### 6.4 Function Calls ¶

A function is a name for a particular calculation. This enables you to 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 Built-in Functions for a list of built-in functions and their descriptions. In addition, you can define functions for use in your program. See User-Defined Functions for instructions on how to do this. Finally, `gawk` lets you write functions in C or C++ that may be called from your program (see Writing Extensions for `gawk`).

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 that 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, just write ‘()’ after the function name. The following examples show function calls with and without arguments:

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

CAUTION: Do not put any space between the function name and the opening parenthesis! A user-defined function name looks just like the name of a variable—a space would make the expression look like concatenation of a variable with an expression inside parentheses. With built-in functions, space before the parenthesis is harmless, 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 of which to take the square root:

```sqrt(argument)
```

Some of the built-in functions have one or more optional arguments. If those arguments are not supplied, the functions use a reasonable default value. See Built-in Functions for full details. If arguments are omitted in calls to user-defined functions, then those arguments are treated as local variables. Such local variables act like the empty string if referenced where a string value is required, and like zero if referenced where a numeric value is required (see User-Defined Functions).

As an advanced feature, `gawk` provides indirect function calls, which is a way to choose the function to call at runtime, instead of when you write the source code to your program. We defer discussion of this feature until later; see Indirect Function Calls.

Like every other expression, the function call has a value, often called the return value, which is computed by the function based on the arguments you give it. In this example, the return value of ‘sqrt(argument)’ is the square root of argument. The following program reads numbers, one number per line, and prints 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
Ctrl-d
```

A function can also have side effects, such as assigning values to certain variables or doing I/O. This program shows how the `match()` function (see String-Manipulation Functions) changes the variables `RSTART` and `RLENGTH`:

```{
if (match(\$1, \$2))
print RSTART, RLENGTH
else
print "no match"
}
```

Here is a sample run:

```\$ awk -f matchit.awk
aaccdd  c+
-| 3 2
foo     bar
-| no match
abcdefg e
-| 5 1
```