1. Lecture 14 Programming with awk II

2. for loop This is a counted loop
Will execute until the counter reaches the target value
Can increment (count up) or decrement (count down)
for also works with the elements of an array

3. Example BEGIN{ FS=":" } { if ($1 ~ /^root$|^uucp$/) line = ""
for (i = NF ; i> 0 ; i--)
line = line ":" $i
print line }

4. while loop The while loop is an example of conditional execution
The loop cycles as long as the condition specified is true
A while loop always checks to see if it should execute
Multiple actions must be enclosed in { }

5. Arrays awk handles one-dimensional array
No array declarations are necessary
The index of an array can be anything, even a string
Array elements are automatically initialized

6. Example Consider the following file f1 (name and age of a person)
Bart 10
Homer 38
Lisa 8
$awk '{age[$1]=$2}
END {for(i in age) print age[i]}' f1
will print the age of each person

7. Example BEGIN { FS = ":" } { fullname = "" ; x = 0 ;
array_length = split($5, name_arr," ") ;
while ( x++ <= array_length ) {
if (x < array_length)
name_arr[x] = name_arr[x] "_" ;
fullname = fullname name_arr[x] ; }
printf fullname

8. Built-in Functions
Built-in functions perform arithmetic and string operations.
Arithmetic functions:
int(x) returns integer value of x
sqrt(x) returns square root of x

9. Built-in Functions String functions
length() returns length of complete line
length(x) returns length of x
awk ‘length($2) < 10’ f1
substr(s, m) returns remaining string from position m in string s
substr(s, m, n) returns portion of string of length n, starting from position m in string s
awk ‘{print substr($1, 6, 2)}’ file

10. Built-in Functions String functions
split(s, arr, ch) splits string s into array arr using ch as delimter; return number of fields
system(“cmd”) runs UNIX command cmd and returns its exit status
e.g.
BEGIN {
system(“tput clear”)
system(“date”) }

11. User-Defined Functions
Complicated awk programs can often be simplified by defining your own functions.
Definitions of functions can appear anywhere in an awk program.
awk reads the entire program before starting to execute any of it. There is no need to put the definition of a function before all uses of the function.
Syntax:
function name(parameter-list)
{ body-of-function }

12. User-Defined Functions
Example:
function myprint(num)
{ printf "%d\n", num }
$3 > 0 { myprint($3) }

13. A Sample Data File Bird, Tweety 81 82 79 89 Cat, Sylvester 93 97 89 91
NAME TEST1 TEST2 TEST3 TEST4
Bird, Tweety
Cat, Sylvester
Coyote, Wiley
Duck, Daffy
Duck, Donald

14. Suppose we want to display only the first names of the students whose test marks are saved in this file.
- first try:
awk ‘{ print NR, $2 }’ class.data
- output:
1 TEST1
2 Tweety
3 Sylvest
4 Wiley
5 Daffy
6 Donald 7

15. Specifically we instruct awk to process only those lines which begin with an upper case letter awk ‘/^[A-Z]/ { print NR, $2 }’ class.data
2 Tweety
3 Sylvester
4 Wiley
5 Daffy
6 Donald

16. Display the current line number after subtracting a 1
Display the current line number after subtracting a 1. awk ‘/^[A-Z]/ { print NR-1, $2 }’ class.data
1 Tweety
2 Sylvester
3 Wiley
4 Daffy
5 Donald

17. To demonstrate awk as a full programming language we tackle the following problem:
Compute the average mark earned by each student in the file.
Display the student’s first name followed by the student’s average.

18. if ( NR > 1 ) # skip the titles line
# averages.awk {
total = 0
count = 0
i = 3
if ( NR > 1 ) # skip the titles line
while ( i <= NF ) # add marks in this line
total += $i
count++
i++
} # while
if ( count > 0 ) # prevent divide by zero
avrg = total/count
print ( $2, avrg ) # display the answer }

19. here is the awk command line: awk –f averages.awk class.data
The Output
Tweety 82.75
Sylvester 92.5
Wiley 81
Daffy 78.25
Donald 87.5

20. A Useful Link