Are there cases where while/do-while must be used instead of for?

Question

This is a long running debate between me and my teacher. Can there be a situation where a for loop absolutely cannot be used in place of a while/do-while loop? In other words, is there a specific case where a for-loop will not work in place of a while loop; is while/do-while in any way "distinct" from for?

Answer 1

No, there is no such situation. Every do-while loop can be written in terms of a while-loop (by executing the body once before the loop) and vice versa. In turn, every while-loop

while (X) {     ... } 

can be written as

for (; X;) {     ... } 

i.e. we omit an initialization and an increment statement. We can also convert from a for back to a while by correctly placing the initialization and increment.

In short, it's always possible to convert from one loop variant to either of the other two. for-loops just give you the benefit of being able to limit the scope of a loop control variable and do any incrementation at the top. It goes without saying that in many cases one particular loop variant makes much more sense to use than the others; each has its specific use cases.

Also note that the fun doesn't just end with loops: it's also possible to convert every loop into a recursive function and vice versa (although in practice there could be limitations to this; for example a loop that worked fine could, when converted to a recursive function, produce a stack overflow error).

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[I]s while/do-while in any way "distinct" from for?

It is not. For instance, the bytecode of the following two snippets is identical:

int x = 0; while (x < 10) {     x++; } 

and

int x = 0; for (; x < 10;) {  // or: for (; x < 10; x++) {}     x++; } 

both become:

   0: iconst_0          1: istore_1          2: goto          8    5: iinc          1, 1    8: iload_1           9: bipush        10   11: if_icmplt     5   14: return  

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There was talk in the comments about for-each loops and that they might be intrinsically different from the other loop types. This is absolutely not true; for-each loops are pure syntactic sugar around iterators (or looping over arrays). Every for-each loop can also be converted to each of the other loop types. Here's an example:

for (String s : l) {  // l is a list of strings     System.out.println(s); } 

and

String s; Iterator<String> iter = l.iterator();  // l is a list of strings while (iter.hasNext()) {     s = iter.next();     System.out.println(s); } 

both become:

  24: invokeinterface #33,  1           // InterfaceMethod java/util/List.iterator:()Ljava/util/Iterator;   29: astore_3         30: goto          50   33: aload_3          34: invokeinterface #39,  1           // InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object;   39: checkcast     #19                 // class java/lang/String   42: astore_2         43: getstatic     #45                 // Field java/lang/System.out:Ljava/io/PrintStream;   46: aload_2          47: invokevirtual #51                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V   50: aload_3          51: invokeinterface #57,  1           // InterfaceMethod java/util/Iterator.hasNext:()Z   56: ifne          33 

Answer 2

No, you can always rewrite a for loop as a while loop, and any while look as a for loop.

<init> while (condition) { ... <increment> } 

is equivalent to:

for (<init>; <condition>; <increment>) { ... }