CSC 4630

1. CSC 4630 Meeting 7 February 7, 2007

2. More Scripting Languages • awk, named for Aho, Weinberger, Kernighan • Script is embedded in a nested looping control structure: for eachpattern {action} do for each input file line do if pattern matches line then action

3. awk Programs • Generally a sequence of pattern {action} statements • If {action} is missing, matched lines are printed (meaning written to STDOUT) • If pattern is missing, action is carried out for all lines

4. Running awk Programs • Short one, composed at keyboard with little thought • awk ‘program’file1 file2 … • Note that awk can take a sequence of files as input. • Long one,composed in editor • awk –f progfile file1 file2 …

5. awk’s View of Files • Input to awk are text files • Divided into lines • Each line divided into fields by blanks or tabs (the default separator) • Each field referenced by relative number, $1, $2, $3, … • $0 refers to the entire line

6. Examples • awk ‘{print $1}’ names • Print the first field in each line of the names file • awk ‘/M/’ names • Print each line of the names file that contains an upper case M

7. Some Built-In Variables • NR, line number of current line of input (runs sequential over all input files) • NF, number of fields in current line • FS, the field separator • FS = “\t” sets the separator to tab, only • FS = “:” sets the separator to colon • FNR, number of the current line (record) in the current input file (resets when a new input file is opened)

8. Examples • {print NR, NF} • {print NR, $0} • {print $NF} • NR == 10 • NF != 3 • NF > 4

9. Patterns • Special patterns • BEGIN Action is done once before any lines of the input file(s) are read • END Action is done once after the last file has been processed • Relational expressions between strings or numbers • Arguments treated as numbers, if possible

10. Comparison Operators < less than > greater than <= less than or equal to >= greater than or equal to == equal to != not equal to ~ matches !~ does not match

11. Regular Expressions • Enclosed in / / • Matches in entire line • Field match specified as $3 ~ /Ab/, for example • Special symbols \ ^ $ . [ ] * + ? ( ) |

12. Examples • /Asia/ • /^.$/ • /a\$/ • /\t/ • $2 !~ /^[0-9]+$/ • /(apple|cherry) (pie|tart)/ (note space)

13. C Escape Sequences \b backspace \f formfeed \n newline \r carriage return \t tab \ddd character whose ASCII value in octal is ddd \” quotation mark \c any other character c literally

14. Actions • Mini C-like programs • Can extend over several lines • Statements terminated by semicolons or newlines. Statements grouped with braces { }. • Variables are either floating point numbers or strings. • Variables are automatically declared and initialized • Strings initialized to “”, the empty string • Numbers initialized to 0

15. Assignment Statements • Simple version: v = e • Variable or field name assigned value of expression • Assignment operators: v op= e means v = v op e • Legal values of op are + - * / % ^ • Used because interpreted code runs faster

16. Increment Operators • Borrowed from C • Prefix or postfix • ++ or – • Example: x = 3. What is the value of k? • k = x++ • k = ++x • k = x-- • k = --x

17. Arithmetic Functions • sin(x) assumes x is in radians • cos(x) assumes x is in radians • atan2(y,x) range from –pi to pi • exp(x) exponential • log(x) natural logarithm of x, so x>0 • sqrt(x) square root of x, so x >= 0 • int(x) truncates fractional part • rand(x) returns a random number in [0,1] • srand(x) sets the seed for rand to x

18. Strings • Literal values enclosed in double quotes “abc” “Wildcats rule” “20 bananas” • Concatenation represented by juxtaposition s = “Villanova” t = “Wildcats” {print s t}

19. String Functions “Standard” string operations (cf. head, tail, firstfew, lastfew, allbut) • length(s) length of s • length = length($0) • index(s,t) if t is a substring of s return position of first character, return 0 otherwise • substr(s,p) returns substring starting at position p if 0<p<=length(s), returns empty string otherwise • substr(s,p,n) returns substring of length n starting at position p

20. String Functions (2) “Editing” functions • sub(r,s) replace r by s in current record (first occurrence only) • sub(r,s,t) replace r by s in t (first occurrence only) • gsub(r,s) replace r by s in current record (globally) • gsub(r,s,t) replace r by s in t (globally) In all cases, return the number of substitutions

21. Control Structures • if (<expression>) <s1> else <s2> <expression> can be any expression; true is defined to be non-zero or non-null <s1> and <s2> can be any group of statements Note the critical parentheses that separate the conditional expression from <s1>

22. Control Structures (2) • while (<expression>) <s1> Same rules as for if-then-else

23. Control Structures (3) • for (<e1>;<e2>;<e3>) <s1> is equivalent to <e1>; while (<e2>) {<s1>;<e3>} • for (k in <array>) <s1> loops over the subscripts of an array but the order of the subscripts is random. Careful: awk allows general subscripting. Strings can be used as subscripts.

24. Control Structures (4) “Go to” structures • break when executed within a for or while statement, causes an immediate exit • continue when executed within a for or while statement, causes immediate execution of the next iteration • next causes the next line (record) of the input file to be read and the sequence of pattern {action} statements executed on it • exit causes the program to jump to the END pattern, execute it, and stop