How can I skip some block content while reading in Perl - perl

Question

I plan to skip the block content which include the start line of "MaterializeU4()" with the subroutin() read_block below. But failed.
# Read a constant definition block from a file handle.
# void return when there is no data left in the file.
# Otherwise return an array ref containing lines to in the block.
sub read_block {
my $fh = shift;
my #lines;
my $block_started = 0;
while( my $line = <$fh> ) {
# how to correct my code below? I don't need the 2nd block content.
$block_started++ if ( ($line =~ /^(status)/) && (index($line, "MaterializeU4") != 0) ) ;
if( $block_started ) {
last if $line =~ /^\s*$/;
push #lines, $line;
}
}
return \#lines if #lines;
return;
}
Data as below:
__DATA__
status DynTest = <dynamic 100>
vid = 10002
name = "DynTest"
units = ""
status VIDNAME9000 = <U4 MaterializeU4()>
vid = 9000
name = "VIDNAME9000"
units = "degC"
status DynTest = <U1 100>
vid = 100
name = "Hello"
units = ""
Output:
<StatusVariables>
<SVID logicalName="DynTest" type="L" value="100" vid="10002" name="DynTest" units=""></SVID>
<SVID logicalName="DynTest" type="L" value="100" vid="100" name="Hello" units=""></SVID>
</StatusVariables>
[Updated]
I print the value of index($line, "MaterializeU4"), it output 25.
Then I updated the code as below
$block_started++ if ( ($line =~ /^(status)/) && (index($line, "MaterializeU4") != 25)
Now it works.
Any comments are welcome about my practice.

Answer 1

Perl already has an operator to keep track of blocks. It's called the "flip-flop" operator:
Try this out:
while ( <DATA> ) {
next if /\Q<U4 MaterializeU4()>\E/../^\s*$/;
push #lines, $_;
}
The value of /\Q<U4 MaterializeU4()>\E/../^\s*$/ will be true when it sees a line that matches the starting regex and it will stop being true after it sees a line matching the second expression.

Answer 2

First, using a regex instead of index is probably better since you can tune it to the exact format of status string if you may decide to be stricter than just "substring exists"
I would suggest as one solution adding a second flag to skip the block contents if it's a MaterializeU4 block, as follows:
# Read a constant definition block from a file handle.
# void return when there is no data left in the file.
# Empty return for skippable (Materialize4U) block!!!
# Otherwise return an array ref containing lines to in the block.
sub read_block {
my $fh = shift;
my #lines = ();
my $block_started = 0;
my $block_ignore = 0;
while (my $line = <$fh> ) {
if ($line =~ /^status.*?((MaterializeU4)?)/) {
$block_started = 1;
$block_ignore = 1 if $1;
}
last if $line =~ /^\s*$/ && $block_started;
push #lines, $line unless $block_ignore;
}
return \#lines if #lines || $block_started;
return;
}
Here's a slightly modified sample I tested using codepad.org:
Code:
use Data::Dumper;
my #all_lines = (
"s 1" ,"b 1" ,""
, "s MaterializeU4" ,"b 2" ,""
, "s 3" ,"b 3" ,""
);
while (#all_lines) {
my $block = read_block();
print Data::Dumper->Dump([$block]);
}
exit 0;
sub read_block {
my #lines = ();
my $block_started = 0;
my $block_ignore = 0;
while (my $line = shift #all_lines) {
if ($line =~ /^s .*?((MaterializeU4)?)/) {
$block_started = 1;
$block_ignore = 1 if $1;
}
last if $line =~ /^\s*$/ && $block_started;
push #lines, $line unless $block_ignore;
}
return \#lines if #lines || $block_started;
return;
}
Output:
$VAR1 = [
's 1',
'b 1'
];
$VAR1 = [];
$VAR1 = [
's 3',
'b 3'
];

Answer 3

On successful match of a substring, index returns the position of the substring, which could be any value >= 0. On "failure", index returns -1.
The way you are using index
index($line, "MaterializeU4") != 0
will be true for all lines except for a line that begins with the string "MaterializeU4".
It looks like you already know a little bit about Perl regular expressions. Why not use one in this case, too?
++$block_started if $line =~ /status/ && $line =~ /MaterializeU4/;
Another issue I see is that you set $block_started to begin capturing lines, but you never set it to zero at the end of the "block", say, when $line is empty. I'm not sure if that's what you wanted to do.