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

Topics about miscellaneous

Miscellaneous: how to use ternary assignment 

Ternary conditional, which may also be called inline-if with assignment, can make your code more concise and easy to maintain.

In bash, ternary conditinal only works for numeric,the syntax is (( assign_condition ? value_when_true : value_when_false )), for example, n=2; nn=$((n>1?3:5))
For string, the wordaround is to use echo: [ condition ] && echo value_when_true || echo value_when_false 

In python, the ternary operator syntax is 'value whentrue if condition else valuewhenfalse'

Tasks:
1.1. assgin value using ternary assignment
2.2. print value with ternary assignment
Bash Perl Python 
  1. n=3

  2. #1.1. assgin value using ternary assignment

  3. s="I have $n `[ $n -eq 1 ] && echo apple || echo apples`"; echo $s

  4. 

  5. #2.2. print value with ternary assignment

  6. echo "I have $n `[ $n -eq 1 ] && echo apple || echo apples`"

  7. printf "I have %d %s\n" $n `[ $n -eq 1 ] && echo apple || echo apples`

  8. 

  9. -------output----------

  1. #1.1. assgin value using ternary assignment

  2. I have 3 apples

  3. 

  4. #2.2. print value with ternary assignment

  5. I have 3 apples

  6. I have 3 apples

  1. $n=3;

  2. #1.1. assgin value using ternary assignment

  3. $s = "I have $n ".(($n==1)? "apple": "apples");print "$s\n";

  4. 

  5. #2.2. print value with ternary assignment

  6. print "I have $n ".(($n==1)? "apple": "apples")."\n";

  7. printf "I have %d %s.\n", $n, ($n == 1) ? "apple" : "apples";

  8. 

  9. -------output----------

  1. #1.1. assgin value using ternary assignment

  2. I have 3 apples

  3. 

  4. #2.2. print value with ternary assignment

  5. I have 3 apples

  6. I have 3 apples.

  1. n=3

  2. #1.1. assgin value using ternary assignment

  3. s="I have "+str(n)+(" apple" if n==1 else " apples")

  4. print s

  5. 

  6. #2.2. print value with ternary assignment

  7. print "I have", n, "apple" if n==1 else "apples"

  8. print "I have %d %s" % (n, "apple" if n==1 else "apples")

  9. 

  10. 

  11. -------output----------

  1. #1.1. assgin value using ternary assignment 

  2. I have 3 apples

  3. 

  4. #2.2. print value with ternary assignment 

  5. I have 3 apples

  6. I have 3 apples

Miscellaneous: how to use block comment 

In vim, multiline codes can be commented out easily.
1. :set number
    :7,11s/^/#
    :7,11s/^#//
2. or
   ctr-v --> select the content to commnet -->shift+I-> # -->ESC
   ctr-v --> select the content to uncomment -->d

In python docstring """....""" or content inside triple double-quotes is used to code documentation. docstring will show up when you using help function.

Tasks:
Bash Perl Python 
  1. echo "--> Here-document is to be used to comment out codes."

  2. :<<'XXXX'

  3. block of codes to be commented

  4. XXXX

  5. echo "--> Note: no string \"XXXX\" should exist inside the here-doc"

  6. 

  7. : '

  8. [bash ~]$declare -A ages

  9. [bash ~]$ages=([dad]=42 [mom]=40 [me]=7)

  10. [bash ~]$echo ${!ages[@]}

  11. mom dad me

  12. [bash ~]$echo ${ages[@]}

  13. 40 42 7

  14. '

  15. 

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

  1. --> Here-document is to be used to comment out codes.

  2. --> Note: no string "XXXX" should exist inside the here-doc

  1. print "--> The '=for x' directive says that only a processor that's formatting for the x(such as =html) output type should heed what's in the following block.\n";

  2. =for comment

  3. block of codes to be commented

  4. =cut

  5. print "--> No processor will format =comment output";

  6. 

  7. 

  8. -------output----------

  1. --> The '=for x' directive says that only a processor that's formatting for the x(such as =html) output type should heed what's in the following block.

  2. --> No processor will format =comment output

  1. print "--> triple quotes can be used for  commenting out."

  2. '''

  3. multiline codes are commented

  4. '''

  5. print "--> but it's only suggested for temprary use"

  6. 

  7. -------output----------

  1. --> triple quotes can be used for  commenting out.

  2. --> but it's only suggested for temprary use