How to avoid returning a Null if there is no “else” condition in an If contruct

Question

I need to use If but with only one option that is if "a" then do "b", else do nothing. So I wrote If[a,b] but the problem is that if it is not a it returns Null in my output. How to avoid this?

Here is the specific example I was working on.

I am looking for the number of comparisons performed by the quick sort algorithm. Here is the code from Rosetta code with my additions

QuickSort[x_List] := 
 Module[{pivot, aa = 0, bb = 0}, If[Length@x <= 1, Return[x]];
  pivot = First[x];
  aa = If [Length[Cases[x, j_ /; j < pivot]] > 1, 
    Length[Cases[x, j_ /; j < pivot]] - 1 , Sequence[]];
  bb = If [Length[Cases[x, j_ /; j < pivot]] > 1, 
    Length[Cases[x, j_ /; j > pivot]] - 1 , Sequence[]]; 
  count = count + aa + bb;
  Flatten@{QuickSort[Cases[x, j_ /; j < pivot]], 
    Cases[x, j_ /; j == pivot], 
    QuickSort[Cases[x, j_ /; j > pivot]]} ; Return[count] ]

now if you run QuickSort[{4, 3, 2, 1, 5}] you will get 2+2 Null instead of 4

Answer 1

It depends what you consider nothing, but you could try something like this

If[a, b, Unevaluated[Sequence[]]]

for example

3 + If[False, 1, Unevaluated[Sequence[]]]

returns 3. Wrapping an argument of a function in Unevaluated is effectively the same as temporarily setting the attribute Hold for that argument meaning that the argument isn't evaluated until after it's inserted in the definition of that function.

By the way, in your definition of QuickSort you're calling Cases[x, j_ /; j < pivot] six times. It's probably more efficient to assign Cases[x, j_ /; j < pivot] to a dummy variable and use that instead.

Answer 2

If does not have the attribute SequenceHold; use the "vanishing function" ##&[] instead of Sequence[]:

If[# > 5, #, ## &[]] & /@ Range[10]

{6, 7, 8, 9, 10}

See this and this for other uses, and SlotSequence if you are confused by ##.

Answer 3

I note that all answers so far try to solve the problem of assigning a potential Null value by manipulating the return value. I feel it would be more appropriate to make the whole assignment conditional. Like this:

If[condition, aa = value]

There's also a small bug in your program (count isn't initialized), and, of course, it doesn't sort at the moment. I assume that you aware of that and that the Return value is used for testing.

The code would then be:

QuickSort[x_List] := 
 Module[{pivot, aa = 0, bb = 0, count = 0}, 
  If[Length@x <= 1, Return[x]];
  pivot = First[x];
  If[Length[Cases[x, j_ /; j < pivot]] > 1,
     aa = Length[Cases[x, j_ /; j < pivot]] - 1
  ];
  If[Length[Cases[x, j_ /; j < pivot]] > 1, 
     bb = Length[Cases[x, j_ /; j > pivot]] - 1
  ];
  count = count + aa + bb;
  Flatten@{QuickSort[Cases[x, j_ /; j < pivot]], 
    Cases[x, j_ /; j == pivot], QuickSort[Cases[x, j_ /; j > pivot]]};
   Return[count]] 

Remove ;Return[count] to let it sort again.

I'm not sure about the j < pivot test in the second If. It depends on your intention with bb, but I guess the test should be j > pivot.

Answer 4

The semantics for if is If[cond,t,f] if f (False) is not given Null is returned.

I am guessing that you need this in a pattern, then you can use Condition:

f[x_] := ppp[x] /; x > 0
f[5]
f[-6]

OK, here is a different version of quick sort (I found on my disk - I am not sure how the author is...)

ClearAll[quickSort1]
quickSort1[lst0_] := 
 Block[{left, right, pivot, lst, $RecursionLimit = 10^6},
  pivot = First[lst0];
  lst = Rest[lst0];
  left = Select[lst, # <= pivot &];
  If[left =!= {}, left = quickSort1[left]];
  right = Select[lst, # > pivot &];
  If[right =!= {}, right = quickSort1[right]];
  Join[left, {pivot}, right]
  ]

If someone feels like adding the counter... my brain feels like gum this morning... please go ahead. The point in the above code is to not use the return value of If but to set it to a variable.

Answer 5

In Mathematica, “not returning anything” is not possible. An expression that does not return anything has a value of Null, even though you only actually see this Null in certain circumstances:

Null
    is a symbol used to indicate the absence of an expression or a result. It is not displayed in ordinary output.

When Null appears as a complete output expression, no output is printed.

So, Null is always returned, but whether it's displayed depend on what you do with this returned value. For example:

In[1]:= If[x > 0, 1]; /. x -> -1
(* nothing is output *)
In[2]:= {If[x > 0, 1]; /. x -> -1, 42}
Out[2]= {Null, 42}