more perl control flow software tools. slide 2 control flow l we have already seen several perl...
TRANSCRIPT
Slide 2
Control Flow We have already seen several Perl
control flow statements: if while for last
Other control flow statements: unless until do while do until foreach next redo
Slide 3
unless The Perl unless statement is like an if statement
with the condition negated:
if($temperature <= 20){print "too cold!\n";
}
unless($temperature > 20){ # same thingprint "too cold!\n";
}-------------------------------------------if(!$hot){
print "too cold!\n";}
unless($hot){ # same thingprint "too cold!\n";
}
Slide 4
unless unless can have else, just like if:
#!/usr/local/bin/perl5 -w
print "Enter temperature: ";
chomp($temperature = <STDIN>);
unless($temperature > 20){
print "too cold!\n";
}else{
print "too hot!\n";
}
Slide 5
unless unless can also have elsif, just like if:
#!/usr/local/bin/perl5 -w
print "Enter temperature: ";
chomp($temperature = <STDIN>);
unless($temperature >= 20){
print "too cold!\n";
}elsif($temperature == 20){
print "ok!\n";
}else{
print "too hot!\n";
}
Slide 6
until The Perl until statement is like a while
statement with the condition negated. Sometimes is is easier to say “until something is
true” rather than “while not this is true”:
while(!$endoffile){...
}
until($endoffile){ # same thing...
}
Slide 7
until The until statement loops indefinitely, until the
condition is true, such as a user-controlled condition:
#!/usr/local/bin/perl5 -w$resp = "no";
until($resp eq "yes"){print "Wakeup [yes/no]? ";chomp($resp = <STDIN>);
}$ test11Wakeup [yes/no]? noWakeup [yes/no]? yWakeup [yes/no]? yes$
Slide 8
do while The do while statement is like the C++ do while
statement. It loops indefinitely, while the condition is true, such
as a user-controlled condition: do while always executes the body of the loop at
least once. #!/usr/local/bin/perl5 -w
do{print "Wakeup [yes/no]? ";chomp($resp = <STDIN>);
}while($resp ne "yes");$ test11Wakeup [yes/no]? noWakeup [yes/no]? yes$
Slide 9
do until The do until statement loops indefinitely, until
the condition is true, such as a user-controlled condition.
do until always executes the body of the loop at least once.
#!/usr/local/bin/perl5 -w do{
print "Wakeup [yes/no]? ";chomp($resp = <STDIN>);
}until($resp eq "yes");$ test11Wakeup [yes/no]? noWakeup [yes/no]? yWakeup [yes/no]? yes$
Slide 10
foreach
foreach takes a list of values and assigns them one by one to a scalar variable.
The body of the loop is executed once for each successive assignment.
foreach is similar to the shell programming’s for statement.
foreach $i (@some_list){...
}
Slide 11
foreach The following example sums the contents of an
array:
$ cat sum#!/usr/local/bin/perl5 -w@a = (21,32,3,44,75,16,19);$sum = 0;foreach $b (@a){
$sum += $b;}print "The array sum is: $sum\n";$ sumThe array sum is: 210 $
Slide 12
foreach foreach allows us to easily print an array in our
own customized way. The following example prints an array with each
element separated by 2 spaces:
$ cat print1#!/usr/local/bin/perl5 -w@a = (1,2,3,4,5);foreach $i (@a){
print "$i ";}print "\n";$ print11 2 3 4 5 $
Slide 13
foreach The following example prints the numbers in reverse
order without changing the array:
$ cat print2#!/usr/local/bin/perl5 -w@a = (1,2,3,4,5);foreach $i (reverse @a){
print "$i ";}print "\n";$ print25 4 3 2 1 $
reverse @a is the same as writing reverse(@a). Parenthesis are always optional on Perl functions.
Slide 14
foreach $_ If you omit the scalar variable in foreach, Perl will
use $_ automatically:
$ cat print3#!/usr/local/bin/perl5 -w@a = (1,2,3,4,5);foreach (reverse @a){
print;}print "\n";$ print354321 $
print (and other Perl functions) use $_ as the default if nothing is specified.
Slide 15
foreach $_ Of course, if you want double spaces, you will
have to use $_ explicitly:
$ cat print3a#!/usr/local/bin/perl5 -w@a = (1,2,3,4,5);foreach (reverse @a){
print "$_ ";}print "\n";$ print3a5 4 3 2 1$
Slide 16
foreach The scalar variable in foreach is an alias for each
variable in the list, not just a copy. (Tricky!) If you modify the scalar variable in foreach, the
aliased element in the list is also changed:$ cat double#!/usr/local/bin/perl5 -w@a = (1,2,3,4);foreach $b (@a){
$b *= 2;}print "@a\n";$ double2 4 6 8 $
Slide 17
last Again The last command only breaks out of while, until, for, and foreach loops.
For mysterious reasons, do while and do until do not count as loops in Perl.
$ cat last1#!/usr/local/bin/perl5 -w
do{print "Wakeup [yes/no]? ";chomp($resp = <STDIN>);if($resp eq "yes"){
last;}
}while(1);$ last1Wakeup [yes/no]? yesCan't "last" outside a block at last1 line 6, <STDIN> chunk 1.$
Slide 18
last Again In the example below, the last statement causes the outer while loop to be broken:
$ cat last2#!/usr/local/bin/perl5 -w
while(1){do{
print "Wakeup [yes/no]? ";chomp($resp = <STDIN>);if($resp eq "yes"){
last;}
}while(1);}$ last2Wakeup [yes/no]? yes$
Slide 19
next The next command causes a jump to the end of the
loop, but without terminating the loop.$ cat next1#!/usr/local/bin/perl5 -w$resp = "no";until($resp eq "quit" || $resp eq "yes"){
print "Wakeup [yes/no/quit]? ";chomp($resp = <STDIN>);if($resp eq "yes"){
next;}print "sleeping...\n";
}$ next1Wakeup [yes/no/quit]? nosleeping...Wakeup [yes/no/quit]? yes$ next1Wakeup [yes/no/quit]? quitsleeping...$
Slide 20
redo The redo command causes a jump back to the
beginning of the loop, but without re-testing the condition (next re-tests the condition).
$ cat redo#!/usr/local/bin/perl5 -w$resp = "no";until($resp eq "quit" || $resp eq "yes"){
print "Wakeup [yes/no/quit]? ";chomp($resp = <STDIN>);if($resp eq "yes"){
redo;}print "sleeping...\n";
}$ redoWakeup [yes/no/quit]? nosleeping...Wakeup [yes/no/quit]? yesWakeup [yes/no/quit]? quitsleeping...$
Slide 21
next and redo Like last, next and redo only break out of while, until, for, and foreach loops (not do while and do until loops).
$ cat redo1#!/usr/local/bin/perl5 -w$resp = "no";do{
print "Wakeup [yes/no/quit]? ";chomp($resp = <STDIN>);if($resp eq "yes"){
redo;}print "sleeping...\n";
}until($resp eq "quit" || $resp eq "yes");$ redo1Wakeup [yes/no/quit]? nosleeping...Wakeup [yes/no/quit]? yesCan't "redo" outside a block at redo1 line 7, <STDIN> chunk 2.$
Slide 22
redo and last/next You can make a while loop using redo and last/next in a “naked” block (a “naked” block is a block { } that is not part of a loop or if statement):
$ cat redo2#!/usr/local/bin/perl5 -w{
print "Wakeup [yes/no]? ";chomp($resp = <STDIN>);if($resp eq "yes"){
last; # could also use "next;"}redo;
}$ redo2Wakeup [yes/no]? noWakeup [yes/no]? yWakeup [yes/no]? yes$
Slide 23
Labeled Blocks What if you want to jump out of two nested loops? In C++, you’d be in trouble. In Perl, you can use labeled loops to break out of an
outer loop using last, next, and redo. For clarity (and other reasons), it is best to choose label
names that are all upper case letters and digits. Labels always end in a colon “:”.
OUTERLOOP: for($i=1; $i<3; $i++){INNERLOOP: for($j=1; $j<5; $j++){
...
Add the label (without the colon) as a parameter to last, for example, if you want to exit OUTERLOOP.
last OUTERLOOP;
Slide 24
Labeled Blocks Example
$ cat label#!/usr/local/bin/perl5 -w$sum = 0;LOOP1: for($i1=1; $i1<=2; $i1++){
LOOP2: for($i2=1; $i2<=2; $i2++){LOOP3: for($i3=1; $i3<=2; $i3++){
$sum += 1;print "sum so far: $sum\n";if($sum > 2){
last LOOP2; }
}$sum += 2;
}$sum += 3;
}print "sum at end: $sum\n";$ labelsum so far: 1sum so far: 2sum so far: 5sum so far: 9sum at end: 12$
Slide 25
Backward if A simple way to write “if this, then that” is:
chomp($user = `whoami`);print("Hi Bill!\n") if($user eq "gates");
is the same as:
chomp($user = `whoami`);if($user eq "gates"){
print "Hi Bill!\n";}
Backward if avoids having to write the curly braces { }.
There can only be one statement inside the block.
Slide 26
Backward if Backward if is a natural and tidy way to exit from
a loop:
$ cat backif#!/usr/local/bin/perl5 -w
while(1){print "Wakeup [yes/no]? ";chomp($resp = <STDIN>);last if $resp eq "yes";
};$ backifWakeup [yes/no]? noWakeup [yes/no]? yWakeup [yes/no]? yes$
Slide 27
Backward unless, while, until
You can also use backward unless, while, and until (if there is only one statement in the block):
$ cat sum#!/usr/local/bin/perl5 -wprint "Enter numbers to sum (0 to quit): \n";$sum = 0;$n = 1;$sum += $n = <STDIN> while($n != 0);print "The sum is: $sum: \n";$ sumEnter numbers to sum (0 to quit): 110The sum is: 2$
Slide 28
&& if Another simple way to write “if this, then that” is:
chomp($user = `whoami`);$user eq "gates" && print("Hi Bill!\n");
is the same as:
chomp($user = `whoami`);if($user eq "gates"){
print("Hi Bill!\n");}
Slide 29
&& if
this && that;
Why does this work? Isn’t && the logical-and operator? Consider what happens when this and that take on values of true and false: If this is true, then the value of the entire expression
is still not known, because it depends on the value of that. So that has to be evaluated.
If this is false, there is no need to look at that, because the value of the whole expression must be false. Since there is no need to evaluate that, Perl skips it.
Slide 30
&& if && if is also a tidy way to exit from a loop:
$ cat backif1#!/usr/local/bin/perl5 -w
while(1){print "Wakeup [yes/no]? ";chomp($resp = <STDIN>);$resp eq "yes" && last;
};$ backif1Wakeup [yes/no]? noWakeup [yes/no]? yWakeup [yes/no]? yes$