Programming Language Concepts - Perl Study Cards

Front 1

What should you put at the top of a perl program to make it a perl program in unix and also allow it to print warnings.

Back 1

#!/usr/bin/perl -w

Front 2

What is the general definition of a scalar in Perl. What are some examples of a scalar?

Back 2

As a general rule, when perl has just one of something, that is a scalar.

Most scallars are either a number (like 255 or 3.25e20) or a string of characters (like "hello" or the getysburg address)

Front 3

What format are numbers stored as internally in Perl?

Back 3

Perl computes ALL numbers with double precision floating-point values. Double precision is defined by the C compiler for the machine that compiled your perl interpretter. Ussually has 15 digits worth of precision and a range e-100 to e+100. This means that there are no integer values as far as perl is concerned.

Front 4

How might you clarify the human reading of a really long integer in Perl?

Back 4

Perl allows you to place underscore '_' characters in a number, these will no be seen by the perl compiler.

example) 61_298_040_283_768

Front 5

How do you specify a Nondecimal Integer literal in base 10, binary, and hex respecively?

Back 5

0377 #377 octal, same as 255 decimal
0xff #FF hex, also 255 decimal
0b11111111 #also 255 decimal, in binary

Front 6

What is 10.5 % 3.2 in Perl?

Back 6

it is 1, because 10.5 and 3.2 are converted to 10 and 3 respectively.

Front 7

What is the exponential operator in Perl?

Back 7

The exponential operator is **, usage is 2**3 (should equal 8).

Front 8

What does perl use to determine when it has reached the end of a string?

Back 8

Perl uses length counting, not a null character, to determine the length of a string. Null in no way indicates the length of a string in perl.

Front 9

What is a single quoted string literal?

Back 9

A single quoted string literal is a sequence of characters enclosed in single quotes. Any character except a backslash and single quote stands for itself inside a string.

Examples:
'fred'
'Don't'
'hello\n' #oddly prints hello\n

Front 10

What is a double quoted string literal?

Back 10

A double quoted string literal is a series of characters enclosed in double quotes. However, now the backslash takes on its full power to specify other characters.

"barney" #just the same as 'barney'
"hello world/n" #hello world and a newline
"coke\tsprite" #coke, a tab, and sprite

Front 11

Is "Barney" the same as 'barney' to Perl?

Back 11

yes, it is like the difference between 255 and octal 377, same thing, just looks different.

Front 12

String concatenation, how is it done?

Back 12

String concatenation is done with the . symbol.

Examples:
"Hello"."world" #same as "Helloworld"
"hello".' '."world" #same as 'hello world'
'hello world'."\n" #same as "hello world\n"

Front 13

What is the string repetition operator? How does it work?

Back 13

The string repetition operator repeats a string a certain number of times.

Example:
"fred" x 3 #"fredfredfred"
"barney" x (4+1) #barneybarneybarneybarneybarney"
5 x 4 # is really "5" x 4, which is "5555"

Front 14

How are scalar variables in perl referenced?

Back 14

Scalar variables in perl are always referenced with the leading $. The shell sometimes you don't use the $VariableName to get the value.

Front 15

What is variable interpolation?
Specifically?
What happens when you use a variable that has not been filled in?
What happens when you mix and match double and single quoted variables?
How do you put a variable into a statement and NOT make the variable evaluate as a variable?

Back 15

If a variable has not been given a article the variable returns the empty string.
It appears that it always become double quoted.
print 'The name is $fred'."\n"; == "The name is \$fred.\n"; # prints a dollar sign

Front 16

What type of mathematical operator precedence does perl follow?

Back 16

For mathematical operators, Perl will evaluate them according to normal mathematical rules. For other operators, consult the table on p32.

Front 17

What types of Algebraic Operators does Perl use?
What types of String Operators does Perl use?

Back 17

Comparison Numeric String
-----------------------------------------
Equal == eq
Not Equal != ne
Less Than < lt
Greater Than > gt
Less than & Equal <= le
Greater Than & Equal >= ge

Front 18

how do you use the if Control Structure in Perl? What is different about the curly braces compared to C++ or Java? What value must be in the evaluation portion of the if?

Back 18

if(Evaluation){
#if true
}else {
#if false
}

Evaluation must be a scalar. Unlike in C++ and Java, the curly braces are required statements.

Front 19

What are the four existent scalar values that are false?

Back 19

undef, 0, '', or '0' are the only false scalar values. All other scalar values are true.

Front 20

How do you get user input in perl?

Back 20

Use the line input operator <STDIN>. The <STDIN> operator gets the next line of text from your system, if there is nothing waiting to be read, it will wait for the next line of input. A newline is generally appended to the end of the line by <STDIN>

Example:
$line = <STDIN>; #puts line of text in $line

Front 21

How function removes the newline character from a string? What does it return? What does it do if there is no newline character?

Back 21

use the chomp function. it will return the number of characters removed. If there is no newline, it does nothing and returns nothing.

Example:
chomp($text = <STDIN>); #gets next line w/out newline character
chomp $text; #for some reason this works as well.

Front 22

What are the specifications for the while control structure?

Back 22

while (THIS_VALUE_IS_TRUE){
#Execute this
}

like if() the block curly braces are required.

Front 23

What is the undef Value?

Back 23

The undef Value is most common the value assigned to variables when they have been declared but have not been given a value. It will evaluate to zero or the empty string depending on context.

Front 24

What does the defined function do? What is it ussually used for?

Back 24

The defined function returns true for all variables that are not set to undef. Because when reading a file, the end of file is marked by returning the undef function, the defined function can be used to determine that no input was available.

Example:
defined($madonna) #true if defined, false if $madone contains undef.