GAWK: Effective AWK Programming

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156 GAWK: Effective AWK Programming8.2.3 Calling User-Defined FunctionsCalling a function means causing the function to run and do its job. A function call is anexpression and its value is the value returned by the function.A function call consists of the function name followed by the arguments in parentheses.awk expressions are what you write in the call for the arguments. Each time the call isexecuted, these expressions are evaluated, and the values are the actual arguments. Forexample, here is a call to foo with three arguments (the first being a string concatenation):foo(x y, "lose", 4 * z)Caution: Whitespace characters (spaces and TABs) are not allowed between the functionname and the open-parenthesis of the argument list. If you write whitespace by mistake,awk might think that you mean to concatenate a variable with an expression in parentheses.However, it notices that you used a function name and not a variable name, and reports anerror.When a function is called, it is given a copy of the values of its arguments. This isknown as call by value. The caller may use a variable as the expression for the argument,but the called function does not know this—it only knows what value the argument had.For example, if you write the following code:foo = "bar"z = myfunc(foo)then you should not think of the argument to myfunc as being “the variable foo.” Instead,think of the argument as the string value "bar". If the function myfunc alters the valuesof its local variables, this has no effect on any other variables. Thus, if myfunc does this:function myfunc(str){print strstr = "zzz"print str}to change its first argument variable str, it does not change the value of foo in the caller.The role of foo in calling myfunc ended when its value ("bar") was computed. If stralso exists outside of myfunc, the function body cannot alter this outer value, because it isshadowed during the execution of myfunc and cannot be seen or changed from there.However, when arrays are the parameters to functions, they are not copied. Instead, thearray itself is made available for direct manipulation by the function. This is usually calledcall by reference. Changes made to an array parameter inside the body of a function arevisible outside that function.NOTE: Changing an array parameter inside a function can be very dangerousif you do not watch what you are doing. For example:function changeit(array, ind, nvalue){array[ind] = nvalue}BEGIN {
Chapter 8: Functions 157}a[1] = 1; a[2] = 2; a[3] = 3changeit(a, 2, "two")printf "a[1] = %s, a[2] = %s, a[3] = %s\n",a[1], a[2], a[3]prints ‘a[1] = 1, a[2] = two, a[3] = 3’, because changeit stores "two" in thesecond element of a.Some awk implementations allow you to call a function that has not been defined. Theyonly report a problem at runtime when the program actually tries to call the function. Forexample:BEGIN {if (0)foo()elsebar()}function bar() { ... }# note that ‘foo’ is not definedBecause the ‘if’ statement will never be true, it is not really a problem that foo has notbeen defined. Usually, though, it is a problem if a program calls an undefined function.If ‘--lint’ is specified (see Section 11.2 [Command-Line Options], page 177), gawkreports calls to undefined functions.Some awk implementations generate a runtime error if you use the next statement (seeSection 6.4.8 [The next Statement], page 108) inside a user-defined function. gawk does nothave this limitation.8.2.4 The return StatementThe body of a user-defined function can contain a return statement. This statement returnscontrol to the calling part of the awk program. It can also be used to return a value for usein the rest of the awk program. It looks like this:return [expression]The expression part is optional. If it is omitted, then the returned value is undefined,and therefore, unpredictable.A return statement with no value expression is assumed at the end of every functiondefinition. So if control reaches the end of the function body, then the function returns anunpredictable value. awk does not warn you if you use the return value of such a function.Sometimes, you want to write a function for what it does, not for what it returns. Sucha function corresponds to a void function in C or to a procedure in Pascal. Thus, it maybe appropriate to not return any value; simply bear in mind that if you use the return valueof such a function, you do so at your own risk.The following is an example of a user-defined function that returns a value for the largestnumber among the elements of an array:function maxelt(vec,{i, ret)
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GAWK: Effective AWK Programming

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