How to sample multiple columns in the 2D matrix by using awk with for loop?-CodePudding

I would like to sample multiple columns in the 2D matrix by using 'awk'.

For instance,

awk -F " " '{print $900, $925, $950, $975 $1000}' [filename].txt > test.txt

I just wrote five columns in the above command for example. In fact the number of columns would be over 40. The column number has a increment, 25, from starting number, $900.

Writing all $(column number) would be painful.

How could I make the command simpler by using for loop? Or Any other suggestion?

Thank you for reading this question.

CodePudding user response：

I would harness GNU AWK for this task following way, let file.txt content be

A B C D E F
AA BB CC DD EE FF
AAA BBB CCC DDD EEE FFF

and say I want to get odd columns starting at 1 that is 1, 3, 5 then

awk 'BEGIN{pitch=2}{for(i=1;i<=NF;i =pitch){printf "%s%s",$i,(i pitch>NF?"\n":" ")}}' file.txt

gives output

A C E
AA CC EE
AAA CCC EEE

Explanation: I do use for loop with increment by pitch which is 2 in example, starting is from 1 and condition is i less equal number of fields (NF), in each turn of loop I use printf once, first element is simply value of ith column ($i), second is newline (\n) - if said element is last in given line or space for all other cases. I use number of current column (i), pitch and number of columns (NF) to calcute if this is last value to be included in current line and then so-called ternary operator condition?valueiftrue:valueiffalse to select fitting character.

(tested in gawk 4.2.1)

CodePudding user response：

jot -s ' ' -w 'Col-%d' 2000 | 

mawk '{ print '"$( jot -s ', ' -w '$%d' 40 900 - 25 )"'"" }'

 1  Col-900
    Col-925
    Col-950
    Col-975
    Col-1000
    Col-1025
    Col-1050
    Col-1075
    Col-1100
    Col-1125
    Col-1150
    Col-1175
    Col-1200
    Col-1225
    Col-1250
    Col-1275
    Col-1300
    Col-1325
    Col-1350
    Col-1375
    Col-1400
    Col-1425
    Col-1450
    Col-1475
    Col-1500
    Col-1525
    Col-1550
    Col-1575
    Col-1600
    Col-1625
    Col-1650
    Col-1675
    Col-1700
    Col-1725
    Col-1750
    Col-1775
    Col-1800
    Col-1825
    Col-1850
    Col-1875

The trick is to use jot (or seq, or something similar) to dynamically generate code with hard-coded column #s :

for the example above, this code is being generated on the fly :

mawk '{
    print $900,  $925,  $950,  $975, $1000, $1025, $1050, $1075,
         $1100, $1125, $1150, $1175, $1200, $1225, $1250, $1275,
         $1300, $1325, $1350, $1375, $1400, $1425, $1450, $1475,
         $1500, $1525, $1550, $1575, $1600, $1625, $1650, $1675,
         $1700, $1725, $1750, $1775, $1800, $1825, $1850, $1875, "" }'