Best Loop Idiom for special casing the last element

I run into this case a lot of times when doing simple text processing and print statements where I am looping over a collection and I want to special case the last element (for example every normal element will be comma separated except for the last case).

Is there some best practice idiom or elegant form that doesn't require duplicating code or shoving in an if, else in the loop.

For example I have a list of strings that I want to print in a comma separated list. (the do while solution already assumes the list has 2 or more elements otherwise it'd be just as bad as the more correct for loop with conditional).

e.g. List = ("dog", "cat", "bat")

I want to print "[dog, cat, bat]"

I present 2 methods the

1. For loop with conditional

   public static String forLoopConditional(String[] items) {
   
   String itemOutput = "[";
   
   for (int i = 0; i < items.length; i++) {
       // Check if we're not at the last element
       if (i < (items.length - 1)) {
           itemOutput += items[i] + ", ";
       } else {
           // last element
           itemOutput += items[i];
       }
   }
   itemOutput += "]";
   
   return itemOutput;
    }
   

2. do while loop priming the loop

   public static String doWhileLoopPrime(String[] items) {
   String itemOutput = "[";
   int i = 0;
   
   itemOutput += items[i++];
   if (i < (items.length)) {
       do {
           itemOutput += ", " + items[i++];
       } while (i < items.length);
   }
   itemOutput += "]";
   
   return itemOutput;
   }
   

Tester class:

    public static void main(String[] args) {
        String[] items = { "dog", "cat", "bat" };

        System.out.println(forLoopConditional(items));
        System.out.println(doWhileLoopPrime(items));

    }

In the Java AbstractCollection class it has the following implementation (a little verbose because it contains all edge case error checking, but not bad).

public String toString() {
    Iterator<E> i = iterator();
if (! i.hasNext())
    return "[]";

StringBuilder sb = new StringBuilder();
sb.append('[');
for (;;) {
    E e = i.next();
    sb.append(e == this ? "(this Collection)" : e);
    if (! i.hasNext())
	return sb.append(']').toString();
    sb.append(", ");
}
}

I usually write it like this:

static String commaSeparated(String[] items) {
    StringBuilder sb = new StringBuilder();
    String sep = "";
    for (String item: items) {
        sb.append(sep);
        sb.append(item);
        sep = ",";
    }
    return sb.toString();
}

There are a lot of for loops in these answers, but I find that an Iterator and while loop reads much more easily. E.g.:

Iterator<String> itemIterator = Arrays.asList(items).iterator();
if (itemIterator.hasNext()) {
  // special-case first item.  in this case, no comma
  while (itemIterator.hasNext()) {
    // process the rest
  }
}

This is the approach taken by Joiner in Google collections and I find it very readable.

string value = "[" + StringUtils.join( items, ',' ) + "]";

My usual take is to test if the index variable is zero, e.g.:

var result = "[ ";
for (var i = 0; i < list.length; ++i) {
    if (i != 0) result += ", ";
    result += list[i];
}
result += " ]";

But of course, that's only if we talk about languages that don't have some Array.join(", ") method. ;-)

I think it is easier to think of the first element as the special case because it is much easier to know if an iteration is the first rather than the last. It does not take any complex or expensive logic to know if something is being done for the first time.

public static String prettyPrint(String[] items) {
    String itemOutput = "[";
    boolean first = true;
    
    for (int i = 0; i < items.length; i++) {
        if (!first) {
            itemOutput += ", ";
        }

        itemOutput += items[i];
        first = false;
    }

    itemOutput += "]";
    return itemOutput;
}

I'd go with your second example, ie. handle the special case outside of the loop, just write it a bit more straightforward:

String itemOutput = "[";

if (items.length > 0) {
    itemOutput += items[0];

    for (int i = 1; i < items.length; i++) {
        itemOutput += ", " + items[i];
    }
}

itemOutput += "]";

Java 8 solution, in case someone is looking for it:

String res = Arrays.stream(items).reduce((t, u) -> t + "," + u).get();

I like to use a flag for the first item.

 ArrayList<String> list = new ArrayList()<String>{{
       add("dog");
       add("cat");
       add("bat");
    }};
    String output = "[";
    boolean first = true;
    for(String word: list){
      if(!first) output += ", ";
      output+= word;
      first = false;
    }
    output += "]";

Since your case is simply processing text, you don't need the conditional inside the loop. A C example:

char* items[] = {"dog", "cat", "bat"};
char* output[STRING_LENGTH] = {0};
char* pStr = &output[1];
int   i;

output[0] = '[';
for (i=0; i < (sizeof(items) / sizeof(char*)); ++i) {
    sprintf(pStr,"%s,",items[i]);
    pStr = &output[0] + strlen(output);
}
output[strlen(output)-1] = ']';

Instead of adding a conditional to avoid generating the trailing comma, go ahead and generate it (to keep your loop simple and conditional-free) and simply overwrite it at the end. Many times, I find it clearer to generate the special case just like any other loop iteration and then manually replace it at the end (although if the "replace it" code is more than a couple of lines, this method can actually become harder to read).

...

String[] items = { "dog", "cat", "bat" };
String res = "[";

for (String s : items) {
   res += (res.length == 1 ? "" : ", ") + s;
}
res += "]";

or so is quite readable. You can put the conditional in a separate if clause, of course. What it makes idiomatic (I think so, at least) is that it uses a foreach loop and does not use a complicated loop header.

Also, no logic is duplicated (i.e. there is only one place where an item from items is actually appended to the output string - in a real world application this might be a more complicated and lengthy formatting operation, so I wouldn't want to repeat the code).

If you are building a string dynamically like that, you shouldn't be using the += operator. The StringBuilder class works much better for repeated dynamic string concatenation.

public String commaSeparate(String[] items, String delim){
    StringBuilder bob = new StringBuilder();
    for(int i=0;i<items.length;i++){
        bob.append(items[i]);
        if(i+1<items.length){
           bob.append(delim);
        }
    }
    return bob.toString();
}

Then call is like this

String[] items = {"one","two","three"};
StringBuilder bob = new StringBuilder();
bob.append("[");
bob.append(commaSeperate(items,","));
bob.append("]");
System.out.print(bob.toString());

In this case, you are essentially concatenating a list of strings using some separator string. You can maybe write something yourself which does this. Then you will get something like:

String[] items = { "dog", "cat", "bat" };
String result = "[" + joinListOfStrings(items, ", ") + "]"

with

public static String joinListOfStrings(String[] items, String sep) {
    StringBuffer result;
    for (int i=0; i<items.length; i++) {
        result.append(items[i]);
        if (i < items.length-1) buffer.append(sep);
    }
    return result.toString();
}

If you have a Collection instead of a String[] you can also use iterators and the hasNext() method to check if this is the last or not.

Generally, my favourite is the multi-level exit. Change

for ( s1; exit-condition; s2 ) {
    doForAll();
    if ( !modified-exit-condition ) 
        doForAllButLast();
}

to

for ( s1;; s2 ) {
    doForAll();
if ( modified-exit-condition ) break;
    doForAllButLast();
}

It eliminates any duplicate code or redundant checks.

Your example:

for (int i = 0;; i++) {
    itemOutput.append(items[i]);
if ( i == items.length - 1) break;
    itemOutput.append(", ");
}

It works for some things better than others. I'm not a huge fan of this for this specific example.

Of course, it gets really tricky for scenarios where the exit condition depends on what happens in doForAll() and not just s2. Using an Iterator is such a case.

Here's a paper from the prof that shamelessly promoted it to his students :-). Read section 5 for exactly what you're talking about.

I think there are two answers to this question: the best idiom for this problem in any language, and the best idiom for this problem in java. I also think the intent of this problem wasn't the tasks of joining strings together, but the pattern in general, so it doesn't really help to show library functions that can do that.

Firstly though the actions of surrounding a string with [] and creating a string separated by commas are two separate actions, and ideally would be two separate functions.

For any language, I think the combination of recursion and pattern matching works best. For example, in haskell I would do this:

join [] = ""
join [x] = x
join (x:xs) = concat [x, ",", join xs]

surround before after str = concat [before, str, after]

yourFunc = surround "[" "]" . join

-- example usage: yourFunc ["dog", "cat"] will output "[dog,cat]"

The benefit of writing it like this is it clearly enumerates the different situations that the function will face, and how it will handle it.

Another very nice way to do this is with an accumulator type function. Eg:

join [] = ""
join strings = foldr1 (\a b -> concat [a, ",", b]) strings 

This can be done in other languages as well, eg c#:

public static string Join(List<string> strings)
{
    if (!strings.Any()) return string.Empty;
    return strings.Aggregate((acc, val) => acc + "," + val);
}

Not very efficient in this situation, but can be useful in other cases (or efficiency may not matter).

Unfortunately, java can't use either of those methods. So in this case I think the best way is to have checks at the top of the function for the exception cases (0 or 1 elements), and then use a for loop to handle the case with more than 1 element:

public static String join(String[] items) {
    if (items.length == 0) return "";
    if (items.length == 1) return items[0];

    StringBuilder result = new StringBuilder();
    for(int i = 0; i < items.length - 1; i++) {
        result.append(items[i]);
        result.append(",");
    }
    result.append(items[items.length - 1]);
    return result.toString();
}

This function clearly shows what happens in the two edge cases (0 or 1 elements). It then uses a loop for all but the last elements, and finally adds the last element on without a comma. The inverse way of handling the non-comma element at the start is also easy to do.

Note that the if (items.length == 1) return items[0]; line isn't actually necessary, however I think it makes what the function does more easier to determine at a glance.

(Note that if anyone wants more explanation on the haskell/c# functions ask and I'll add it in)

It can be achieved using Java 8 lambda and Collectors.joining() as -

List<String> items = Arrays.asList("dog", "cat", "bat");
String result = items.stream().collect(Collectors.joining(", ", "[", "]"));
System.out.println(result);

A third alternative is the following

StringBuilder output = new StringBuilder();
for (int i = 0; i < items.length - 1; i++) {
    output.append(items[i]);
    output.append(",");
}
if (items.length > 0) output.append(items[items.length - 1]);

But the best is to use a join()-like method. For Java there's a String.join in third party libraries, that way your code becomes:

StringUtils.join(items,',');

FWIW, the join() method (line 3232 onwards) in Apache Commons does use an if within a loop though:

public static String join(Object[] array, char separator, int startIndex, int endIndex)     {
        if (array == null) {
            return null;
        }
        int bufSize = (endIndex - startIndex);
        if (bufSize <= 0) {
            return EMPTY;
        }

        bufSize *= ((array[startIndex] == null ? 16 : array[startIndex].toString().length()) + 1);
        StringBuilder buf = new StringBuilder(bufSize);

        for (int i = startIndex; i < endIndex; i++) {
            if (i > startIndex) {
                buf.append(separator);
            }
            if (array[i] != null) {
                buf.append(array[i]);
            }
        }
        return buf.toString();
    }

I usually write a for loop like this:

public static String forLoopConditional(String[] items) {
	StringBuilder builder = new StringBuilder();         
	                                                     
	builder.append("[");                                 
	                                                     
	for (int i = 0; i < items.length - 1; i++) {         
		builder.append(items[i] + ", ");                 
	}                                                    
	                                                     
	if (items.length > 0) {                              
		builder.append(items[items.length - 1]);         
	}                                                    
	                                                     
	builder.append("]");                                 
	                                                     
	return builder.toString();                           
}       

                                             

If you are just looking for a comma seperated list of like this: "[The, Cat, in, the, Hat]", don't even waste time writing your own method. Just use List.toString:

List<String> strings = Arrays.asList("The", "Cat", "in", "the", "Hat);

System.out.println(strings.toString());

Provided the generic type of the List has a toString with the value you want to display, just call List.toString:

public class Dog {
    private String name;

    public Dog(String name){
         this.name = name;
    }

    public String toString(){
        return name;
    }
}

Then you can do:

List<Dog> dogs = Arrays.asList(new Dog("Frank"), new Dog("Hal"));
System.out.println(dogs);

And you'll get: [Frank, Hal]