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Re: Function returns null

  • To: mathgroup at
  • Subject: [mg104739] Re: [mg104689] Function returns null
  • From: Leonid Shifrin <lshifr at>
  • Date: Sun, 8 Nov 2009 06:47:58 -0500 (EST)
  • References: <>

Hi Michael,

You are obviously trying to use the syntax of some C-like language in
Mathematica code, and that is the main source of confusion.

Specifically, your code returned Null because it was missing a semicolon
after a closing bracket of the <If> statement (indeed, coming from C or Java
placing such a semicolon is the last thing to think about).

A few pointers:

1. All commands (including If, For, other control flow constructs) are
statements in Mathematica. Some of them return nothing - this means they
retun Null.

2. Curly braces mean lists - a most common data structure in Mathematica.
Here they are *not* what they are in C or Java (where they are used to
block-structure the code) - use parentheses instead (not always needed). In
your code, you could have omitted any explicit block-structuring.

Here is a version of your code that works (I didn't get the purpose of the
line sodv = sOd[sodv] in the original version - I guess it's a mistake):

sOd[x0_] :=
 Module[{sodv, x, digit},
  x = IntegerPart[x0];
  sodv = x;
  If[(x > 9),
   sodv = 0;
   While[x > 0, digit = (FractionalPart[x/10])*10;
    sodv = sodv + digit;
    x = IntegerPart[x/10];]];

Note that in most cases procedural programming is not the best way to code
in Mathematica. Here is how I would code the function you need:

sOdAlt[x_Integer] := Total@IntegerDigits[x];
sOdAlt[x_?NumericQ] := sOdAlt[IntegerPart[x]];

If you plan to do any serious work/programming in Mathematica,
a good time investment would be to have a closer look at the online
documentation, Mathematica book or other resources to get familiar with the
Mathematica way of doing things.

Hope this helps.


On Sat, Nov 7, 2009 at 3:46 AM, Michael Greene <mgreene at> wrote:

> I'm getting an unexpected result when I remove a print statement from my
> function. The function computes a single digit sum of digits and returns
> what I expect as long as the print statement just prior to the function
> exit
> is present. I can verify that by invoking the function within a print
> statement, e.g.,
> Print["sOd returned - ",sOd[3329]]
> When I remove the print  within the function, the function returns a null
> result. What simple point am I missing ? Does the := operator have
> something
> to do with this?
> Here is the function: (with embedded print statement)
> sOd[x0_] := Module[{sodv, x, digit},
>  x = IntegerPart[x0]; (* ensure we have an integer input*)
>  sodv = x;
>  If[(x > 9), {              (* sum digits if input has two or more digits*)
>     sodv = 0;
>     While[x > 0,
>      digit = (FractionalPart[x /10])*10;
>      sodv = sodv + digit;
>      x = IntegerPart[x/10];
>      ];
>     sodv = sOd[sodv];  (* check for multiple digit sum  *)
>     }
>    ]
>   Print["sOd returns -", sodv];  (* remove this and function returns null*)
>  Return[sodv];             (* return single digit sum *)
>  ]
> Thanks,
> Michael Greene

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