本章节实在是简单的不想写，太多雷同的部分了。

 understanding conditional statements

1. conditional execute code selectively

if(condition){
          statements )

2. conditionals my provide alternative code

if (condition){
          statement_1 }else{
          statement_2 }

beginning with version 5.10, perl also provides a switch statement

　　-use given and when

　　-requires the latest version of perl

 if.pl

#!/usr/bin/perl use strict; use warnings; use constant{
          TRUE =>1,     FALSE => '' }; main(@ARGV); sub main {
      if(1){
      #replace 1 with 0/142/"this is true"/""/undef/5+5/5-5 and veiw the result.         message("this is true.");     } } sub message {
      my $m = shift or return; print("$m\n"); } sub error {
      my $e = shift || 'unkown error'; print("$0: $e\n"); exit 0; }

else.pl

#!/usr/bin/perl use strict; use warnings; use constant{
          TRUE =>1,     FALSE => '' }; main(@ARGV); sub main {
      if(TRUE){
              message("This is true.");     }else{
              message("This is false.");         } } sub message {
      my $m = shift or return; print("$m\n"); } sub error {
      my $e = shift || 'unkown error'; print("$0: $e\n"); exit 0; }

elsif.pl

#!/usr/bin/perl use strict; use warnings; use constant{
          TRUE =>1,     FALSE => '' }; main(@ARGV); sub main {
      if(TRUE){
              message("This is true.");     }elsif(FALSE){
              message("but this one is true");     } else{
              message("This is false.");     } } sub message {
      my $m = shift or return; print("$m\n"); } sub error {
      my $e = shift || 'unkown error'; print("$0: $e\n"); exit 0; }

postfix.pl

#!/usr/bin/perl # use strict; use warnings; main(@ARGV); sub main {
      my $x =42;     message("number is in range") if($x>25 and $x < 100); } sub message {
      my $m = shift or return; print("$m\n"); } sub error {
      my $e = shift || 'unkown error'; print("$0: $e\n"); exit 0; }

这里我们用一个很cool的代码书写方式：将control结构放在后面，简洁，漂亮！

unless.pl

#!/usr/bin/perl use strict; use warnings; main(@ARGV); sub main {
      my $x =24;     message("number is out of range") unless $x>25 and $x < 100;  } sub message {
      my $m = shift or return; print("$m\n"); } sub error {
      my $e = shift || 'unkown error'; print("$0: $e\n"); exit 0; }

同样简洁！

ternary.pl

#!/usr/bin/perl use strict; use warnings; main(@ARGV); sub main {
      my $n= 24; my $s = ($n>25 and $n <100)?"in range":"out of range";     message("number is ". $s); } sub message {
      my $m = shift or return; print("$m\n"); } sub error {
      my $e = shift || 'unkown error'; print("$0: $e\n"); exit 0; }

学会用和C不同的风格写条件控制语句也是一件有趣的事情！