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hpr_transcripts/hpr2163.txt

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+Episode: 2163
+Title: HPR2163: Gnu Awk - Part 4
+Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr2163/hpr2163.mp3
+Transcribed: 2025-10-18 15:08:36
+
+---
+
+This is HPR episode 2,163 entitled Genoaq Part 4 and is part of the series Learning Auk.
+It is hosted by Dave Morris and is about 31 minutes long.
+The summer is recapping the last episode and looking at variables in an Auk program.
+This episode of HPR is brought to you by AnanasThost.com.
+Get 15% discount on all shared hosting with the offer code HPR15.
+That's HPR15.
+Better web hosting that's honest and fair at AnanasThost.com.
+Hi everyone, this is Dave Morris and this is episode 4 in the Genoaq series.
+Be easy and I are progressing with this and we've now, well this is episode 4 as I say,
+so it means we now have a series which we've called Learning Auk so they're all joined together
+so you can find them easier and that sort of thing.
+Okay, so what I'm going to start with this time is a recap of the previous episode
+and then I'm going to go into a bit more detail about variables in Auk.
+So in the last episode you saw logical operators, they're also called Boolean operators.
+If that means anything to you, Boolean algebra and that type of thing.
+Boolean algebra has not and an all operators.
+Well in Auk the double ampersand means and double vertical bar or pipe symbol means Auk.
+One that wasn't covered was the not operator which is an exclamation mark.
+So we can generate some quite complex Boolean expressions with this but
+I'll leave that or we'll leave that, I'm not sure who's going to do this but we'll leave it
+till later because we want to deal with this in the context of other Auk statements in
+in an Auk program walk script so we'll expand on this a bit later on.
+You also saw last episode the next statement and this we discovered is a way of stopping
+processing on the current input record so it really does abort everything.
+No more patterns are tested against it.
+The pattern that's currently executing in the current rule I should say the actions
+in the current rule are finished with stopped at that point.
+It's a statement in a similar way to things like print and so you can't use it anywhere else
+other than in the action part of a rule and you can't use it in begin or end rules either
+and I'm going to talk about that in a minute.
+So beginning and end, beginning and end are actually patterns.
+They're in capitals, capital B E G I N and E N D.
+They're patterns which are special and they have to have to work with an action.
+You can't have either of them without an action.
+The action is being in curly brackets as you know and the whole Shibang
+begin an action, end an action, make up rule in the same way as we've seen with the pattern
+action sequences.
+So the begin stuff is run before the main pattern action rules are processed.
+That is the input file or files are read.
+End rules are run after everything's been been read and processed and you can have more than one
+begin and more than one end doesn't actually matter which order they occur in in terms of
+the begins versus the ends but if you have multiple begins then they are executed in the order
+that they are encountered similarly with end.
+So in the last episode we also started to look at variables.
+It's it's difficult when describing this sort of issue.
+This is effectively a language we're talking about here or
+and you can't really start at the beginning and work through it at the end because there
+isn't really a beginning you know because it's it's quite difficult to find a linear path through
+it. So we're sort of going ahead into areas that haven't really been explained yet just to
+demonstrate certain functions and processes and so on.
+So there was a bunch of things that were commented on that were shown last episode.
+Variables arrays and loops and that sort of thing.
+So we're going to look at all of these in a bit more detail in this episode so I'm trying
+to consolidate them all. Okay that was a quick recap of where we were from last episode
+and I now want to start talking about variables in relation to ORC.
+They've already seen things like NR capital NR capital NF which is the record number and the
+field number in the early part of the series and in the last episode you saw that you can
+create your own variables too. So what's a variable? Well as you find in most other programming
+languages it's a named storage area that can hold a value and it has certain rules about how
+you construct the name. It consists of letters, digits and the underscore in the case of ORC
+and it mustn't start with the digit. The case of the letters is significant so lower case sum
+and capital S, lower case UM and capital S capital M are three variables that you would
+speak them the same but they're different. The other name for these types of variables they
+can just hold a single value they're called scalars. You might see that name I'm mentioning
+these because you might see them if you look in the manual. So variable in ORC can contain a numeric
+value or a string value. ORC deals with the conversion of one of these to the other as appropriate.
+Sometimes it might mistake if you like to put it that way what it was you intended it might
+need some assistance but we'll refer to these later. Now one of the things you learn as a
+somebody learning programming or did so back in the day when I was learning this sort of stuff is
+when you create a variable in the language you need to initialize it because there's no
+definition of what it contains before you use it but in ORC that's not so. All variables begin
+as an empty string and an empty string is the equivalent of zero if you need to use it as a number.
+So how do you set variables to values? Well you do it as you do in most languages you
+use an assignment so I've given an example here count equals three that's an assignment count is
+the name of the variable the equals is the assignment operator three is the value you're going to put
+into it and last episode saw an assignment like used usd plus equals dollar three what this actually
+means is increment the contents of variable used the variable with the name used by the contents
+of field three so uses the variable plus equals is this special type of assignment and dollar three as
+you already know means field three there is an assumption here that dollar three contains a
+numeric value but we'll come on to what would happen if it didn't a bit later on it's a
+shorthand version of used equals used plus dollar two so what that means is add the contents of
+used to the contents of field three and then save the result back in the variable used. So the first
+time the variable is incremented its contents are taken to be zero and as I've said it used to be
+that if you were writing in C or Fortran or Pascal or one of those sorts of older languages
+compiled languages you you could not get away with it but in Orc and many other scripting languages
+these days it's it's not a problem so we've started down the road of looking at arithmetic operators
+so I thought we would stop and look at the whole the whole list it's a pretty short list but I'll
+just go through them briefly there's a table in the in the long notes here which you can refer to
+if you need to but you've had any experience of programming most of these will be very very obvious
+to you one thing to note before we proceed is that all numbers in Orc are floating point numbers
+that is they have a decimal point in them this can catch you out in some edge cases because comparing
+floating point numbers for equality doesn't always give you the result that you would expect one
+but we'll we'll highlight these as we go along what I've done here is to put together a list
+bake based on what's in the Gnuwark user's guide and as before there's a reference to it if you
+want to go and examine it yourself I've listed in them as they do in the order of their precedence
+from highest to lowest so the first one is the circumflex character which is exponentiation so
+x circumflex y means x raised the power of y so something like two circumflex three that's
+two to the power of three which has the value eight in Orc there is a double asterisk operator which
+is does the same job but it's not the standard version Gnuwark and it is slightly different from
+standard Orc so we're trying to stick to pretty much the mainstream stuff as much as possible
+because otherwise you you might get caught out if you try and run your Orc script on a different
+machine a different system a BSD system wasn't another perhaps a Mac or something so we're not
+going to use the double asterisk operator so a minus sign in trying to put a variable or a number
+obviously negates it plus sign in front of one is unri plus and that's actually a way in which
+you can tell Orc to treat a variable as a number and I was typing this out I was trying to think
+of cases where you'd want to do that and I couldn't come up with any but hopefully some will
+occur to me as we go along the asterisk is multiplication the forward slash is division and there's
+a note here which is that because all numbers in Orc are floating point the result is not rounded
+to an integer so three divided by four which would be written as three forward slash four it has
+the value 0.75 whereas if you did the same thing in bash for example which is purely integer you
+typed something like echo dollar open parenthesis open rendsis three slash four close parenthesis
+you'd get the answer zero because it's rounded it to an integer to all number the percent symbol
+is the remainder after division so x percent y is the the remainder after x has been divided by
+one so three percent four is three so it doesn't it can't be divided by four there's and the
+remainder is three five percent two is one because two goes into five twice leaving one remainder the
+plus sign is also addition so x plus y so you'll be meaning and the hyphen
+the minor sign is subtraction x minus one so pretty obvious so if you've already seen
+the plus equals operator this is an assignment operator these are shorthand forms of more verbose
+assignments which is we've already looked at in one particular case so I put together a table
+which is a modification of the GNUORC user's guide table showing all of these operators so you might
+do plus equals minus equals asterisk equals slash equals percent equals and circumflex equals and
+I think you probably get what that means in in all of the cases let's just look at the last one
+circumflex equals so if you wrote variable circumflex equals power so you might
+might type x circumflex equals two what that means is raise x to the power of two so x becomes x
+squared I wrote a little script just to demonstrate these things and it's available if you want it
+and it's called arithmetic assignment operators dot org and it's I've listed its contents and it's
+simply a bunch of expressions statements which use these various operators and print out the
+result yet the whole thing is in a begin rule because we don't want the script to actually
+do any file processing it's just doing a little demonstration of its internal computation
+capabilities as I've written it say for example the first line after the begin it is x equals 42
+semi-colon print quotes x is close quotes comma x so there are there are two statements there one
+is the assignment statement which sets x to 42 the second one is a print which prints out x is
+the string x is followed by the contents of x so there there's a semicolon between them if you
+write two statements on a line then you need semicolons between them they could have been written
+on two successive lines but I just thought a little bit of need to doing it this way so you need
+semicolon statement separators if there are multiple statements on a line but you don't need them
+if there if there's only one statement per line so there's no semicolon on the end if you're used to
+other languages where this is necessary then orc doesn't make it so it doesn't matter if you
+put a semicolon on the end of the line as well if you want to there's something to be said for
+doing that I guess but you don't need to okay so I've got an example here of what happened when
+you run this and I'm not reading you that because it's pretty obvious so let's talk about type
+conversion so variable can contain a numeric value or a string at any point in time as we've seen
+when converting from a number to a string then what you get is a string containing the number
+a little bit more to it than that but we'll leave that for another time converting from a string
+to a number on the other hand well there needs to be something that can be interpreted as a number
+within the string in other words it needs to begin with a digit sequence so my little example here
+uses the string nine gag dot com and if you set into a variable called s and then I set x equal
+to s plus one and print x so the answer is ten because or pulled the nine off the front of this
+address IP address and simply added one to it so the the nine off the front was converted to
+number and then one was added to it if there's no valid number in a string when you come to do
+this type of conversion then orc will treat it as zero so orc will handle strings containing
+all sorts of numbers so it'll handle energy numbers like number 42 floating point numbers like
+4.2 and also exponential numbers and the notation for this which is common in many languages
+one e three i've used a capital E in this case but it could also be a lower case one e three
+means one times ten to the to the three so it's a thousand so i've got a little example of
+these three strings being fed to a print f statement and printed out and the print f uses the
+g format control letter which we haven't really looked at we're going to spend some time on these
+control letters a bit later on but the g one is for printing general numbers so it prints 42
+as 42 4.2 as 4.2 and one e three comes out as a thousand also in last the last episode
+these are used some operators which consisted of two operators together plus plus i think he used
+and these are called increment and decrement operators and they increment or decrement the value
+of a variable by one and if you've been following my series on on bash and parameter expansion
+or various expansions i covered arithmetic expansion where i talked about these in the bash
+context you can look at episode 1951 if you've gotten or if you're interested so again i've produced
+a list of the various variables the various operators i should say so for example plus plus
+variable name means increment the variable returning the new value as the value of expression
+so plus plus variable is different from variable plus plus because the first one plus plus
+means add one to it and then return the result variable plus plus it's called a post increment in
+this case returns the contents the variable before it's had one added to it then adds one to it okay
+so this is in a similar pair minus minus variable which decrements it and then returns that value
+and variable minus minus which returns the value and then subtract one from it there's some
+examples of how this might be used a little bit later on in the notes so that's scalar variables
+and but there's also a whole bunch of other capabilities in the shape of arrays within
+orc or provides one dimensional arrays now there's a little note here to the effect that what does
+actually allow you to have multi-dimensional arrays traditional orc offers this by a sort of
+a hacky solution. Gnu orc provides true arrays of arrays but i'm not sure that we're going to
+cover that in this particular series because it's pretty much on the edge if i wanted to do this
+personally i would not be using orc to to do it but you you may think otherwise of course but to
+think it might be a better we'd simply point you at the manual to to go further with this but
+i thought it was worth just pointing out that there's quite a lot in Gnu orc. The thing about
+arrays in orc is that they are so-called associative arrays that which also known as hashes so let's talk
+about what an array is it has a name and its name it's got to conform to the rules we talked about
+for scalars you can't have an array called the same thing as a as a scalar variable an array can
+store multiple values and to get at them you use an index since this is a scripting language
+it's different from compiled languages the arrays can be any length and can be expanded it can
+contract it at will so given an array let's call it a we might store a value in it so we type a
+open square bracket one closed square bracket equals and then a string i've put hpr in double
+quotes double quotes is the way you define a string in in orc by the way so the array name is a
+the index is one and the contents of a square brackets one is the string hpr so if you if you
+used to using arrays in other languages you might assume that the index is numeric but it's not
+it's a string all array indices are strings because orc arrays are these types of things they're
+associative you use a string as the index into it so it's an associative array or a hash
+their index but arbitrary string values and they make up a sort of a lookup table it's actually
+quite powerful capability so in one of the examples in last episode we saw this is just an extract
+from an from an example nr not equal to one that was a patent open curly bracket a square bracket
+dollar two closed square bracket plus plus close curly brace so we saw that and here the
+orc script was being used to produce a frequency count of colors and we were looking through the file
+file 1.txt which you already have a copy of would imagine field two in this file is the name
+of a color so what we're doing here is we're using the color name as an index and we're simply
+incrementing that array element so I've tried to explain it in text and here is what I've typed
+means index the array a by the string contents of field two if the element doesn't exist
+created so like this this thing can be used even before there there is an a an array a or an array
+a with that particular element in it since orcs very relaxed about initialization this array element
+will be taken to be zero when it's created and then the plus plus on the end will increment it to
+one if the element already exists then its previous value will be incremented so if you ran
+this particular bit of code was in the last episode it just went through all of the rows in the
+file one file one dot txt file then if you could look at the insides of that the array when it
+had finished you'd find an index with the string brown and the contents would be two meaning
+that there were two instances of the the color brown so there's an out that means there's
+an element a open square brackets open double quotes brown close double quotes close square
+bracket and in that array element there's a number two I also noted that a square brackets dollar
+two plus plus is the same as a square bracket dollar two close square bracket plus equals one
+both mean the same thing I don't know you're already there ahead of me we also saw last time
+the concept of looping through an array to print it out and we had I'll just read this out quickly
+without going into a lot of details for b in a print b a brackets b so this was a case of sort of
+rushing ahead into areas that we hadn't really explained yet but it was necessary to get some of
+the precursor concepts sorted out we haven't looked at looping and other such statements in all
+yet but we need to look at this one now so we can understand how you would process an entire array
+so briefly the four statement provides a way to repeat a given set of statements a number of time
+we'll have a look at this and other related things like while and do and so forth later on
+this particular variant of the four statement allows the processing of arrays and it consists of
+the word four followed by in parenthesis variable name followed by the word in followed by the name
+of an array so it's saying for every element of this array and then the four statement is followed
+by one or more statements which are being controlled by it so the expression variable in array
+results in all of the index values in the nominated array being provided one at a time and while the
+loop runs the variable is set to the successive index values and the body the body part the dependent
+statements are executed now the body part can be a single statement or a group if a group is used
+then you have to put curly braces around them but if there's only one statement you only need
+you don't need any any curly braces now one thing about the way or works is that the order in which
+the array index values are provided is not defined so they sort of come out in a in a sort of random
+order it's not really random but it's a it's an arbitrary undefined order different orc versions
+will use different orders in the way it processes this now GNU org does can have extensions which
+allow the ordering of this the index values to be controlled but we'll deal with this later so
+let's just look at the example from the last episode and I've made some modifications to it
+change names and that type of thing later that slightly differently just to demonstrate the concept
+a little bit more clearly and this particular example is in a file which you can download from
+the hpl website and it's called color underscore count org I've used the american spelling because
+be easy what had used it throughout his example and I know it's basically his example I've stolen
+and hacked around so the array has been renamed from a to count because it holds counts or frequencies
+of the number of times of colors encountered the raised index by the names of the colors in field
+two and when we look through the array in the end rule we use the variable color to store the
+latest index and I took out semi-colons and curly braces that were not really necessary just to
+really demonstrate that they could be removed without any any problem so I'll not read this one
+out because you've seen the just of it last time so you might want to check this one out just to
+see how different it is in terms of its layout and use of use of variable names and so on
+so when it runs it does the same as the previous version does it prints out a list of it actually runs
+against the file called file1.csv and it prints out a csv list comma section separated variable list
+that should be consisting of the headline color comma count and brown comma two purple comma two
+etc so it's just the count of the number of occurrences of those colors so to finish then I want to
+just mention the built-in variables that we've seen so far and we saw another one added to the list
+in the last episode so this this one is called FS capital F capital S and it stands for field
+separator and it's the internal variable within the org script that matches the the minus capital
+F option that you give to the command so for example minus capital F and then in double quotes
+a comma on the command line is the same as assigning FS equals double quotes comma close double
+inside the script the statement FS equals etc needs to be in a begin rule in order that it can be
+set early enough in the script you can't put it in a pattern action style rule because that happens
+too late you've already grabbed the first record by then most likely or a record at least and
+and and that that was separated out based on whatever the default separator is which is a space
+so I've just put a little example here where org hyphen f double quotes comma quote begin curly
+bracket print quotes FS is close quotes comma FS close curly bracket close quote and you get the answer
+FS is comma so you can see see there it's just to demonstrate the point really we also saw
+OFS the variable which is all in capital's OFS which is the output field separator and it controls
+the the format of the output record produced when you use the print statement and normally it's set
+to a space so we're giving example here where org is run it and there's a begin rule within it
+and it simply consists of print followed by in double quotes hello comma and then in double
+quotes world so there's two two arguments to the print command separated by a comma and the answer
+produced is hello space world because the comma whenever whenever you put a comma in one of these
+things it it tells the print statement to output the OFS variable contents which is a space
+by default so if you were to do my second example which is pretty much the same except that instead
+of there being a comma between hello and world two separate strings there's nothing then you get
+hello world with no space in between them and that's because all because seen these two strings as
+the arguments to print and it can coordinate them together and given print one argument which
+consists of the string hello followed by world no spaces OFS variable can be set to a string if
+you want to so I did a rather silly example where I set in a begin rule OFS equals double quotes space
+blog close a space double quotes semicolon print hello comma world as before and then you get
+out instead of a space between hello world you get the word blog just proves the point can be
+useful sometime now the OFS variable only affects the behavior of print not print f so there's an
+example here showing OFS being set to a tab character then we print out using print f hello world
+and it comes out without a tab in it as no effect so with this just reiterating that print f is
+always followed by at least one argument and that first argument has got to be the control
+the format string which specifies how what it's to print out and how it's to be formatted and then
+it can be followed by any number of further arguments separated by commas so this one has got
+the first argument is this string that's got percent s space percent s backslash in in order to get
+a new line the end of it and then comma then the string hello comma and then the string world
+so the three arguments in total
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