Introduction to Programming Basics

1. Introduction to Programming Basics • This review introduces the basic concepts to programming. • Note that most (if not all) modern programming languages support these concepts • This review is intended for students who have little or no programming experience • "Learning Perl" is probably your best option for a reference book (in the syllabus).

2. Assignment operator (=)Variables and Memory In most programming languages, we want to "store" values (integers, characters, strings, etc.) for later use, or for using in a calculation. To store values, we use "variables" with the assignment operator ( = ) In perl, all (or almost all) variables are preceded by a dollar-sign ($). Examples: $test = 32; $value = 4 * 2; $test = $value + 1; $test = $test + 5;

3. Rules for Evaluation $x = 3 + 5; $test = $value + (1 * $x); $test = $test + 5; $sequence = "ATGCCT"; Generally, the expressions on the right-hand-side (RHS) of the assignment operator (=) are evaluated FIRST, from inner-most parentheses, to outer-most parentheses, left-to-right. The resulting value is then assigned to the variable on the left-hand-side (LHS).

4. Rules of Evaluation (cont.) $test = 5; #line 1 $test = $test + 2; #line 2 Because of this rule, it is possible to first read and use the value held within a variable, before it is assigned a new value. For example, in "line 2" above, on the RHS, $test has a value of 5. $test + 2 evaluates to a value of 7 on the RHS. Then 7 is assigned to $test on the LHS. After the completion of "line 2", $test now contains the value of 7 Note that it would be incorrect to put something other than a variable on the LHS (this is NOT algebra): 5 = $x + 2; # INCORRECT

5. Logical Expression • What does it mean if an expression is "true" or "false"? • For example: 2 < 5 • 6 != 4 • Is "45" "true" or "false"?

6. Conditional Operators == "equal" for numerical expressions eq > < != "not equal" ne >= <= zero is "false" (0, 8<2, 5 == 4, 5 != 5, etc) Everything else is "true" (1, 5, 8>2, 5 == 5, etc)

7. Conditional statement • if <condition true> • if <condition> … else … • if <condition> … elsif … else … if($a == 5) { print "$a \n"; $a=6; } $a=0;

8. Conditional statement if($a == 5) { print "$a \n"; $a=6; } else{ print "$a \n"; } $a=0;

9. Conditional statement Note -- code blocks Any number of "if" statements may be strung together in this way Conditional == > < >= <= != true/false (0 == false) if($a == 5) { print "$a \n"; $a=6; } elsif($b > 10) { print "$a \n"; } else { $b=5; } $a=0;

10. repeating statements/loops -- while while(condition true) { statement 1 statement 2 : } Statement (outside of while loop)

11. Formal Statements • Assignment $a = 5; $b = $a + 1; • "for" iterative statement (for-loop) informally: for i = 1 to 5

12. For-loop for(initialize statement;condition;post statement) { # loop body statement 1 statement 2 : } Out of loop statement

13. For-example (Perl) for($i=0;$i<5;$i=$i+1) { $a=5; $j=$i+5; print "$j\n"; } $a=6;

14. Logical Operators Not (!) operator 1 = 1 0 = 0 !1 = 0 !0 = 1 Example: if( !(!0)) evaluates to false if(!(5>4)) evaluates to false

15. Logical operator: && (and) || (or) 1 && 1 == 1 1 && 0 == 0 0 && 1 == 0 0 && 0 == 0 1 || 1 == 1 1 || 0 == 1 0 || 1 == 1 0 || 0 == 0 Example: if(( 5>4) && (0)) || (4 != 5) evaluates to TRUE

16. "call" • Call (function, subroutine, procedure) • Change where the algorithm (program) is running

17. Subroutine Example statement 1 statement 2 subroutine() statement 3 #end program subroutine() statement a statement b return()

18. Subroutine Example -- pass by value statement 1 statement 2 subroutine(x) statement 3 #end program subroutine() newValue = x statement a statement b return()

19. Subroutine Example -- return statement 1 statement 2 y=subroutine(x) statement 3 #end program subroutine() newValue=x statement a statement b return(z)

20. Subroutine #!/usr/bin/perl $a = 5; $b = 6; print "a = $a\n"; if($a==10) { $a=$b; } print_hello(); if($b == 6) { print "b = $b\n"; } # END of program sub print_hello { $i=11; print "Now in print_hello subroutine\n"; if($i==11) { $i=0; } return(); }

21. Passing a value "in" #!/usr/bin/perl $a = 5; $b = 6; print "a = $a\n"; if($a==10) { $a=$b; } print_hello($a); if($b == 6) { print "b = $b\n"; } # END of program sub print_hello { $i=$_[0] print "Now in print_hello subroutine\n"; if($i==5) { print "$i = $i\n"; } return(); }

22. Return Value too #!/usr/bin/perl $a = 5; $b = 6; print "a = $a\n"; if($a==10) { $a=$b; } $a = print_hello($a); print "a = $a\n"; if($b == 6) { print "b = $b\n"; } # END of program sub print_hello { $i=$_[0]; print "Now in print_hello subroutine\n"; if($i==5) { print "i = $i\n"; } $i = $i+1; return($i); }