end-for ^Z 1 (0b0001) has bits [ 0 ] set 2 (0b0010) has bits [ 1 ] set 3 (0b0011) has bits [ 0 1 ] set 4 (0b0100) has bits [ 2 ] set 5 (0b0101) has bits [ 0 2 ] set 6 (0b0110) has bits [ 1 2 ] set 7 (0b0111) has bits [ 0 1 2 ] set 8 (0b1000) has bits [ 3 ] set 9 (0b1001) has bits [ 0 3 ] set 10 (0b1010) has bits [ 1 3 ] set 11 (0b1011) has bits [ 0 1 3 ] set 12 (0b1100) has bits [ 2 3 ] set 13 (0b1101) has bits [ 0 2 3 ] set 14 (0b1110) has bits [ 1 2 3 ] set 15 (0b1111) has bits [ 0 1 2 3 ] set [download] The first column is the value of $i in decimal. The second is the value of $i as binary. The numbers in []s are the positions (numbered right to left, from zero) of the bits that are set in the current value of $i. Examine what is said, not who speaks. "But you should never overestimate the ingenuity of the sceptics to come up with a counter-argument." -Myles Allen "Think for yourself!" - Abigail "Time is a poor substitute for thought"--theorbtwo "Efficiency is intelligent laziness." -David Dunham "Memory, processor, disk in that order on the hardware side. Algorithm, algorithm, algorithm on the code side." - tachyon	[reply] [d/l] [select]
Re^2: understanding for loops by rsiedl (Friar) on Nov 21, 2004 at 11:14 UTC
Thanks BrowserUk!	[reply]
[DONE ;-)] by Luca Benini (Scribe) on Nov 21, 2004 at 11:33 UTC
& is the bit bit operator == is the logical operator for compare The first if check if the bit of position $j was setted in $i. The second checsk if $i is 2**$j	[reply]
Re: understanding if conditions by pg (Canon) on Nov 21, 2004 at 18:20 UTC
A slight change to the code reveals its purpose: `my $n = 4; for my $i (1..2$n-1) { my $str = ""; for my $j (0..$n-1) { $str = (($i & 2$j) ? '1' : '0') .$str; } print "$i = $str\n" }` [download] This prints: `1 = 0001 2 = 0010 3 = 0011 4 = 0100 5 = 0101 6 = 0110 7 = 0111 8 = 1000 9 = 1001 10 = 1010 11 = 1011 12 = 1100 13 = 1101 14 = 1110 15 = 1111` [download]	[reply] [d/l] [select]
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