Introduction to Perl Based Loosely on Learning Perl, 2nd Ed by Randal Schwartz & Tom Christiansen
Cogs, Flywheels and Rubberbands (version 1.2) IT Computing Services February 1998 February 2002 Part I expression, decisions and flowcharting if while 3 House cleaning item: print ( … ); vs. printf … ;
• Up until now, I have been using print in the following way: print (”blah”); but you can also omit the parentheses: print ”blah”; which is nice and simple.
I would prefer to include the parentheses because it is more C-like and less ambiguous (in my mind) about what the parameters are.
• I would also like to begin using the printf() function instead of the print() function. As you will see later, printf() is more flexible than print() . 4 House Cleaning: Other Operators I “Neglected” to Mention
• I had talked about operators last time such as “+”, “-”, “.” and “=”. There is a class of operators that I have left out: the relational operators:
Numerical Relational Operators == equal != not equal < less than > more than <= less than or equal to >= greater than or equal to 5 … Operators (cont’d)
String Relational Operators eq equal ne not equal lt less than gt more than le less than or equal to ge greater than or equal to
and there will be more.
• A relational operator makes a comparison between two values and returns a TRUE or FALSE . 6 Examples of Using Relational Operators
• Suppose $a=3, $b=4, $i=1 and $first=“Bill”, $last=“Clinton”, $full=“Bill Clinton”
expression result expression result $a == 3 TRUE $b >= $a TRUE $a > $b FALSE $b-$a == $i TRUE
expression result $first eq “Bill” TRUE $first . “ “ . “$last eq “Bill Clinton” TRUE $last < $first FALSE 7 What is truth?
• In other words, what is considered TRUE?
TRUE is defined in Perl (as well as many other languages) as a non-zero value:
expression result 1 == 1 TRUE 1 == 0 FALSE
1 TRUE 3 TRUE 0 FALSE 8 A Slight but Important Diversion: Flowcharts
• Give us a visual representation or mapping of how a program flows. • Flowcharting is just an interesting way of thinking how to program. • It’s a useful tool especially for learning, but as you will soon see, it can’t be scaled up into full-blown programs. You can try... 9 A Process Statement m a r g o r p e h t
do something f o w o l f
• With process statements, we do something. A good example of one is the printf statement: printf (“Hello there.\n”); 10 Grouping: Braces around Bodies of Boxes • There will be times when we want to group many m a
process statements. We r
pull out g o usually do this because we cigarette r p
are grouping smaller e h t light processes to form a larger f o cigarette
process. And to do this, we w o l usually use curly-braces: puff f { and } . cigarette 11 The Decision Box 1 2 • Up until now every statement y y t t i that we’ve seen and used has i l l i i b b i been one path. So, the i s s s s o programs we’ve written have o p p been like a long river: very w w o o l linear. Pretty boring. l f f Test • And the Decision Box changes False the flow of a program. This is a lot like a forked river. True 12 Example: A Simple Decison with Outcome • Decisions Should Have Outcomes! A decision without an outcome is not a $name eq F useful decision! “Barry”
T • Notice the elements of the this: Decision and Outcome when TRUE. printf (“Hello Barry.\n”); • What happens if the decision is not TRUE? 13 A Decision w/ Outcome in Perl
• Notice the extra syntax reminders added here! Notice the curly-braces for the bodies of process statements. Notice also the $name eq F parentheses that surround the test. Always “Barry” surround the test with parentheses! ( ) T • A decision with an outcome in Perl takes the form: if (expr) {body} printf (“Hello Barry.\n”); • And as in our example: if ($name eq “Barry”) { printf (“Hello Barry.\n”); } A More Complicated Decision: 14 Two Outcomes
What does this do?
$name eq F “Barry”
T printf (“You’re not Barry.\n”); printf (“Hello Barry.\n”); ... And now in Perl 15 if (expr) {body1} else {body2}
$name eq F ( “Barry” ) T printf (“You’re not Barry.\n”); printf (“Hello Barry.\n”);
• and we write this in Perl as:
if ($name eq “Barry”) { printf (“Hello Barry.\n”); } else { printf (“You’re not Barry.\n”); } 16 Beware of the Two Types of Equality • This $a==3 This tests for equality. is different from $a=3 This assigns the value of 3 to $a .
• So, don’t ever write:
if ($a=3) { printf (“a is now 3!\n”); }
( Make this $a==3 instead! ) (I made this dumb mistake when writing this presentation.) 17 Beware of Another Kind of Equality • This $name==“Barry” This tests for equality for numbers and is wrong!. is different from $name eq “Barry” This assigns the equality in strings and is correct.
• So, don’t ever write:
if ($name==“Barry”) { printf (“Hello Barry!\n”); }
(I made this mistake too when writing this presentation.) 18 Excercises with if’s
• 1) Write the code fragment for this rather contrived and meaningless example:
T $catch printf ( !=22 “Rock.\n”)
F printf ( “Hard place.\n”); And how does it work? What happens if $catch is 11? 22? 0? 19 Excercises with if’s (cont’d)
• 2) What if you did not have else in your language like some older computer languages? Rewrite this code fragment:
if ($name eq “Barry”) { printf (“Hi Barry.\n”); } else { printf (“You’re not Barry.\n”); }
• 3) Suppose you had this variable called $indian, and you had to printf the word “One” if the value of $indian is 1 and “Two” if the value of $indian is 2 and so on up to 5. Write the code fragment for that. 20 A New & More Interesting Kind of Decision Structure
$num_indian =0 T printf ( $num_indian $num_indian “$num_indian != 10 ++ ( ){ little indian.\n}”)
• What will this do? F printf ( “Ten little indian boys.\n”) • Here is another way which we can diverge from the linear way of programming. The if statement allowed us to just branch off to another possible flow. This new way allows us to return to the decision again some time later. This is known as a loop. 21 The while (expr) {body} Loop
$num_indian =0 T printf ( $num_indian $num_indian “$num_indian != 10 ++ ( ){ little indian.\}n”)
F printf ( “Ten little • And this type of loop is known indian boys.\n”) as the while loop. Here it is in Perl:
• $num_indian=0; while ($num_indian!=10) { $num_indian++; printf (“$num_indian little indian.\n); } printf (“Ten little indian boys.”); Four Areas of Interest of the 22 while (expr) {body} Loop
Area 1 Initialization Body of Loop T Area 4 Area 2 Area 3 Increment or Test ) Do something Update State ( { of Loop }
F Outside of Loop
• Area 1: Before the loop stars, start off with some set-up or initial conditions. Area 2: Make a test, go into loop (again) or continue on? Area 3: Do something (whatever you wrote the loop for) Area 4: Increment or change the state of the loop. Outside the Loop: Loop finishes. Not very interesting. Sometimes needed to repackage the answer for another part of the code. Four Areas of Interest of the 23 while (expr) {body} Loop (cont’d)
Area 1 Initialization Body of Loop T Area 4 Area 2 Area 3 Increment or Test ) Do something Update State ( { of Loop }
F Outside of Loop
• Sometimes Area 4 might seem like it can be included in Area 3, but it serves an important enough part that it can stand on its own. Area 4 is like the housekeeping part of the loop.
You could say that Area 3 does all the “important” work of the loop and Area 4 has the responsibility of keeping the loop going. 24 There’s more than one way to write a loop...
• The example of the Ten-little-indian program doesn’t fit our example. Notice that the increment is at the beginning of the loop; and my model of loops puts it at the end.
Let’s rewrite the loop so that it does follow that form. There’s a good reason for this. I’ll say more 25 A Matter of Style
#!/opt/local/bin/perl I (personally) like to write my $num_indian=0; perl scripts this way. This follows a style in C called while ($num_indian!=10) { K&R style. Notice the $num_indian++; printf (“$num_indian little indian.\n); beginning brace begins on the end of the line of the test and } the closing brace lines up with printf (“Ten little indian boys.”); the line of the test. It’s great at conserving screen space.
#!/opt/local/bin/perl $num_indian=0; while ($num_indian!=10) This is also a perfectly valid way to indent your programs. And this is { more logical for beginners. A $num_indian++; beginning brace begins on the next printf (“$num_indian little indian.\n); line of a loop and the closing brace lines up with the beginning brace. } But you’ll see me use K&R style. printf (“Ten little indian boys.”); Sorry. 26 A Matter of Style II
Style is not only curly-braces. Indentation is helps you keep track of multiple loops. We won’t get into this now, but I want to sufficiently scare you into good habits. Notice the two loops (one inside the other) and how easy it is to see them: Main Program $minute=10; while ($minute!=0) { Outer Loop printf (“Beginning minute $minute .\n”); second=60; while ($second!=0) { Inner Loop printf (“\tsecond $second .\n”); $second--; } print (“Ending minute $minute .\n”); $minute--; }
Indent your programs too!!! Or you’ll be sorry... 27 while (expr) {body} Benchpresses
1) Write the code fragment for this flowchart:
printf (“The shuttle is preparing for take off.\n”); $time= 30; T printf (“T minus $time!=0 $time and $time--; ( ){ counting.\n); } F What new concept is in printf (“Blast this fragment/flowchart? off!\n); 28 while (expr) {body} Benchpresses: More Reps
• 2) Have the user enter in ten numbers and have the computer compute the sum and average of those ten numbers.
• 3) Do the same except the user gets to choose how many numbers the computer must sum and average. I’m leaving this one vague. 29 All You Need is while and if
• You can do just about everything you want with only while (expr) {body} and if (expr) {body} constructs!
• All other loop constructs we will be talking about are variants of or are made of different combinations of while and if. Sometimes it takes more work and you have to be creative about how you code some situations… , but it can be done! We will talk about the variants soon. 30 Suggested Readings
• Chapter 4: Control Structures (2nd Ed)
sections: Statement Blocks (pg. 58) The if/unless Statement (pgs. 59 - 60) Skip the part about unless; we’ll come back later. The while/until Statement (pg. 61) Skip the part about until; we’ll come back later.
Skip the section on for and foreach, that’s next time & later. 31 Suggested Readings
• Chapter 2: Scalar Data (3rd Ed)
sections: Comparison Operators (pg. 33) if Control Structure (pg. 34) while Control Structure (pg. 37) Boolean Values (pg. 34) Getting User Input (pg. 35) 32 Suggested Homework Send solutions (code and output) to [email protected] • PROBLEM 0: Type in every program in this presentation and try them out. 1 little indian • PROBLEMS 1, 2, 3: Do in Learning Perl, Chapter 4: 2 little indian 3 little indian pg. 65 (2nd ed.) 4 little indian 5 little indian 6 little indian • PROBLEM 4: There was a small problem with the 7 little indian 8 little indian Ten-Little-Indian program. It would count indians in 9 little indian 10 little indian numbers (1, 2, 3...) and at the end, it would say “Ten Ten little indian boys
little indian boys.” That’s inconsistent. 1 little indian 2 little indian 3 little indian a) Easy(?): Change the program so that at the last line 4 little indian 5 little indian “Ten little indian boys.” is now consistent with the 6 little indian 7 little indian counting part of the program. Think about what I’m 8 little indian 9 little indian asking you to do. Keep a spare copy of the original program before you 10 little indian do (a) because problem (b) has nothing to do with (a). Fix last line! 10 little indian boys 33 Suggested Homework (cont’d)
One little indian Fix these! Two little indian Three little indian b) More challenging: Change the program so that the Four little indian Five little indian counting is now consistent with the last line “Ten little Six little indian Seven little indian indian boys”. You’ll need a lot of if statements inside Eight little indian Nine little indian the while loop. Ten little indian boys Make an extra spare copy of your finished (b); parts (c) and (d) depend on (b). Ten little indian boys
One little indian c) Notice that “Ten little indian boys” is repeated twice. Two little indian Three little indian That’s not how the song goes. Modify the test in the Four little indian Five little indian loop so that you only get the last line once now. Modify Six little indian Seven little indian the test. Eight little indian Nine little indian Ten little indian boys 34 Suggested Homework (cont’d) One little indian Two little indian d) There is another way to print “Ten little indian Three little indian Four little indian boys” once and it has nothing to do with the loop Five little indian Six little indian or the test. How do you modify the program? Seven little indian Eight little indian Modify it. Nine little indian Ten little indian boys • PROBLEM 5: a) Write a program that prints out 1 to 10 (eg: 1,2,3…) and prints out the value as it is counting. Use a while loop. b) Now have it print out from 2 to 20 by 2’s (eg: 2,4,8,…) without changing the test condition. You may only change one line of code. Hint: try to do (c) first. c) Now have it print out from 0.5 to 5.0 by 1/2’s (eg: 0.5,1.0,1.5,…) without changing the test condition. You may only change one line of code. 35 Suggested Homework (cont’d)
• PROBLEM 6: CHALLENGE (Make brownie points with the instructor! :)
The computer picks a random number between 1 and 100. Make a user try to guess the computer’s random number. The program will continue in the loop as long as the user hasn’t guessed the correct random number.
If the user’s number is too high, the computer will tell the user “Lower” and vice-versa. After the user guesses the computer’s random number, the program will congratulate the user and terminate. 36 Suggested Homework (cont’d)
Hint: To pick a random number in Perl: $random_number= int (rand(1)*100)+ 1 ;
Go Applesoft BASIC!!! :) Hint: The overall structure of the program could go something like this.
Pick a random number $user_is_correct= 0; # “0” means incorrect; # we assume the user is wrong while ($user_is_correct==0) { # while (user’s guess is not correct) Get guess from user. Decide if the guess is correct or not and modify $user_is_correct. Give user feedback if his/her guess is too high or low. } Congratulate the user for winning End Part I