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print "NOMATCH: " $0;
}
}
ThewhileStatement
The
while
statementlooksjustlikethe
if
statement.Forexample:
{
i=4
if ($0 ~ /Mary/) {
while (i) {
print i ":" $0;
i--;
}
}
}
AsinC,youcanskiptheremainingcodeinthebodyofa
while
loopbycallingthe
continue
function.
Thefor Statement
The
for
statementsyntaxhasaspectsofboththeCsyntaxandtheshellscriptsyntax.TheClanguageform
ofthe
for
statementisasfollows:
for (pre_expression; while_expression; post_expression) statement
Thisstatementisequivalenttothefollowing:
pre_expression;
while (while_expression) {
statement;
post_expression;
}
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Thefirstexpression,whichexecutesbeforeenteringthe
while
loop,usuallyinitializesoneormoreloop
iterators.Thesecondexpressionisthentestedfortruth.Whileitistrue,thestatementexecutes.Aftereach
iterationthroughtheloop,thethirdexpressionexecutes.Thisusuallyincrementsordecrementstheloop
iterator.
AsinC,youcanskiptheremainingcodeinthebodyofa
for
loopbycallingthe
continue
function.
Forexample,thefollowingcodeprintseachlinethatmatches“Mary”threetimes.Thesearenumbered1,2,
and4.Itskipsthecasewhere
i==2
,andthusthenumber3isneverprinted.
{
if ($0 ~ /Mary/) {
for (i=0; i<4; i++) {
if (i==2) continue;
print i+1 ":" $0;
}
}
}
Inaddition,AWKsupportsashell-like(really,Perl-like)versionofthe
for
loop,inwhichitactsasanarray
iterator.Thearrayiterationsyntaxis:
for (key_variable in array) statement
ThissyntaxisdescribedinmoredetailinWorkingwithArraysinAWK (page 133).
SkippingRecordsandFiles
Atanypointinyourfilterrules,youcanskipprocessingofallremainingrules(effectivelyskippingtothenext
record)byusingthe
next
statement.Forexample:
if (i > 4) next;
Likewise,atanytime,youcanskipprocessingoftheremainderofaninputfilebyusingthe
nextfile
statement.
Forexample:
if (i > 4) nextfile;
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The
if
statementsyntaxisdescribedinControlStatementsinAWK (page 130).
Functions in AWK
Inadditiontoprovidinganumberofstandardfunctions(describedinthemanualpagefor
awk
),theAWK
languageallowsyoutodefineyourowncustomfunctions.Thesyntaxforafunctiondeclarationis:
function function_name(parameter1 [, parameter2, ...]) {
action
}
Becausevariablesareintheglobalscopeexceptforfunctionparameters,ifyouwanttodefinealocalvariable
inafunction,youmust declareitasanextraparametertothefunction.Youdonothavetopassinavalue.If
youdonotdeclarethevariableasaparameter,itaffectsexecutionoutsideofthefunctionanditsvalueis
persistentacrossmultipleinvocationsofthefunction.
Forexample,thisfunctiontakestwoparameters,subtractsthem,andthenaddsone(
1
):
function subtractAndAddOne(a, b, c) {
c = 1
return (a-b+c);
}
BEGIN {
print subtractAndAddOne(3, 2);
}
Important:  Whenyoucallafunction,youmustnotputaspacebeforetheopeningparenthesis.InAWK
scripts,aspaceisusedforstringconcatenation,soaddingaspaceislikelytocauseasyntaxerror.However,
itmightinsteadresultinratherstrangebehaviorincertaincontexts.
Working withArrays in AWK
ArraysinAWKscriptsaresyntacticallyverysimilartoarraysinC.Don’tletthatfoolyou,though.Underthe
hood,theybehaveverydifferently.
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ArraysinAWKscriptsareassociative.Thismeansthateacharrayelementisstoredasakey-valuepair,resulting
inthreemajordifferenceswhencomparedtoC:
Arraysareallocatedandgrowdynamicallyasspaceisneeded.
Arrayscanbesparse;youcanhaveanarraywithavalueatindex
711
andavalueatindex
1116
with
nothingbetweenthem.
Youcannotpopulateanarrayinasingleoperationexceptbysplittingastring.
Therearetwowaystocreateanarray.Thefirstisbysimplyusingit.Thesecondisbyusingthe
split
function.
Thesemethodsaredescribedinthesectionsthatfollow,alongwithusefultipsaboutworkingwitharrays.
ArrayBasics
Thefollowingcodecreatesandprintsanarraycalled
my_array
containingthevalues“Partridge”,“tree”,
“pear”,and“Cassidy”:
BEGIN {
my_array[0] = "Partridge";
my_array[1] = "pear";
my_array[2] = "tree";
my_array["David"] = "Cassidy";
for ( my_index in my_array ) {
print my_index "=" my_array[my_index];
}
}
Thefirstthingyouwillnoticeisthatthearrayisnotprintedinorder.Infact,itisprintedintheorderinwhich
theunderlyingdataisstoredinternally.Ifyouwanttoprintthevaluesinkeyorder,youmustwalkthrough
theindexnumericallyinstead.
Thesecondthingyouwillnoticeisthatthe
for
statementcanbeusedtoiteratethroughallofthekeysinthe
array.Inthisusage,the
for
statementinAWKscriptsislikethe
for
statementinashellscript.The
for
statementarray-iteratorusageis:
for (key_variable in array_name) statement
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Note:  Unlikethe
for
or
foreach
statementsinmostotherlanguages,thearray-iterator-style
for
statementinAWKscriptsiteratesthroughthearraykeys(indices)ratherthanthroughthearray
values.Thus,itissimilartothefollowingPerlstatement:
foreach my $key_variable (keys %assoc_array) { ... }
Becausekey_variable containsthekeyfromeachkey-valuepairratherthanthevalue,youmust
explicitlyusethekeyasanarrayindexifyouwanttotoobtainthevaluesinthearray.Forexample:
for ( i in arr ) {
print arr[i];
}
Thethirdthingyouwillnoticeisthat,unlikeC,arrayelementscantakearbitrarystringsastheirkey(array
index).Ifyouneedtoiteratethroughthearrayinkeyorder,however,youshouldlimityourselftonumeric
keys.
Asasideeffect,thekeysarealways storedasastringeveniftheyonlycontainnumbers.Thus,ifyouwantto
comparethemnumericallytoeachother(forexample,tofindthesmallestkeyforwhichavalueexists),you
mustaddzero(0)tothekeypriortomakingthecomparison.
Forexample,thefollowingcodeiteratesthroughthissparsearrayinkeyorderbyfindingtheminimumand
maximumkeyvaluesandtheniteratingfromtheminimumtothemaximum:
BEGIN {
my_array[0] = "Partridge";
my_array[1] = "pear";
my_array[2] = "tree";
my_array[13] = "Cassidy";
min = 0; max = 0;
for ( my_index in my_array ) {
if (my_index+0 < min) min = my_index;
if (my_index+0 > max) max = my_index;
}
for (i=min; i<= max; i++) {
if (i in my_array) {
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print i "=" my_array[i];
}
if (!(i in my_array)) {
print i " is unset.";
}
}
}
Inthisexample,youshouldnotethe
if
statementsyntaxneartheend.Beforeprintinganarrayvalue,the
examplecheckstoseeifavaluehaseverbeenstoredforthatkeyvalue:
if (i in my_array) { ... }
Aswithanyexpression,youcaninvertmatchingwithanexclamationpoint.Forexample,tochecktoseeifa
particularindexhasneverbeenstoredinanarray,youcouldwritethefollowing:
if (!(i in my_array)) { ... }
Note:  Generallyspeaking,theAWKlanguageisdesignedundertheassumptionthatyouwilldo
anyarraysortingexternally(afterthe
awk
interpreterhasfinished)usingthe
sort
toolorsimilar
tools;forperformancereasons,youshouldgenerallydoso.
CreatingArrayswithsplit
Assigningarrayelementsindividuallycanbeverytedious.Amorecommon(read“lesspainful”)waytocreate
anarrayiswiththe
split
function.The
split
syntaxisasfollows:
count = split( string, array_name, regexp );
Forexample,thefollowingcodesplitsthestring“Marylambfreezer”intowordsseparatedbyspaces.
BEGIN {
arr_len = split( "Mary lamb freezer", my_array, / / );
}
HowAWK-ward
WorkingwithArraysinAWK
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136
Theresultisthat
arr_len
containsthenumberthree(
3
).Thevariable
my_array[1]
contains“Mary”,
my_array[2]
contains“lamb”,andsoon.
CopyingandJoininganArray
TheAWKlanguagedoesnotsupportassignmentofarrays.Thus,tocopyanarray,youmustcopytheindividual
valuesfromonearraytothenext.Forexample,thefollowingcodeinitializes
my_array
andthencopiesits
contentsto
copy_array
beforeprintingthearray:
BEGIN {
arr_len = split( "Mary lamb freezer", my_array, / / );
for (word in my_array) {
copy_array[word] = my_array[word];
}
for (word in copy_array) {
print copy_array[word];
}
}
Similarly,theAWKlanguagedoesnotprovidefunctionstojoinanarray.Tojoinanarray,youshouldwritea
simplefunctionlikethisone:
function join(input_array, separator) {
string = "";
first = 1;
# Note: the array items are in no particular
# order when joined with this function.
for (i in input_array) {
if (first) first = 0;
else string = string separator;
string = string input_array[i];
}
return string;
}
BEGIN {
arr_len = split( "foo bar baz", my_array, / /);
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for (word in my_array) {
print my_array[word];
}
print join(my_array, " ");
}
Likeallarrayfunctionswrittenusingthearray-iteratorformofthe
for
statement,thisjoindoesnotoccurin
anyparticularorder.Ifyouneedtojointhearrayvaluesinaparticularorder,youmustwriteyourowncustom
join
functioneitherusinganumericiteratororamanuallyspecifiedlistoffields.Forexample:
function count_elements(input_array)
{
counter=0;
for (word in input_array) {
counter++;
}
return counter;
}
function join(input_array, separator) {
string = "";
first = 1;
# Note: this preserves order, but does not
# work with nonnumeric or sparse arrays.
for (i=1; i<=count_elements(input_array); i++) {
if (first) first = 0;
else string = string separator;
string = string input_array[i];
}
return string;
}
BEGIN {
arr_len = split( "foo bar baz", my_array, / /);
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for (word in my_array) {
print my_array[word];
}
print join(my_array, " ");
}
Compatibility Note:  Previousversionsofthisscriptusedthebuilt-in
length
functiontoobtain
thenumberofelementsinanarray(insteadofthe
count_elements
function).Whilethistechnique
worksinmostversionsofAWKreleasedsince2002,itdoesnotworkinGNUAWKoritsderivatives
withinthecontextofafunctionifthearraywaspassedasoneofthefunction’sarguments.
AlthoughthisbughasbeenfixedintheofficialGNUAWKsourcerepositoryandshouldbefixedin
versionsofGNUAWKafterversion3.1.6,formaximumportability,youshouldstillavoidusingthe
length
functioninthisway.
DeletingArrayElements
AsyousawinArrayBasics (page 134),youcanaddvaluestoanarrayusingarbitrarykeys.Youcanalsocheck
toseeifavalueexistsforagivenkeyusingthe
if (key in array)
syntax.
Ifyouneedtodeleteakey-valuepair,youcouldassignanemptyvalue.However,the
if (key in array)
syntaxstillevaluatestotruebecausethereisstillavalueforthatkey(albeitanemptyvalue).Thus,youprobably
wanttoremovethekeyentirely.
TheAWKprogramminglanguagesolvesthisproblemwiththe
delete
function.Thesyntaxfor
delete
is:
delete array_name[key];
Forexample,thefollowingscriptprintsonlythekey-valuepairs“purple=Partridge”and“majesties=tree”.
BEGIN {
my_array["purple"] = "Partridge";
my_array["mountain"] = "pear";
my_array["majesties"] = "tree";
my_array["fruited"] = "Cassidy";
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mykey = "fruited";
delete my_array["mountain"];
delete my_array[mykey];
for (i in my_array) {
print i "=" my_array[i];
}
}
Ifyouneedtoclearallvaluesfromanarraysimultaneously,though,youdon’thavetodeletethemoneata
time.Instead,youcansimplydothefollowing:
delete array_name;
Thisstatementleavesthearrayspecifiedbyarray_nameemptyforfutureuse.Youmightdothisif,forexample,
youwantanarraytoberesetforeachrecord.
FileInputand Output
TheAWKprogramminglanguagewasprimarilyintendedasafilterbetweenoneormoreinputfiles(orstandard
input)andstandardoutput.However,itdoesprovidesomebasicinputandoutputcapability.
Asinshellscripts,anyprintstatementcanbewrittentoafileusingtheredirection(
>
)operator(whichdestroys
anypreviouscontentsofthefile)orconcatenatedontotheendofanexistingfileusingtheconcatenation(
>>
)
operator.
Also,asinshellscripts,anyprintstatementcanbepipedtoanoutsidetoolusingthepipe(
|
)operator.
Pipesandredirections,however,behavedifferentlyinAWKscriptsthaninshellscripts;theyremainopenfor
futureuseuntilyouexplicitlyclosethemor
awk
exits.Thismeans,amongotherthings,thattheconcatenation
(
>>
)operatorisonlynecessaryifyouwanttoretainanexistingfileandisnotnecessarytocontinueadding
toafilethatyoucreatein
awk
.
Forexample,thisscriptdoesthefollowing:
Sendstwostringsto
/bin/tail -n 1
.The
tail
toolprintsthelastlinesent(whichcontainsthesecond
string).Thisdemonstratesthatthefirsttwoprintstatementsbothsenttheiroutputtothesameinstance
of
tail
.
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