Useful alternative control structures?

Question

Sometimes when I am programming, I find that some particular control structure would be very useful to me, but is not directly available in my programming language. I think my most common desire is something like a "split while" (I have no idea what to actually call this):

{
    foo();
} split_while( condition ) {
    bar();
}

The semantics of this code would be that foo() is always run, and then the condition is checked. If true, then bar() is run and we go back to the first block (thus running foo() again, etc). Thanks to a comment by reddit user zxqdms, I have learned that Donald E. Knuth writes about this structure in his paper "Structured programming with go to statements" (see page 279).

What alternative control structures do you think are a useful way of organizing computation?

My goal here is to give myself and others new ways of thinking about structuring code, in order to improve chunking and reasoning.

Note: I'm not asking about how to generalize all possible control structures, whether by using jne, if/goto, Lisp macros, continuations, monads, combinators, quarks, or whatever else. I'm asking what specializations are useful in describing code.

Answer 1

One that's fairly common is the infinite loop. I'd like to write it like this:

forever {
  // ...
}

Answer 2

Sometimes, I need to have a foreach loop with an index. It could be written like this:

foreach (index i) (var item in list) {
  // ...
}

(I'm not particularly fond of this syntax, but you get the idea)

Answer 3

Most languages have built-in functions to cover the common cases, but "fencepost" loops are always a chore: loops where you want to do something on each iteration and also do something else between iterations. For example, joining strings with a separator:

string result = "";
for (int i = 0; i < items.Count; i++) {
    result += items[i];
    if (i < items.Count - 1) result += ", "; // This is gross.
    // What if I can't access items by index?
    // I have off-by-one errors *every* time I do this.
}

I know folds can cover this case, but sometimes you want something imperative. It would be cool if you could do:

string result = "";
foreach (var item in items) {
    result += item;
} between {
    result += ", ";
}

Answer 4

Loop with else:

while (condition) {
  // ...
}
else {
  // the else runs if the loop didn't run
}

Answer 5

{
    foo();
} split_while( condition ) {
    bar();
}

You can accomplish that pretty easily using a regular while:

while (true) {
    foo();
    if (!condition) break;
    bar();
}

I do that pretty frequently now that I got over my irrational distaste for break.

Answer 6

If you look at Haskell, although there is special syntax for various control structures, control flow is often captured by types. The most common kind of such control types are Monads, Arrows and applicative functors. So if you want a special type of control flow, it's usually some kind of higher-order function and either you can write it yourself or find one in Haskells package database (Hackage) wich is quite big.

Such functions are usually in the Control namespace where you can find modules for parallel execution to errorhandling. Many of the control structures usually found in procedural languages have a function counterpart in Control.Monad, among these are loops and if statements. If-else is a keyworded expression in haskell, if without an else doesn't make sense in an expression, but perfect sense in a monad, so the if statements without an else is captured by the functions when and unless.

Another common case is doing list operation in a more general context. Functional languages are quite fond of fold, and the Specialized versions like map and filter. If you have a monad then there is a natural extension of fold to it. This is called foldM, and therefor there are also extensions of any specialized version of fold you can think of, like mapM and filterM.

Answer 7

This is just a general idea and syntax:

if (cond)
   //do something
else (cond)
   //do something
also (cond)
   //do something
else
   //do something
end

ALSO condition is always evaluated. ELSE works as usual.

It works for case too. Probably it is a good way to eliminate break statement:

case (exp)
   also (const)
      //do something
   else (const)
      //do something
   also (const)
      //do something
   else
      //do something
end

can be read as:

switch (exp)
   case (const)
      //do something
   case (const)
      //do something
      break
   case (const)
      //do something
   default
      //do something
end

I don't know if this is useful or simple to read but it's an example.

Answer 8

With (lisp-style) macros, tail-calls, and continuations all of this is quaint.

With macros, if the standard control flow constructs are not sufficient for a given application, the programmer can write their own (and so much more). It would only require a simple macro to implement the constructs you gave as an example.

With tail-calls, one can factor out complex control flow patters (such as implementing a state machine) into functions.

Continuations are a powerful control flow primitive (try/catch are a restricted version of them). Combined with tail-calls and macros, complex control flow patterns (backtracking, parsing, etc.) become straight-forward. In addition, they are useful in web programming as with them you can invert the inversion of control; you can have a function that asks the user for some input, do some processing, asks the user for more input, etc.

To paraphrase the Scheme standard, instead of piling more features onto your language, you should seek to remove the limitations that make the other features appear necessary.

Answer 9

if not:

unless (condition) {
  // ...
}

while not:

until (condition) {
  // ...
}

Answer 10

Labeled loops are something I find myself missing sometimes from mainstream languages. e.g.,

int i, j;
for outer ( i = 0; i < M; ++i )
    for ( j = 0; j < N; ++j )
        if ( l1[ i ] == l2[ j ] )
           break outer;

Yes, I can usually simulate this with a goto, but an equivalent for continue would require you to move the increment to the end of loop body after the label, hurting the readability. You can also do this by setting a flag in the inner loop and checking it at each iteration of the outer loop, but it always looks clumsy.

(Bonus: I'd sometimes like to have a redo to go along with continue and break. It would return to the start of the loop without evaluating the increment.)

Answer 11

I propose the "then" operator. It returns the left operand on the first iteration and the right operand on all other iterations:

var result = "";
foreach (var item in items) {
    result += "" then ", ";
    result += item;
}

in the first iteration it adds "" to the result in all others it adds ", ", so you get a string that contains each item separated by commas.

Answer 12

if (cond)
   //do something
else (cond)
   //do something
else (cond)
   //do something
first
   //do something
then
   //do something
else (cond)
   //do something
else
   //do something
end

FIRST and THEN blocks runs if any of 3 conditionals are evaluated to true. FIRST block runs before the conditional block and THEN runs after the conditional block has ran.

ELSE conditional or final write following FIRST and THEN statement are independent from these blocks.

It can read as :

if (cond)
   first()
   //do something
   then()
else (cond)
   first()
   //do something
   then()
else (cond)
   first()
   //do something
   then()
else (cond)
   //do something
else
   //do something
end


function first()
   //do something
return
function then()
   //do something
return

These functions are just a form to read. They wouldn't create scope. It's more like a gosub/return from Basic.

Usefulness and readability as matter of discussion.

Answer 13

How about

alternate {
    statement 1,
    statement 2,
    [statement 3,...]
}

for cycling through the available statements on each successive pass.

Edit: trivial examples

table_row_color = alternate(RED, GREEN, BLUE);

player_color = alternate(color_list); // cycles through list items

alternate(
    led_on(),
    led_off()
);

Edit 2: In the third example above the syntax is maybe a bit confusing as it looks like a function. In fact, only one statement is evaluated on each pass, not both. A better syntax might be something like

alternate {
    led_on();
}
then {
    led_off();
}

Or something to that effect. However I do like the idea that the result of which ever is called can be used if desired (as in the color examples).

Answer 14

D's scope guards are a useful control structure that isn't seen very often.

Answer 15

I think I should mention CityScript (the scripting language of CityDesk) which has some really fancy looping constructs.

From the help file:

{$ forEach n var in (condition) sort-order $}
... text which appears for each item ....
{$ between $}
.. text which appears between each two items ....
{$ odd $}
.. text which appears for every other item, including the first ....
{$ even $}
.. text which appears for every other item, starting with the second ....
{$ else $}
.. text which appears if there are no items matching condition ....
{$ before $}
..text which appears before the loop, only if there are items matching condition
{$ after $}
..text which appears after the loop, only of there are items matching condition
{$ next $}

Answer 16

Also note that many control structures get a new meaning in monadic context, depending on the particular monad - look at mapM, filterM, whileM, sequence etc. in Haskell.

Answer 17

ignoring - To ignore exceptions occuring in a certain block of code.

try {
  foo()
} catch {
  case ex: SomeException => /* ignore */
  case ex: SomeOtherException => /* ignore */
}

With an ignoring control construct, you could write it more concisely and more readably as:

ignoring(classOf[SomeException], classOf[SomeOtherException]) {
  foo()
}

[ Scala provides this (and many other Exception handling control constructs) in its standard library, in util.control package. ]

Answer 18

Something that replaces

bool found = false;
for (int i = 0; i < N; i++) {
  if (hasProperty(A[i])) {
    found = true;
    DoSomething(A[i]);
    break;
  }
}
if (!found) {
  ...
}

like

for (int i = 0; i < N; i++) {
  if (hasProperty(A[i])) {
    DoSomething(A[i]);
    break;
  }
} ifnotinterrupted {
  ...
}

I always feel that there must be a better way than introducing a flag just to execute something after the last (regular) execution of the loop body. One could check !(i < N), but i is out of scope after the loop.

Answer 19

I'd like to see a keyword for grouping output. Instead of this:

        int lastValue = 0;

        foreach (var val in dataSource)
        {
            if (lastValue != val.CustomerID)
            {                    
                WriteFooter(lastValue);
                WriteHeader(val);
                lastValue = val.CustomerID;
            }
            WriteRow(val);
        }
        if (lastValue != 0)
        {
            WriteFooter(lastValue);
        }

how about something like this:

        foreach(var val in dataSource)
        groupon(val.CustomerID)
        {            
            startgroup
            {
                WriteHeader(val);
            }
            endgroup
            {
                WriteFooter(val)
            }
        }
        each
        {
            WriteRow(val);
        }

If you have a decent platform, controls, and/or reporting formatting you won't need to write this code. But it's amazing how often I find myself doing this. The most annoying part is the footer after the last iteration - it's hard to do this in a real life example without duplicating code.

Answer 20

This is a bit of a joke, but you can get the behavior you want like this:

#include <iostream>
#include <cstdlib>

int main (int argc, char *argv[])
{
  int N = std::strtol(argv[1], 0, 10); // Danger!
  int state = 0;
  switch (state%2) // Similar to Duff's device.
  {
    do {
      case 1: std::cout << (2*state) << " B" << std::endl;
      case 0: std::cout << (2*state+1) << " A" << std::endl; ++state;
    } while (state <= N);
      default: break;
  }

  return 0;
}

p.s. formatting this was a bit difficult and I'm definitely not happy with it; however, emacs does even worse. Anyone care to try vim?

Answer 21

Generators, in Python, are genuinely novel if you've mostly worked with non-functional languages. More generally: continuations, co-routines, lazy lists.

Answer 22

This probably doesn't count, but in Python, I was upset there was no do loop.

Anto ensure I get no upvotes for this answer, I wind up annoyed at any language I work in for any period of time that lacks goto's.

Answer 23

for int i := 0 [down]to UpperBound() [step 2]

Missing in every C-derived language.

Please consider before you vote or write a comment:
This is not redundant to for (int i = 0; i <= UpperBound(); i++), it has different semantics:

1. UpperBound() is evaluated only once

2. The case UpperBound() == MAX_INT does not produce an infinite loop

Answer 24

foo();

while(condition)
{
   bar();
   foo();
}

Answer 25

This is similar to the response by @Paul Keister.

(mumble, mumble) years ago, the application I was working on had lots of variations of so-called control-break processing -- all that logic that goes into breaking sorted rows of data into groups and subgroups with headers and footers. As the application was written in LISP, we had captured the common idioms in a macro called WITH-CONTROL-BREAKS. If I were to transpose that syntax into the ever-popular squiggly form, it might look something like this:

withControlBreaks (x, y, z : readSortedRecords()) {
  first (x) :     { emitHeader(x); subcount = 0; }
  first (x, y) :  { emitSubheader(x, y); zTotal = 0; }
  all (x, y, z) : { emitDetail(x, y, z); ztotal += z; }
  last (x, y) :   { emitSubfooter(x, y, zTotal); ++subCount; }
  last (x) :      { emitFooter(x, subcount); }
}

In this modern era, with widespread SQL, XQuery, LINQ and so on, this need does not seem to arise as much as it used to. But from time to time, I wish that I had that control structure at hand.

Answer 26

How about PL/I style "for" loop ranges? The VB equivalent would be:

' Counts 1, 2, ... 49, 50, 23, 999, 998, ..., 991, 990
  For I = 1 to 50, 23, 999 to 990 Step -1

The most common usage I can see would be to have a loop run for a list of indices, and then throw in one more. BTW, a For-Each usage could also be handy:

' Bar1, Bar2, Bar3 are an IEnum(Wazoo); Boz is a Wazoo
  For Each Foo as Wazoo in Bar1, Bar2, Enumerable.One(Boz), Bar3

This would run the loop on all items in Bar1, all items in Bar2, Boz, and Bar3. Linq would probably allow this without too much difficulty, but intrinsic language support might be a little more efficient.

Answer 27

One of the control structures that isn't available in many languages is the case-in type structure. Similar to a switch type structure, it allows you to have a neatly formatted list of possible options, but matches the first one that's true (rather then the first one that matches the input). A LISP of such such (which does have it):

(cond
   ((evenp a) a)        ;if a is even return a
   ((> a 7) (/ a 2))    ;else if a is bigger than 7 return a/2
   ((< a 5) (- a 1))    ;else if a is smaller than 5 return a-1
   (t 17))              ;else return 17

Or, for those that would prefer a more C-like format

cond 
    (a % 2 == 0): 
        a;     break;
    (a > 7):
        a / 2; break;
    (a < 5):
        a - 1; break;
    default:
        17;    break;

It's basically a more accurate representation of the if/elseif/elseif/else construct than a switch is, and it can come in extremely handing in expressing that logic in a clean, readable way.

Answer 28

How about iterating with a moving window (of n elements instead of 1) through a list? This is tangentially related @munificent's answer, I think.

Something like

#python
#sum of adjacent elements
for x,y in pairs(list):
    print x + y

def pairs(l):              
    i=0                    
    while i < len(l)-1:    
        yield (l[i],l[i+1])
        i+=1               

It is useful for certain types of things. Don't get me wrong, this is easy to implement as a function, but I think a lot of people try to bring out for and while loops when there are more specific/descriptive tools for the job.