printing elements in a nice format

Question

Hi, I have a simple, general question regarding a real small issue that bothers me: I'm printing a list of elements on the fly, so I don't have prior knowledge about the number of printed elements. I want a simple format where the elements are separated by a comma (elem1, elem2...) or something similar. Now, if I use a simple loop like:

while(elements to check) {
  if (elem should be printed) {
     print elem . ","
  }
}

I get a comma after the last element... I know this sounds quite stupid, but is there a way to handle this?

Answer 1

Java does not have a build-in join, but if you don't want to reinvent the wheel, you can use Guava's Joiner. It can skipNulls, or useForNull(something).

An object which joins pieces of text (specified as an array, Iterable, varargs or even a Map) with a separator. It either appends the results to an Appendable or returns them as a String. Example:

Joiner joiner = Joiner.on("; ").skipNulls();
return joiner.join("Harry", null, "Ron", "Hermione");

This returns the string "Harry; Ron; Hermione". Note that all input elements are converted to strings using Object.toString() before being appended.

Answer 2

I guess the simplest way is to create a new array containing only the elements from the original array that you need to print (i.e. a filter operation). Then print the newly created array, preferably using your language's built-in array/vector print/join function.

(In Perl)

@orig=("a","bc","d","ef","g");
@new_list=();
for $x(@orig){
    push(@new_list,$x) if (length($x)==1);
}
print join(',',@new_list)."\n";

(In Java)

List<String> orig=Arrays.asList(new String[]{"a","bc","d","ef","g"});
List<String> new_list=new ArrayList<String>();
for(String x: orig){
    if (x.length()==1)
        new_list.add(x);
}
System.out.println(new_list);

Answer 3

Why not add a comma BEFORE each element (but the first one)? Pseudo-code:

is_first = true
loop element over element_array
BEGIN LOOP
   if (! is_first)
       print ","
   end if
   print element
   is_first = false
END
print NEWLINE

Answer 4

the way of having all your data in an array and then

 print join(',', @yourarray)

is a good one.

You can also, after looping for your concatenation

declare eltToPrint
 while (LOOP on elt) {
       eltToPrint .= elt.','
}

remove the last comma with a regex :

eltToPrint =~s/,$//;

ps : works also if you put the comma at the beginning

eltToPrint =~s/^,//;

Answer 5

Let's assume that "should be printed" means "at least one non-whitespace character. In Perl, the idiomatic way to write this would be (you'll need to adjust the grep to taste):

print join "," => grep { /\S/ } @elements;

The "grep" is like "filter" in other languages and the /S/ is a regex matching one non-whitespace character. Only elements matching the expression are returned. The "join" should be self-explanatory.

perldoc -f join
perldoc -f grep