AWK how records and fields are executed and read

问题:

I am getting the right result for the awk program below. But I dont understand how does AWK process lines of code for the below program :

{
    for(i = 1; i <= NF; i++)
    {
    if (min[i]==""){ print "initial min " $i; min[i]=$i;} #line1
    if (max[i]==""){ print "initial max " $i; max[i]=$i;} #line2
    if ($i<min[i]) { print "New min " $i; min[i]=$i;}     #line3
    if ($i>max[i]) { print "New max " $i; max[i]=$i;}     #line4
    }
}
END
{
   OFS="\t";
   print "min","max";
   for(i = 1; i <= NF; i++)
   {
   print min[i],max[i];
   }     
}

Dataset fields are separated using space

0.4 1.4 2.4 3.4
0.3 1.3 2.3 3.3 
0.1 1.1 2.1 3.1
0.2 1.2 2.2 3.2
0.5 1.5 2.5 3.5

Output

initial min 0.4
initial max 0.4
initial min 1.4
initial max 1.4
initial min 2.4
initial max 2.4
initial min 3.4
initial max 3.4
New min 0.3
New min 1.3
New min 2.3
New min 3.3
New min 0.1
New min 1.1
New min 2.1
New min 3.1
New max 0.5
New max 1.5
New max 2.5
New max 3.5

min max
0.1 0.5
1.1 1.5
2.1 2.5
3.1 3.5

Line 1 and 2 are printed alternatively(that is, initial min and max) But line 3 and line 4 are executed after setting new Min or Max for all the Fields(or column) So how does awk really work?

回答1:

I edited your code into:

{
    for(i = 1; i <= NF; i++)
    {
    if (min[i]==""){ print "initial min["i"] " $i; min[i]=$i;} #line1
    if (max[i]==""){ print "initial max["i"] " $i; max[i]=$i;} #line2
    if ($i<min[i]) { print "New min["i"] " $i; min[i]=$i;}     #line3
    if ($i>max[i]) { print "New max["i"] " $i; max[i]=$i;}     #line4
    }
}
END {
   OFS="\t";
   print "min","max";
   for(i = 1; i <= NF; i++)
   {
   print min[i],max[i];
   }     
}

Now, with its output you should understand what happens:

initial min[1] 0.3
initial max[1] 0.3
initial min[2] 3.3
initial max[2] 3.3
initial min[3] 0.5
initial max[3] 0.5
initial min[4] 3.6
initial max[4] 3.6
New max[1] 0.9
New max[2] 4.7
New max[3] 2.5
New min[4] 1.6
New min[1] 0.2
New min[2] 2.7
New max[3] 6.3
New max[4] 9.3
New min[2] 1.6
New max[3] 8.9
min     max
0.2     0.9
1.6     4.7
0.5     8.9
1.6     9.3