Arrays, Built-in Variables, and Built-in Functions in AWK

when first referenced and are initialized to null. New expressions are composed with the following operators in order of increasing precedence. assignment = += -= *= /= %= ^= conditional ? : logical or || logical and && array membership in matching ~ !~ relational < > <= >= == != concatenation (no explicit operator) add ops + - mul ops * / % unary + - logical not ! exponentiation ^ inc and dec ++ -- (both post and pre) field $ Assignment, conditional and exponentiation associate right to left; the other operators associate left to right. Any expression can be parenthesized. 6. Arrays Awk provides one‐dimensional arrays. Array elements are expressed as array[expr]. Expr is internally converted to string type, so, for example, A[1] and A["1"] are the same element and the actual index is "1". Ar‐ rays indexed by strings are called associative arrays. Initially an array is empty; elements exist when first accessed. An expression, expr in array evaluates to 1 if array[expr] exists, else to 0. There is a form of the for statement that loops over each index of an array. for ( var in array ) statement sets var to each index of array and executes statement. The order that var transverses the indices of array is not defined. The statement, delete array[expr], causes array[expr] not to exist. mawk supports the delete array feature, which deletes all elements of array. Multidimensional arrays are synthesized with concatenation using the built‐in variable SUBSEP. array[expr1,expr2] is equivalent to array[expr1 SUBSEP expr2]. Testing for a multidimensional element uses a parenthe‐ sized index, such as if ( (i, j) in A ) print A[i, j] 7. Builtin‐variables The following variables are built‐in and initialized before program execution. ARGC number of command line arguments. ARGV array of command line arguments, 0..ARGC-1. CONVFMT format for internal conversion of numbers to string, initially = "%.6g". ENVIRON array indexed by environment variables. An environment string, var=value is stored as ENVIRON[var] = value. FILENAME name of the current input file. FNR current record number in FILENAME. FS splits records into fields as a regular expression. NF number of fields in the current record. NR current record number in the total input stream. OFMT format for printing numbers; initially = "%.6g". OFS inserted between fields on output, initially = " ". ORS terminates each record on output, initially = "\n". RLENGTH length set by the last call to the built‐in function, match(). RS input record separator, initially = "\n". RSTART index set by the last call to match(). SUBSEP used to build multiple array subscripts, initially = "\034". 8. Built‐in functions String functions gsub(r,s,t) gsub(r,s) Global substitution, every match of regular expression r in variable t is replaced by string s. The number of replacements is returned. If t is omitted, $0 is used. An & in the replacement string s is replaced by the matched substring of t. \& and \\ put literal & and \, respectively, in the replacement string. index(s,t) If t is a substring of s, then the position where t starts is returned, else 0 is returned.

This section explains arrays in AWK, including how elements are accessed, the concept of associative arrays, and looping through array indices with the `for` statement. It also covers deleting array elements. Multidimensional arrays are synthesized using `SUBSEP`. The section then details built-in variables like `ARGC`, `ARGV`, `CONVFMT`, `ENVIRON`, `FILENAME`, `FNR`, `FS`, `NF`, `NR`, `OFMT`, `OFS`, `ORS`, `RLENGTH`, `RS`, `RSTART`, and `SUBSEP`, explaining their purpose and initial values. Finally, it introduces built-in string functions such as `gsub` and `index` with explanations of their behavior and parameters.