CSC 4630

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CSC 4630

• Meeting 7
• February 7, 2007

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More Scripting Languages

• awk, named for Aho, Weinberger, Kernighan
• Script is embedded in a nested looping control
  structure
• for each pattern action do
• for each input file line do
• if pattern matches line then action

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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

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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

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awks 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

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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

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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)

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Examples

• print NR, NF
• print NR, 0
• print NF
• NR 10
• NF ! 3
• NF gt 4

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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

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Comparison Operators

• lt less than
• gt greater than
• lt less than or equal to
• gt greater than or equal to
• equal to
• ! not equal to
• matches
• ! does not match

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Regular Expressions

• Enclosed in / /
• Matches in entire line
• Field match specified as 3 /Ab/, for example
• Special symbols
• \ . ? ( )

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Examples

• /Asia/
• /./
• /a\/
• /\t/
• 2 ! /0-9/
• /(applecherry) (pietart)/ (note space)

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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

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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

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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

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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

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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 xgt0
• sqrt(x) square root of x, so x gt 0
• int(x) truncates fractional part
• rand(x) returns a random number in 0,1
• srand(x) sets the seed for rand to x

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Strings

• Literal values enclosed in double quotes
• abc Wildcats rule 20 bananas
• Concatenation represented by juxtaposition
• s Villanova
• t Wildcats
• print s t

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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 0ltpltlength(s), returns empty
  string otherwise
• substr(s,p,n) returns substring of length n
  starting at position p

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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

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Control Structures

• if (ltexpressiongt) lts1gt else lts2gt
• ltexpressiongt can be any expression true is
  defined to be non-zero or non-null
• lts1gt and lts2gt can be any group of statements
• Note the critical parentheses that separate the
  conditional expression from lts1gt

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Control Structures (2)

• while (ltexpressiongt) lts1gt
• Same rules as for if-then-else

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Control Structures (3)

• for (lte1gtlte2gtlte3gt) lts1gt is equivalent to
• lte1gt while (lte2gt) lts1gtlte3gt
• for (k in ltarraygt) lts1gt 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.

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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