How to Append a String to a Variable That Either Exists or Not

how to append a string to a variable that either exists or not?

I would probably do it like this:

@results = @results.to_s + "run"

This works because NilClass defines a #to_s method that returns a zero-length String, and because instance variables are automatically initialized to nil.

Appending string to variable name

The right was is to use either an array or an object, depending on if the id is going to be sequential or named.

var card_links_grid = {};
var card_id = "the_hanged_man";
card_links_grid[card_id] = "some value";

or

var card_links_grid = [];
card_links_grid.push("some value");

Variable variables are possible, but only with some hard to maintain and debug approaches that should be avoided. Use object types designed for the data structure you want to represent.

Append suffix to string only if it is present (or not nil)

string.presence&.concat('suffix')

In case you care for nils only and not overall presence, you can simplify it:

string&.concat('suffix')

If you don't want to mutate the original value, you can instead do the somewhat cryptic:

string&.+('suffix')

perl: append variable to string if it exists

To shift a variable from @_ and insert it, if it's there

print $var1 . (shift // '') . $var2;

For example, with a space inserted

sub concat {
   my ($var1, $var2) = ("Hello", "Friend");
   print $var1 . (shift // ' ') . $var2, "\n";
}

concat();
concat(" my dear ");

There may be questions of design with spaces, please change as suitable. This prints

Hello Friend
Hello my dear Friend

If the angle brackets need be inserted around the variable, with $var2 already declared

print "$var1 " . (defined $var2 ? "<$var2>" : '') . "$var3\n";

In a sub, and without declaring an extra variable

sub concat {
    my ($var1, $var2) = ("Hello", "Friend");
    print $var1 . (@_ ? ' <' . join(' ', @_) . '> ' : ' ') . $var2, "\n";
}

concat();
concat("my dear");
concat("my", "very", "dear");

Prints

Hello Friend
Hello <my dear> Friend
Hello <my very dear> Friend

Again, change the handling of spaces as suitable for the application.

You can also use substr conditionally on an existing string but that is more cumbersome.

Append to string variable

Like this:

var str = 'blah blah blah';
str += ' blah';

str += ' ' + 'and some more blah';

how to concat string with dynamic value if that value exists

Try this:

var numb = 12;

var msg = "Hello x" + (numb > 5 ? (', ' + numb + ' is your promo code') : '');

console.log(msg);

'a' // ' ' really produces  'a '

result = result // ' '

result = trim(result) // ' '

  character(4) :: str

  str = "a"
  str = trim(str) // "bcd"
  print *, str
end

character(:), allocatable:: result
result = 'a'  !now contains 'a' and has length 1
result = result // 'b' !now contains 'ab' and has length 2

character(:), allocatable:: result
result = ' '  !now contains ' ' and has length 1
result = result // ' ' !now contains '  ' and has length 2

s=test1
s="${s}test2"

s=test1
s+=test2