University:
Boston University
Course:
GRS CS 910 | Directed Study: Systems
Academic year:

2024
Views:

139

Pages:

12
Author:

ksaveranwsu

V. Conditionals and loops Conditional statements: if ● ● Of course, the power of programs comes from their input to react differently to different data. The conditional statement is the key to this. The if statement has two parts: a condition, which is evaluated, and a block, which is executed if the condition evaluates to be true. if ($price > 100) { print "Too expensive!\n"; } Conditional statements: elsif ● An if statement can optionally be followed by an elsif statement. This is evaluated after the if statement only if the previous condition evaluated (the if statement's) was false. The two blocks will never both be exectuted. if ($price > 100) {print "expensive"} elsif ($price < 10) {print "bargain!"} elsif ($price < 40) {print "cheap"} ● Again: If the if statement evaluates to true, the elsif statements are just skipped. Similarly, if the first elsif evaluates to true, the second is skipped. Conditional statements: else ● An else statement is a special kind of elsif that covers all remaining conditions. In linguistics, this is known as "the elsewhere condition". if ($grade < 60) { print "Failing grade!: $grade\n"; } else {print "Grade is $grade.\n"} ● Note that there is no condition to be evaluated in an else statement, because it's the "garbabe can" category and is always true. Loops ● ● A major reason we use computers is that they can do the same thing many times without getting tired or needing coffee. We get them to repeat actions using iterative or looping constucts. There are three main types of loop statement, which are all related but specialized for different uses: for, foreach, and while statements. 'while' loops ● While statements are the least complex of the three types, so we'll start there. They are similar to conditional statements, because they have a condition and a block. If the condition evaluates to true, the block is executed. Then, the condition is evaluated again, and if it's still true, the block is This represents executed again, and so on. some user-defined subroutine, which presumably checks for more money and returns the value as $pay. while ($money < 1000) { $pay = Get_Paycheck (); $money = $money + $pay; } print "Ok, we can pay the rent!\n"; 'until' loops ● Until statements are a special type of while statement in which the block is executed until the condition evaluates as true. It's equivalent to "while NOT (condition)...". until ($money < 1000) { $money = Eat_Out ($money); } print "Time to eat noodles!\n"; 'for' loops ● ● For statements are very useful, because they allow you to do something a preset number of times, or until a condition becomes true. Their structure is more complex than a while statement's: for ($i = 1; $i <= 100; $i ++) { print "We've reached $i\n"; } We've reached 1 We've reached 2 ... We've reached 100 'for' loops This sets the initial value of the variable: its value on the first pass through the loop. This sets the test that determines when to exit the loop: as long as this is true, the loop will be rerun. This sets how the variable will be incremented on each pass through the loop. for ($i = 1; $i <= 100; $i ++) { print "We've reached $i\n"; } We've reached 1 We've reached 2 ... We've reached 100 'foreach' loops ● Foreach statements allow us to perform the same operations on every element of an array or hash in series. They temporarily assign the value of each element to a scalar that we can work with. foreach $grade (@grades) { if ($grade < 60) { print "Failing grade!: $grade\n"; $fail_tally ++; } } print "We have $fail_tally failing students!\n"; Exiting the loop ● ● ● ● Sometimes you want to exit the loop via a method other than the standard one. We do this using the statements next, redo, and last. next means "skip over everything else in the block, increment the counter, and evaluate the conditional again." redo means "skip over everything else in the block and evaluate the conditional again, without incrementing the counter." last means "exit the block and never come back." Exiting the loop ● Here's an example that uses both next and last: foreach $student (@students) { if ($student eq "END_REGISTERED") { last; } elsif ($student eq "Silber"){ next; } else { $grade = Check_Grade ($student); } print "$student: $grade\n"; }