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6.3 Output to STDOUT

Perl uses the print and printf functions to write to standard output. Let's look at how they are used.

6.3.1 Using print for Normal Output

We've already used print to display text on standard output. Let's expand on that usage a bit.

The print function takes a list of strings and sends each string to standard output in turn, without any intervening or trailing characters added. What might not be obvious is that print is really just a function that takes a list of arguments, and returns a value like any other function. In other words:

$a = print("hello ", "world", "\n");

would be another way to say hello world . The return value of print is a true or false value, indicating the success of the print. The print nearly always succeeds, unless you get some I/O error, so $a in this case will usually be 1.

Sometimes you'll need to add parentheses to print as shown in the example given below, especially when the first thing you want to print starts with a left parenthesis, as in:

print (2+3),"hello";   # wrong! prints 5, ignores "hello"
print ((2+3),"hello"); # right, prints 5hello
print 2+3,"hello";     # also right, prints 5hello

6.3.2 Using printf for Formatted Output

You may wish to have a little more control over your output than print provides. In fact, you may be accustomed to the formatted output of C's printf function. Fear not: Perl provides a comparable operation with the same name.

The printf function takes a list of arguments (enclosed in optional parentheses, like the print function). The first argument is a format control string, defining how to print the remaining arguments. Here's an example:

 printf "%15s %5d %10.2f\n", $s, $n, $r;

This function prints $s in a 15-character field, then a space, then $n as a decimal integer in a 5-character field, then another space, then $r as a floating-point value with 2 decimal places in a 10-character field, and finally a newline.

Among the many formats supported by Perl's printf() and sprintf() functions are the following commonly used ones:

%%

Percent sign

%c

Character with the given number

%s

String

%d

Signed integer, in decimal

%u

Unsigned integer, in decimal

%o

Unsigned integer, in octal

%x

Unsigned integer, in hexadecimal

%e

Floating-point number, in scientific notation

%f

Floating-point number, in fixed decimal notation

%g

Floating-point number, in %e or %f notation

Between the percent and the format character, you may place one or more of the following flags:

space

Prefix positive number with a space

+

Prefix positive number with a plus sign

-

Left justify within the field

0

Use zeros, not spaces, to right justify

number

Minimum field width

.number

Precision: digits after decimal point for floating-point number, maximum length for string, minimum length for integer











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