UNIX for Dummies Questions & Answers

Hi, I'm in dire need of some help for AWK. I'm a college student and my statistics professor decided he'd teach AWK in the last two days of our class, even when it's not programming class and we don't even have computers in class to experiment. Anyway, I tried looking at AWK tutorials, but it doesn't necessarily teach me in the sequence our professor taught it. This is the code our professor wrote on the board for printing lines. I can't figure the thing out. If anyone can interpret it or correct me (if I miscopied the code), thank you.

Abe M 70
Bea F 65
Cathy F 67
Dave M 69

Code:

{if ($2 == "M"){
s=s+$3
n=n+1
}END{
print s, n, s/n, "avg height"
}

I have trouble understand the "s=s+$3" and "n=n+1" lines. What do those mean? Also, I have trouble understanding the "print" line. Obviously it means to print, but what would it print? "s" "n" and "s/n?" What is the "s/n" line?

Here's another one.

Abe M 70
Bea F 65
Cathy F 67
Dave M 69

Code:

{
if ($N / [fF] /) {
fsum = fsum + 3
fnum = fsum + 1
}
if ($2N/[mM]){
msum = msum + $3
mnum = mnum + 1
}
}

What does "if ($N / [fF] /)" or "($2N/[mM])" mean?

THX


I've studied HTML, Javascript and CSS in the past, and I like to know what each line or code does. I'm very OCD with programming, but it just frustrates me that the professor is simply teaching it this way WITHOUT (i repeat) a computer. This has got to be, by far, the most knuckleheaded professor I've probably taken in this college. Thx for the help.

Code:

{if ($2 == "M"){  if the second fiedl is "M"
s=s+$3           add the value stored in s to the value in the third column, store sum in s
n=n+1            add one to the count of records
}END{
print s, n, s/n, "avg height"  When done with file: print sum, count, sum divided by count, the literal "avg height"
}

Your awk program isn't valid.
The is a problem with the syntax of the two if staments.
If you try to execute it, you will get the following errors (with GNU awk):

Code:

$ awk -f coll.awk inputfile
awk: coll.awk:3:    if ($N / [fF] /) {
awk: coll.awk:3:             ^ syntax error
awk: coll.awk:3:    if ($N / [fF] /) {
awk: coll.awk:3:                   ^ unterminated regexp
awk: coll.awk:7:    if ($2N/[mM]){
awk: coll.awk:7:    ^ syntax error
awk: coll.awk:7:    if ($2N/[mM]){
awk: coll.awk:7:            ^ syntax error
awk: coll.awk:7:    if ($2N/[mM]){
awk: coll.awk:7:                 ^ syntax error
awk: coll.awk:9:       msum = msum + $3
awk: coll.awk:9:                       ^ unexpected newline or end of string
awk: coll.awk:10:       mnum = mnum + 1
awk: coll.awk:10:                      ^ unexpected newline or end of string
$

Jean-Pierre.

{if ($2 == "M"){
s=s+$3
n=n+1
}END{
print s, n, s/n, "avg height"
}

hi
I am new to the forum and I am first time posting any reply.So if any thing is wrong forgive me fr that .

now the answer to the first question

My friend you have to first understand the basic rule for these lines and then you will understand the whole syntax easily . lets take it one by one.
if ($2 == "M")----- /* awk goes to every line of file 1 by 1 and executes the command repetedly fr every line so, $2 is compared with the 2nd field that is 'M' since its boolean output is true then it goes to next line */

{
s=s+$3 -----/* Note after the block i.e '{' you can assign a variable (here it is s & n) and if that variable is not initialized then in shellscript uninitialized variable is given the value 0 so your values fr the first line scanning are s=0+70 ===> s=70
n=0+1 ===> n=1
}

and again for the second time you get if ($2 == "M") =true then you are assign
s=s+$3 => s=70+69 = 139
n=n+1 => n=1+1 =2


now once all the lines are scannned in a file
then there is final action taken on the results obtained during scanning of file
this is done by
END{final action}
so you have now variables and now you can get them printed the way you like i.e
END{
print s, n, s/n, "avg height"
}


thanx

Another explanation to accompany. I wrote to a work colleague which might find useful:
--------------
Basically, an awk script is this:
------------
BEGIN{}
/<search string>/
{ <put your code in here> }
/<search string 2>/
{ <put your code in here> }
------------

And that's it!

Now, if you then run this awk file
(using "awk -f your.awk file.to.process > file.to.output")....

suppose your file to process is 3 lines of data:
ABC
DEF
GHI


When you run the command, the process is basically like this:
process reads in line 'ABC', goes to awk.file and says 'begin.....ok......search string...ok....search string 2....ok....finish'
then, process reads in line 'DEF', goes to awk.file and says 'begin.....ok.....search string....ok....search string 2......ok....finish'
then ......
contine until the end of the input file!

That's all it does!


So now, you can do stuff like:


BEGIN{}
/ABC/{
printf("go away")
}

/GHI/{
printf("hello again")
}


So basically, awk scripts treat each line in the file like a record.
The search strings are basically - is 'ABC' in this line? - if it is, carry out the code underneath

Within the record, each record has fields.
In awk, fields are delimited by space (which you can set to something else if req.)
So this line
"grep 'who is your daddy?'"
will have five fields
$1 = grep
$2 = 'who
$3 = is
$4 = your
$5 = daddy?'

$0 is the whole record.

So you can do stuff like
printf($4) and all that kind of stuff.

That's it.