Speed up script that determines if all columns in a row are the same or not

Question

I need to speed up a script that essentially determines whether or not all the "columns" for each row are the same, then writes a new file containing either one of the identical elements, or a "no_match". The file is comma delimited, consists of around 15,000 rows, and contains varying numbers of "columns".

For example:

1-69
4-59,4-59,4-59,4-61,4-61,4-61
1-46,1-46
4-59,4-59,4-59,4-61,4-61,4-61
6-1,6-1
5-51,5-51
4-59,4-59

Writes a new file:

1-69
no_match
1-46
no_match
6-1
5-51
4-59

Deleting the second and fourth rows because they contain non-identical columns.

Here is my far from elegant script:

#!/bin/bash

ind=$1 #file in
num=`wc -l "$ind"|cut -d' ' -f1` #number of lines in 'file in'
echo "alleles" > same_alleles.txt #new file to write to

#loop over every line of 'file in'
for (( i =2; i <= "$num"; i++));do
    #take first column of row being looped over (string to check match of other columns with)
    match=`awk "FNR=="$i" {print}" "$ind"|cut -d, -f1`
    #counts how many matches there are in the looped row
    match_num=`awk "FNR=="$i" {print}" "$ind"|grep -o "$match"|wc -l|cut -d' ' -f1`
    #counts number of commas in each looped row
    comma_num=`awk "FNR=="$i" {print}" "$ind"|grep -o ","|wc -l|cut -d' ' -f1`
    #number of columns in each row
    tot_num=$((comma_num + 1))
    #writes one of the identical elements if all contents of row are identical, or writes "no_match" otherwise
    if [ "$tot_num" == "$match_num" ]; then
            echo $match >> same_alleles.txt
    else
            echo "no_match" >> same_alleles.txt
    fi
done

#END

Currently, the script takes around 11 min to do all ~15,000 rows. I'm not really sure how to speed this up (I'm honestly surprised I could even get it to work). Any time knocked off would be fantastic. Below is a smaller excerpt of 100 rows that could be used:

allele
4-39
1-46,1-46,1-46
4-39
4-4,4-4,4-4,4-4
3-23,3-23,3-23
3-21,3-21
4-34,4-34
3-33
4-4,4-4,4-4
4-59,4-59
3-23,3-23,3-23
1-45
1-46,1-46
3-23,3-23,3-23
4-61
1-8
3-7
4-4
4-59,4-59,4-59
1-18,1-18
3-21,3-21
3-23,3-23,3-23
3-23,3-23,3-23
3-30,3-30-3
4-39,4-39
4-61
2-70
4-38-2,4-38-2
1-69,1-69,1-69,1-69,1-69
1-69
4-59,4-59,4-59,4-61,4-61,4-61
1-46,1-46
4-59,4-59,4-59,4-61,4-61,4-61
6-1,6-1
5-51,5-51
4-59,4-59
1-18
3-7
1-69
4-30-4
4-39
1-69
1-69
4-39
3-23,3-23,3-23
4-39
2-5
3-30-3
4-59,4-59,4-59
3-21,3-21
4-59,4-59
3-9
4-59,4-59,4-59
4-31,4-31
1-46,1-46
1-46,1-46,1-46
5-51,5-51
3-48
4-31,4-31
3-7
4-61
4-59,4-59,4-59,4-61,4-61,4-61
4-38-2,4-38-2
3-21,3-21
1-69,1-69,1-69
3-23,3-23,3-23
4-59,4-59
3-48
3-48
1-46,1-46
3-23,3-23,3-23
3-30-3,3-30-3
1-46,1-46,1-46
3-64
3-73,3-73
4-4
1-18
3-7
1-46,1-46
1-3
4-61
2-70
4-59,4-59
5-51,5-51
3-49,3-49
4-4,4-4,4-4
4-31,4-31
1-69
1-69,1-69,1-69
4-39
3-21,3-21
3-33
3-9
3-48
4-59,4-59
4-59,4-59
4-39,4-39
3-21,3-21
1-18

My script takes ~ 7 sec to complete this.

Answer 1

$ awk -F, '{ for (i=2; i<=NF; ++i) if ($i != $1) { print "no_match"; next } print $1 }' file
1-69
no_match
1-46
no_match
6-1
5-51
4-59

I'm sorry, but I did not even look at your code, there was too much going on. When you find yourself calling awk three times in the body of a loop on the same data, you will have to look at other ways to do it more efficiently. Also, if you involve awk, you don't need grep and cut as awk would easily be able to do their tasks (which are not needed in this case though).

The awk script above reads a comma-delimited line at a time and compares each field with the first field. If any of the tests fails, the string no_match is printed and the script continues with the next line. If the loop finishes (without finding a mismatch), the first field is printed.

As a script:

#!/usr/bin/awk -f

BEGIN { FS = "," }

{
    for (i=2; i<=NF; ++i)
        if ($i != $1) {
            print "no_match"
            next
        }

    print $1
}

• FS is the input field separator, also settable with the -F option on the command line. awk will split each line on this character to create the fields.
• NF is the number of fields in the current record ("columns on the line").
• $i refers the the i:th field in the current record, where i may be a variable or a constant (as in $1).

Related:

• Why is using a shell loop to process text considered bad practice?

---

DRY variation:

#!/usr/bin/awk -f

BEGIN { FS = "," }

{
    output = $1

    for (i=2; i<=NF; ++i)
        if ($i != output) {
            output = "no_match"
            break
        }

    print output
}

Answer 2

Awk is a full programming language. You already use it. But don't use it just for simple tasks with multiple invocations per line, use it for the whole task. Use the field delimiter in awk, don't use cut. Do the full processing in awk.

awk -F',' '
{ 
  eq=1; 
  for (i = 2; i <= NF; i++)
    if ($1 != $i)
      eq=0;
  print eq ? $1 : "no_match";
}
' $1

Answer 3

With perl List::MoreUtils, by evaluating the distinct / uniq elements in scalar context:

perl -MList::MoreUtils=distinct -F, -lne '
  print( (distinct @F) > 1 ? "no_match" : $F[0])
' example 
1-69
no_match
1-46
no_match
6-1
5-51
4-59

Answer 4

You could do this using the sed editor also, like as shown:

sed -e '
    s/^\([^,]*\)\(,\1\)*$/\1/;t
    s/.*/NOMATCH/
' input.csv

Here we rely on the regex to multiplicate itself and reach the end of line. If it is able to do so, then terminate with the first field otherwise flash NOMATCH.

Explanation:

This is what goes on in my head when seeing this pbm:
Think of the comma-separated fields as stones of different colors. And picture them whether they can be arranged in a row as a repetition of the first stone, with a comma prefixing them.

Something like:

STONEA ,STONEA ,STONEA ,STONEA ... all the way to end of line

Now in terms of regex terminology, it becomes:

^ (STONEA) (,\1) (,\1) (,\1) ... all the way to end of line

^ (STONEA) (,\1)* $

Output:

1-69
NOMATCH
1-46
NOMATCH
6-1
5-51
4-59