## Function Calls

A function is a name for a particular calculation. Because it has a name, you can ask for it by name at any point in the program. For example, the function sqrt computes the square root of a number.

A fixed set of functions are built-in, which means they are available in every awk program. The sqrt function is one of these. See section Built-in Functions, for a list of built-in functions and their descriptions. In addition, you can define your own functions for use in your program. See section User-defined Functions, for how to do this.

The way to use a function is with a function call expression, which consists of the function name followed immediately by a list of arguments in parentheses. The arguments are expressions which provide the raw materials for the function's calculations. When there is more than one argument, they are separated by commas. If there are no arguments, write just `()' after the function name. Here are some examples:

sqrt(x^2 + y^2)        one argument
atan2(y, x)            two arguments
rand()                 no arguments

Do not put any space between the function name and the open-parenthesis! A user-defined function name looks just like the name of a variable, and space would make the expression look like concatenation of a variable with an expression inside parentheses. Space before the parenthesis is harmless with built-in functions, but it is best not to get into the habit of using space to avoid mistakes with user-defined functions.

Each function expects a particular number of arguments. For example, the sqrt function must be called with a single argument, the number to take the square root of:

sqrt(argument)

Some of the built-in functions allow you to omit the final argument. If you do so, they use a reasonable default. See section Built-in Functions, for full details. If arguments are omitted in calls to user-defined functions, then those arguments are treated as local variables, initialized to the empty string (see section User-defined Functions).

Like every other expression, the function call has a value, which is computed by the function based on the arguments you give it. In this example, the value of `sqrt(argument)' is the square root of argument. A function can also have side effects, such as assigning values to certain variables or doing I/O.

Here is a command to read numbers, one number per line, and print the square root of each one:

\$ awk '{ print "The square root of", \$1, "is", sqrt(\$1) }'
1
-| The square root of 1 is 1
3
-| The square root of 3 is 1.73205
5
-| The square root of 5 is 2.23607
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