Coding Bash vs. Perl vs. Python (by Evan Liu)

Topics about function

Function: define and call function 

In bash: function definition has no parameters within the parentheses, arguments are obtained by the positional parameters: $1,$2. the return value is obtained by 'echo $?' and must be numeric

In perl:arguments in subroutines are passed in the @_ array. the word 'subroutine' is the same as function, they are used interchangeably.

Tasks:
1.1. define a function that prints information about the program and arguments
2.2. call the function and print the return value
Bash Perl Python 
  1. #1.1. define a function that prints information about the program and arguments

  2. func1() for w ; do echo "$w"; done

  3. func2() { for w ; do echo "$w"; done; echo $$; return 255; }

  4. ##print the func definition

  5. declare -f func1

  6. declare -f func2

  7. 

  8. #2.2. call the function and print the return value 

  9. echo $$

  10. ##function will be called in the same shell with a return code

  11. func2 param{1..3}  #or func param1 param2 param3

  12. echo "the return value is $?"

  13. echo $$

  14. 

  15. 

  16. -------output----------

  1. #1.1. define a function that prints information about the program and arguments

  2. ##print the func definition

  3. func1 () 

  4. { 

  5.     for w in "$@";

  6.     do

  7.         echo "$w";

  8.     done

  9. }

  10. func2 () 

  11. { 

  12.     for w in "$@";

  13.     do

  14.         echo "$w";

  15.     done;

  16.     echo $$;

  17.     return 255

  18. }

  19. 

  20. #2.2. call the function and print the return value 

  21. 18824

  22. ##function will be called in the same shell with a return code

  23. param1

  24. param2

  25. param3

  26. 18824

  27. the return value is 255

  28. 18824

  1. #1.1. define a function that prints information about the program and arguments

  2. sub func{\

  3.   print "\$0 returns the program name: $0\n";\

  4.   print "\$_[0] returns the 1st argument: $_[0]\n";\

  5.   print "\@_ returns the argument list: @_\n";\

  6.   print "\$\$ returns the shell PID: $$\n";\

  7.   print "\$#_ returns the argument number : 1+$#_\n";\

  8.   return scalar(@_);}

  9. #  or func (param1,param2,param3);

  10. $ret=func (param1..param3);

  11. print "the return value is $ret";

  12. 

  13. 

  14. -------output----------

  1. #1.1. define a function that prints information about the program and arguments

  2. #  or func (param1,param2,param3);

  3. $0 returns the program name: -e

  4. $_[0] returns the 1st argument: param1

  5. @_ returns the argument list: param1 param2 param3

  6. $$ returns the shell PID: 18825

  7. $#_ returns the argument number : 1+2

  8. the return value is 3

  1. #  function, no positional parameters?

  2. def func(p1,p2):

  3.     print(p1, p2)

  4. func("param1","param2")

  5. #  or 

  6. tup=("param1","param2"); func(*tup)

  7. #  use tuple as parameter

  8. def func2(*prms): 

  9.   print(prms)  #*prms is tuple

  10. func2("one",2,3.0)

  11. tup=("one",2,3.0); func2(*tup)

  12. #  use dict as parameter

  13. def func3(**ages): 

  14.   print(ages)  #**ages is dictionary

  15. func3(dad=42,mom=48,lisa=7)

  16. #  or

  17. dic={'dad':42,'mom':48,'lisa':7}; func3(**dic)

  18. #  parameters with default value

  19. def func4(p1='tst1',p2='tst2'): 

  20.   print(p1+" "+p2)

  21. func4()

  22. 

  23. 

  24. -------output----------

  1. #  function, no positional parameters? 

  2. ('param1', 'param2')

  3. #  or  

  4. ('param1', 'param2')

  5. #  use tuple as parameter 

  6. ('one', 2, 3.0)

  7. ('one', 2, 3.0)

  8. #  use dict as parameter 

  9. {'dad': 42, 'lisa': 7, 'mom': 48}

  10. #  or 

  11. {'dad': 42, 'lisa': 7, 'mom': 48}

  12. #  parameters with default value 

  13. tst1 tst2

Function: generators and yield keyword 

In python, generator is a "function", which will return a generator type, which will yield a series of values , instead of returning only a single value. 
generator function -> return type generator-->yield values by next() method
normal function     -> return a singal value

Tasks:
Bash Perl Python 
  1. #not applicable

  2. 

  3. -------output----------

  1. #not applicable

  1. #not applicable

  2. 

  3. -------output----------

  1. #not applicable

  1. def simple_generator():

  2.     yield("Venus")

  3.     yield("Mars")

  4.     yield("Jupiter")

  5.     yield("Saturn")

  6. # use the next() method

  7. x = simple_generator()

  8. print "the type of x is: ", type(x);

  9. print x.next(),x.next(),x.next(),x.next()

  10. 

  11. # use for loop

  12. y = simple_generator()

  13. for i in y:

  14.     print i,

  15. 

  16. -------output----------

  1. # use the next() method 

  2. the type of x is:  <type 'generator'>

  3. Venus Mars Jupiter Saturn

  4. # use for loop 

  5. Venus Mars Jupiter Saturn