Linguistic Computing with UNIX Tools - Natural Language Processing and Text Mining - page 238

Note that the output of context is, essentially, eleven times the size of the
input for the example just listed. It may be advisable to incorporate any desired,
subsequent pattern matching for the strings that are printed by context into an
extended version of this program.

Control Structures

awk has two special control structures next and exit . In addition, awk has the
usual control structures: if , for and while .
next is a statement that starts processing the next input line immediately from
the top of the awk program. next is the analogue of the d operator in sed . exit is
a statement that causes awk to terminate immediately. exit is the analogue of the
q operator in sed .
The if statement looks the same as in C [31, p. 55]:
if ( conditional ){ action1 }
else { action2 }
conditional can be any of the types of conditionals we defined above for address
patterns including Boolean combinations of comparison of algebraic expressions in-
cluding the use of variables. If a regular expression / regExpr / is intended to match
or not to match the entire pattern space $0 in conditional , then this has to be de-
noted explicitely using the match-operator ~ . Thus, one has to use $0~/ regExpr / or
$0!~/ regExpr / respectively. The else part of the if statement can be omitted or
can follow on the same line.
Example: The use of a for -statement in connection with an if -statement is
shown in the next example. We shall refer to the following program as
findFourLetterWords . It shows a typical use of for and if , i.e. , looping over all
fields with for , and on condition determined by if taking some action on the fields.

1: #!/bin/sh
2: # findFourLetterWords (awk version)
3: leaveOnlyWords $1 |

4: awk ' { for(f=1;f<=NF;f++) {

5: if($(f)!~/^[A-Za-z][a-z][a-z][a-z]$/) { $(f)="" }

6: }

7: }

8: /[^ ]/ { print NR , $0 }

9: ' - | adjustBlankTabs -

Explanation: The for -loop in line 4 processes every field (addressed as $(f) in
line 5, f the counter variable) from left to right. If a field $(f) does not match the
pattern /^[A-Za-z][a-z][a-z][a-z]$/ , then it is set to the empty string in line 5.
In case the pattern space stays non-white ( /[^ ]/ ) after this procedure, it is printed
in line 8 with a leading line-number NR . Finally, blanks are properly adjusted in the
output by adjustBlankTabs .
The technique how to loop over associative arrays has already been demonstrated
in the listing of the program countFrequencies in Section 12.2.2.
Similar to the for statement, the while statement also looks the same as in C
[31, p. 60]: while ( conditional ){ action } .