6.23. User-Defined Functions (nawk)
A user-defined function can be placed anywhere in the script that a pattern action rule can be placed.
FORMAT
function name ( parameter, parameter, parameter, ... ) {
statements
return expression
(The return statement and expression are optional)
}
Variables are passed by value and are local to the function where they are used. Only copies of the variables are used. Arrays are passed by address or by reference, so array elements can be directly changed within the function. Any variable used within the function that has not been passed in the parameter list is considered a global variable; that is, it is visible to the entire nawk program, and if changed in the function, is changed throughout the program. The only way to provide local variables within a function is to include them in the parameter list. Such parameters are usually placed at the end of the list. If there is not a formal parameter provided in the function call, the parameter is initially set to null. The return statement returns control and possibly a value to the caller.
Example 6.160.
(The Command Line Display of grades File before Sort)
% cat grades
44 55 66 22 77 99
100 22 77 99 33 66
55 66 100 99 88 45
(The Script)
% cat sorter.sc
# Scriptname: sorter
# It sorts numbers in ascending order
1 function sort ( scores, num_elements, temp, i, j ) {
# temp, i, and j will be local and private,
# with an initial value of null.
2 for( i = 2; i <= num_elements ; ++i ) {
3 for ( j = i; scores [j–1] > scores[j]; ––j ){
temp = scores[j]
scores[j] = scores[j–1]
scores[j–1] = temp
}
4 }
5 }
6 {for ( i = 1; i <= NF; i++)
grades[i]=$i
7 sort(grades, NF) # Two arguments are passed
8 for( j = 1; j <= NF; ++j )
printf( "%d ", grades[j] )
printf("\n")
}
(After the Sort)
% nawk –f sorter.sc grades
22 44 55 66 77 99
22 33 66 77 99 100
45 55 66 88 99 100
EXPLANATION
The function called sort is defined. The function can be defined anywhere in the script. All variables, except those passed as parameters, are global in scope. If changed in the function, they will be changed throughout the nawk script. Arrays are passed by reference. Five formal arguments are enclosed within the parentheses. The array scores will be passed by reference, so that if any of the elements of the array are modified within the function, the original array will be modified. The variable num_elements is a local variable, a copy of the original. The variables temp, i, and j are local variables in the function. The outer for loop will iterate through an array of numbers, as long as there are at least two numbers to compare. The inner for loop will compare the current number with the previous number, scores[j –1]). If the previous array element is larger than the current one, temp will be assigned the value of the current array element, and the current array element will be assigned the value of the previous element. The outer loop block ends. This is the end of the function definition. The first action block of the script starts here. The for loop iterates through each field of the current record, creating an array of numbers. The sort function is called, passing the array of numbers from the current record and the number of fields in the current record. When the sort function has completed, program control starts here. The for loop prints the elements in the sorted array.
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