Why does awk seem to randomize the array?

Question

If you look at output of this awk test, you see that array in awk seems to be printed at some random pattern. It seems to be in same order for same number of input. Why does it do so?

echo "one two three four five six" | awk '{for (i=1;i<=NF;i++) a[i]=$i} END {for (j in a) print j,a[j]}'
4 four
5 five
6 six
1 one
2 two
3 three

echo "P04637 1A1U 1AIE 1C26 1DT7 1GZH 1H26 1HS5 1JSP 1KZY 1MA3 1OLG 1OLH 1PES 1PET 1SAE 1SAF 1SAK 1SAL 1TSR 1TUP 1UOL 1XQH 1YC5 1YCQ" | awk '{for (i=1;i<=NF;i++) a[i]=$i} END {for (j in a) print j,a[j]}'
17 1SAF
4 1C26
18 1SAK
5 1DT7
19 1SAL
6 1GZH
7 1H26
8 1HS5
9 1JSP
10 1KZY
20 1TSR
11 1MA3
21 1TUP
12 1OLG
22 1UOL
13 1OLH
23 1XQH
14 1PES
1 P04637
24 1YC5
15 1PET
2 1A1U
25 1YCQ
16 1SAE
3 1AIE

Why does it do so, is there rule for this?

Answer 1

From 8. Arrays in awk --> 8.5 Scanning All Elements of an Array in the GNU Awk user's guide when referring to the for (value in array) syntax:

The order in which elements of the array are accessed by this statement is determined by the internal arrangement of the array elements within awk and cannot be controlled or changed. This can lead to problems if new elements are added to array by statements in the loop body; it is not predictable whether or not the for loop will reach them. Similarly, changing var inside the loop may produce strange results. It is best to avoid such things.

---

So if you want to print the array in the order you store it, then you have to use the classical for loop:

for (j=1; j<=NF; j++) print j,a[j]

Example:

$ awk '{for (i=1;i<=NF;i++) a[i]=$i} END {for (j=1; j<=NF; j++) print j,a[j]}' <<< "P04637 1A1U 1AIE 1C26 1DT7 1GZH 1H26 1HS5 1JSP 1KZY 1MA3 1OLG 1OLH 1PES 1PET 1SAE 1SAF 1SAK 1SAL 1TSR 1TUP 1UOL 1XQH 1YC5 1YCQ"
1 P04637
2 1A1U
3 1AIE
4 1C26
5 1DT7
6 1GZH
7 1H26
8 1HS5
9 1JSP
10 1KZY
11 1MA3
12 1OLG
13 1OLH
14 1PES
15 1PET
16 1SAE
17 1SAF
18 1SAK
19 1SAL
20 1TSR
21 1TUP
22 1UOL
23 1XQH
24 1YC5
25 1YCQ