Unit 1-scalar expressions and control structures
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The document discusses scalar expressions and control structures in Perl programming. It covers arithmetic, string, comparison, logical and conditional operators. It also covers control flow structures like if/else, loops (while, until, do-while, for), and loop refinements (last, next, redo). Scalar variables can be combined with operators to form expressions, and control structures allow conditional and repetitive execution of code blocks.
Unit 1-scalar expressions and control structures
- 1. SCALAR EXPRESSIONS Scalar data items are combined into expressions using operators. $c=12; $d=3+($c); 1) ARITHMETIC OPERATORS Perl provides usual arithmetic operator including auto-increment and auto-decrement. Operator Example Result Definition + 7 + 7 = 14 Addition - 7 - 7 = 0 Subtraction * 7 * 7 = 49 Multiplication / 7 / 7 = 1 Division ** 7 ** 7 = 823543 Exponents % 7 % 7 = 0 Modulus $c=17; $d=++$c; //increment then assign $c=12; $d=$c ++; //assign then increment Binary arithmetic operation: $a+=3; $a=a+3;
- 2. 2)String Operators In Perl most of the processing is done by using built-in functions and regular expressions. Perl uses period (.) for concatenation of strings The other string operator is (x), used to replicate strings. $a=“hello” x 3 $a=“hellohellohello” $foo.= “ ” Arithmetic Operators used in String Context. 1. Auto-increment This operator can be applied to a variable. If a variable is assigned with string of letters and digits then auto increment operation is applied on the string from rightmost character. eg: $a=‘all12’; print ++$a; result: bmm23 2. Unary minus If this is applied to string which starts with a plus or minus character, then it returns the same string with opposite sign. eg. $name=“sia” -$name=“-sia”
- 3. 3) Comparison operator Values of comparison is returned in numbers. 1--- if true and (“ ”) – if false These are classified into two kinds. One for numbers and other for strings. Numbers: ==,!=,<,>,<=,>=,<=>(comparison operator) Strings: eq,ne,lt,gt,le,ge,cmp 4)Logical operator not ---- ! and ---- && or ---- || print “OKn” if $a<10 and $b<12; 5) Conditional expressions it is the one whose value is chosen from one of the alternatives at runtime depending on the outcome of test. test? True_exp:false_exp eg: $a=($a<0)?0:$a;
- 4. CONTROL STRUCTURES A control structure is a block of programming that analyzes variables and chooses a direction in which to go based on given parameters. The term flow control details the direction the program takes (which way program control "flows"). Blocks: It is a sequence of one or more statements enclosed in curly braces. eg: { $positive=1; $negative=-1; } Conditions: They make use of relational operators. It is a Perl expression which is evaluated in Boolean context. If it evaluates to --- 0 or (“ “) --- condition is false, else it is treated as true. $total > 50 and $total <100 A condition can be negated using ! Operator, we can specify that as !($total > 50 and $total<100)
- 5. CONDITIONAL EXPRESSIONS Conditional Expressions should be in brackets. If-then-else statement: if($total> 0){ print “$totaln” } else { print “wrong total ! n” } If-elsif statement if($total> 70) { $grade=“A”;} elsif($total > 56){ $grade =“B”;} else {$grade=“C”); } Alternative to if-then-else statements are ‘conditional expression’ and using ‘or’ operator. In perl a single statement can be followed by a conditional modifier. print “OKn” if $volts>=1.5;
- 6. REPETITION Testing loops and counting loops can be used for repetition mechanisms. Testing loops: while($a!=$b){ if($a > $b){ $a=$a-$b; }else { $b=$b-$a; } } An until loop statement in Perl programming language repeatedly executes a target statement as long as a given condition is false. Syntax The syntax of an until loop in Perl programming language is − until(condition) { statement(s); } Here statement(s) may be a single statement or a block of statements. The condition may be any expression. The loop iterates until the condition becomes true. When the condition becomes true, the program control passes to the line immediately following the loop. $a+=2 while $a < $b; (and) $a+=2 until $a < $b; Although condition is specified after the written statement, it is evaluated before the statement executed.
- 7. Do loop: It is built-in function rather than a syntactic construct.Overhere the condition is tested after the execution of the block, so the block is executed at least once. do{ ... }while $a!=$b; Counting Loops: They use same syntax as c for($i=1;$i<=10;$i++){ $i_square=$i*$i; $i_cube=$i**3; print “ $it$i_squaret$i_cuben”; } foreach $i (1..10) { $i_square=$i*$i; $i_cube=$i**3; print “ $it$i_squaret$i_cuben”; }
- 8. LOOP REFINEMENTS Perl provides three loop commands : last , next and redo . The last and next command are similar to break and continue statements in c language. last breaks out of the loop and next forces the next iteration of a loop. Ex: to terminate input processing if a line contains ‘quit’ is read we write while <STDIN> { last if /quit/; ... } The redo command repeats the current iteration from the beginning. Output: 3 4 5 6 7