Scanning an Array (The GNU Awk User’s Guide)
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8.1.5 Scanning All Elements of an Array
In programs that use arrays, it is often necessary to use a loop that executes once for each element of an array. In other languages, where arrays are contiguous and indices are limited to nonnegative integers, this is easy: all the valid indices can be found by counting from the lowest index up to the highest. This technique won’t do the job in awk
, because any number or string can be an array index. So awk
has a special kind of for
statement for scanning an array:
for (var in array) body
This loop executes body
once for each index in array
that the program has previously used, with the variable var
set to that index.
The following program uses this form of the for
statement. The first rule scans the input records and notes which words appear (at least once) in the input, by storing a one into the array used
with the word as the index. The second rule scans the elements of used
to find all the distinct words that appear in the input. It prints each word that is more than 10 characters long and also prints the number of such words. See section String-Manipulation Functions for more information on the built-in function length()
.
# Record a 1 for each word that is used at least once { for (i = 1; i <= NF; i++) used[$i] = 1 }
# Find number of distinct words more than 10 characters long END { for (x in used) { if (length(x) > 10) { ++num_long_words print x } } print num_long_words, "words longer than 10 characters" }
See section Generating Word-Usage Counts for a more detailed example of this type.
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 in standard awk
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 the for
loop will reach them. Similarly, changing var
inside the loop may produce strange results. It is best to avoid such things.
As a point of information, gawk
sets up the list of elements to be iterated over before the loop starts, and does not change it. But not all awk
versions do so. Consider this program, named loopcheck.awk
:
BEGIN { a["here"] = "here" a["is"] = "is" a["a"] = "a" a["loop"] = "loop" for (i in a) { j++ a[j] = j print i } }
Here is what happens when run with gawk
(and mawk
):
$ gawk -f loopcheck.awk -| here -| loop -| a -| is
Contrast this to BWK awk
:
$ nawk -f loopcheck.awk -| loop -| here -| is -| a -| 1
Next: Controlling Scanning, Previous: Array Example, Up: Array Basics [Contents][Index]