awk: find minimum and maximum in column - bash

Question

I'm using awk to deal with a simple .dat file, which contains several lines of data and each line has 4 columns separated by a single space.
I want to find the minimum and maximum of the first column.
The data file looks like this:
9 30 8.58939 167.759
9 38 1.3709 164.318
10 30 6.69505 169.529
10 31 7.05698 169.425
11 30 6.03872 169.095
11 31 5.5398 167.902
12 30 3.66257 168.689
12 31 9.6747 167.049
4 30 10.7602 169.611
4 31 8.25869 169.637
5 30 7.08504 170.212
5 31 11.5508 168.409
6 31 5.57599 168.903
6 32 6.37579 168.283
7 30 11.8416 168.538
7 31 -2.70843 167.116
8 30 47.1137 126.085
8 31 4.73017 169.496
The commands I used are as follows.
min=`awk 'BEGIN{a=1000}{if ($1<a) a=$1 fi} END{print a}' mydata.dat`
max=`awk 'BEGIN{a= 0}{if ($1>a) a=$1 fi} END{print a}' mydata.dat`
However, the output is min=10 and max=9.
(The similar commands can return me the right minimum and maximum of the second column.)
Could someone tell me where I was wrong? Thank you!

Answer 1

Awk guesses the type.
String "10" is less than string "4" because character "1" comes before "4".
Force a type conversion, using addition of zero:
min=`awk 'BEGIN{a=1000}{if ($1<0+a) a=$1} END{print a}' mydata.dat`
max=`awk 'BEGIN{a= 0}{if ($1>0+a) a=$1} END{print a}' mydata.dat`

Answer 2

a non-awk answer:
cut -d" " -f1 file |
sort -n |
tee >(echo "min=$(head -1)") \
> >(echo "max=$(tail -1)")
That tee command is perhaps a bit much too clever. tee duplicates its stdin stream to the files names as arguments, plus it streams the same data to stdout. I'm using process substitutions to filter the streams.
The same effect can be used (with less flourish) to extract the first and last lines of a stream of data:
cut -d" " -f1 file | sort -n | sed -n '1s/^/min=/p; $s/^/max=/p'
or
cut -d" " -f1 file | sort -n | {
read line
echo "min=$line"
while read line; do max=$line; done
echo "max=$max"
}

Answer 3

Your problem was simply that in your script you had:
if ($1<a) a=$1 fi
and that final fi is not part of awk syntax so it is treated as a variable so a=$1 fi is string concatenation and so you are TELLING awk that a contains a string, not a number and hence the string comparison instead of numeric in the $1<a.
More importantly in general, never start with some guessed value for max/min, just use the first value read as the seed. Here's the correct way to write the script:
$ cat tst.awk
BEGIN { min = max = "NaN" }
{
min = (NR==1 || $1<min ? $1 : min)
max = (NR==1 || $1>max ? $1 : max)
}
END { print min, max }
$ awk -f tst.awk file
4 12
$ awk -f tst.awk /dev/null
NaN NaN
$ a=( $( awk -f tst.awk file ) )
$ echo "${a[0]}"
4
$ echo "${a[1]}"
12
If you don't like NaN pick whatever you'd prefer to print when the input file is empty.

Answer 4

late but a shorter command and with more precision without initial assumption:
awk '(NR==1){Min=$1;Max=$1};(NR>=2){if(Min>$1) Min=$1;if(Max<$1) Max=$1} END {printf "The Min is %d ,Max is %d",Min,Max}' FileName.dat

Answer 5

A very straightforward solution (if it's not compulsory to use awk):
Find Min --> sort -n -r numbers.txt | tail -n1
Find Max --> sort -n -r numbers.txt | head -n1
You can use a combination of sort, head, tail to get the desired output as shown above.
(PS: In case if you want to extract the first column/any desired column you can use the cut command i.e. to extract the first column cut -d " " -f 1 sample.dat)

Answer 6

#minimum
cat your_data_file.dat | sort -nk3,3 | head -1
#this fill find minumum of column 3
#maximun
cat your_data_file.dat | sort -nk3,3 | tail -1
#this will find maximum of column 3
#to find in column 2 , use -nk2,2
#assing to a variable and use
min_col=`cat your_data_file.dat | sort -nk3,3 | head -1 | awk '{print $3}'`