Handling missing data in inputs

Missing data can be very annoying to a programmer. In fact, it is so annoying that very often we'll write separate programs to clean up data and eliminate unpleasant conditions so that the main program doesn't have to deal with it. Here, I'll show some examples of the kind of problems we see.

Let's take a comman data format, a TAB delimited file. A simplistic Perl program to read such a file might be:

#!/usr/bin/perl
while (<>) {
 #split on tab into @x array
 @x=split /\t/;
 #print first three elements
 print "$x[0]\t$x[1]\t$x[2]\n";
}
 

An equivalent shell script might be

IFS="(tab here)"
while read a b c d
do
echo "$a        $b      $c"
done
 

The Perl script works, but the shell script doesn't. Here's the output if the input file looks like this:

$ cat t;hexdump -c t
1       2       3       4
1               3       4
        2       3       4
1       2               4
                3       4
0000000   1  \t   2  \t   3  \t   4  \n   1  \t  \t   3  \t   4  \n  \t
0000010   2  \t   3  \t   4  \n   1  \t   2  \t  \t   4  \n  \t  \t   3
0000020  \t   4  \n                                                    
0000023
 

The Perl script produces

1       2       3
1               3
        2       3
1       2
                3
 

but the shell script messes up:

1       2        3
1       3        4
2       3        4
1       2        4
3       4 
 

If this were a problem with Perl, we'd handle it like this:

#!/usr/bin/perl
while (<>) {
 # make sure there is at least one space between adjacent tabs
 s/\t\t/\t \t/g;
 #split on tab into @x array
 @x=split /\t/;
 #print first three elements
 print "$x[0]\t$x[1]\t$x[2]\n";
}
 

But things can be worse. For example, if we are processing what was once a report format, we may have no delimiters, just empty space. We might see something like this:

Date          Customer             Phone           Terms     Balance

09/04/04      ABCD Corp.                           PPD          0.00
09/04/04      Abba Corp.          555-5555         Net 30     985.00
 

#!/usr/bin/perl
while(<>) {
@x=unpack("A8A6A20A17A9A12",$_);
print "$x[0]:$x[2]:$x[3]:$x[4]:$x[5]\n";
}
 

Which will produce:

Date:Customer: Phone:Terms: Balance
::::
09/04/04:ABCD Corp.::PPD:    0.00
09/04/04:Abba Corp.:555-5555:Net 30:  985.00
 

Comma separated value files can be annoying if they also contain commas within quoted fields. You can't use split because of that. There are at least two ways to handle that: either use the Text::Parsewords module:

#!/usr/bin/perl
use Text::ParseWords;
while(<>) {
 @x=quotewords(",",0,$_);
 foreach (@x) {
  print " $_";
 }
print "\n";
}
 

Or (assuming the data is regular enough), replace commas not inside quotes with a different delimiter and then split it. I think ParseWords is easier.

But sometimes none of that is going to work either. I'm working on a project right now where the input data can have up to three fields, but any of the three can be missing and there are no delimiters and no spacing. The only way to determine what we have is to know that the field one, if present, is alpha, field two is a whole integer, and field three will always have decimal points. So

ABC  982.00
8
15.45
 

means that I have 1 and 3 on line 1, only 2 on line 2, and only 3 on line 3. It's actually much worse than this; there are other fields, some of which are always present and some which are not, and it is quite a challenge to normalize this stuff to be able to massage the data. The way to handle it is to do splits on / /, and then determine what we got. So it's something like this:

#!/usr/bin/perl
while(<>) {
s/\s+/ /g;
@x=split / /;
foreach (@x) {
  .. determine what we have based on previous field(s) seen and content
}
 

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3 comments

Thu Mar 17 20:34:15 2005: 186   anonymous



with this prime number programme .
how do i find the last prime number that did not go into the prime number.i thought it would be something like

printf(" %-8.3F\n", $value++);but it do not work.

cat prime
#!/usr/bin/perl
print "enter a number> ";
$number = <STDIN>;
chomp( $number );
if ( $number !~ /^\d+$/ )
{
print "invalid input\n";
exit 1;
}
$prime = 1;
for( $value = 2; $value < $number; $value++ )
{
if ( $number % $value == 0 )
{
$prime = 0;
break;
}
}
if ( $prime == 1 )
{
print "prime number\n";
}
else
{
print "not a prime number\n";
}
exit 0;






Thu Mar 17 20:35:56 2005: 187   anonymous



with this prime number programme .
how do i find the last prime number that did not go into the prime number.i thought it would be something like

printf(" %-8.3F\n", $value++);but it do not work.

cat prime
#!/usr/bin/perl
print "enter a number> ";
$number = <STDIN>;
chomp( $number );
if ( $number !~ /^\d+$/ )
{
print "invalid input\n";
exit 1;
}
$prime = 1;
for( $value = 2; $value < $number; $value++ )
{
if ( $number % $value == 0 )
{
$prime = 0;
break;
}
}
if ( $prime == 1 )
{
print "prime number\n";
}
else
{
print "not a prime number\n";
}
exit 0;
colin_richard_weaver@hotmail.com
weaverc1@cardiff.ac.uk



Thu Mar 17 22:01:19 2005: 189   TonyLawrence


I don't think you understand the code. This is NOT looping through prime numbers, so there is no "last prime number".

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