awk: find minimum and maximum in column

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"
}