Why is this returning 0?

Question

Perhaps I have spent one too many nights up late, but I had the following code :

MessageBox.Show(
  Convert.ToInt32(
    Math.Round(
      (double)( ( i+1 )/r.Count )*100 ,
      0
    )
  ).ToString()
);

Which always returns 0, even when i=120000 and r.Count=150000. This is a simple progress calculation where i is the current iteration in the loop, and r is the collection ( List() ).

I have separated the code, and even the following still returns 0 :

Convert.ToInt32( 
           Math.Round( 
                      (double)(( 120000+1 )/167428 )*100
                      , 0 )
                )

Not sure whats going on here so any insight as to what I am doing wrong (probably a forehead slapper). It should be (using above numbers), returning 71.

@Habib's flag as duplicate :

that duplicate you claim -- is actually different, even the solutions don't apply here as this was a casting issue to double, and not adding 'm' with additional decimal points to each variable in play.

so what you guys are saying (reading answers below), is that I need to cast the integers both to double first, and that .NET doesn't do any magic when using mathematical operations (eg, float the number if divising two integers when needed) ? — Kraang Prime, Jul 09 '15 at 17:11
no just casting one will do, but do it before division not after. — Chaitanya Gadkari, Jul 09 '15 at 17:14
@SanuelJackson It can't guess what you want, and in some cases integer division *is* what you want. So no, it doesn't do magic. — Borgleader, Jul 09 '15 at 17:14
Got it. Makes sense. Adding a footnote, this wasn't as much of a forehead slapper as I thought it would be -- the math was technically correct, just the values weren't auto-converting to double. Easy fix, but this is one of those moments I was expecting .NET to take care of more than it actually does :) . — Kraang Prime, Jul 09 '15 at 17:17
lol. love the downers tho. -.- ... like why down-vote when the question is legit, and clear. sometimes this boggles me as to why people down-vote easy questions (or difficult ones). PS --- that duplicate you claim Habib -- is actually different, even the solutions don't apply here as this was a casting issue to `double`, and not adding 'm' with additional decimal points to each variable in play. — Kraang Prime, Jul 09 '15 at 17:19

Michael Dunlap · Answer 1 · 2015-07-09T17:30:03.390

9

Because ( 120000+1 )/167428 is still being done using integer math. Instead of ~0.716, it's 0.

If you want to avoid this in the future, make sure you're casting the numerator and denominator to a double as well.

( (double)( i+1 )/(double)r.Count )*100

edited Jul 09 '15 at 17:30

answered Jul 09 '15 at 17:09

Michael Dunlap

4,300
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The answer is right, but I think you meant `~0.716`. Either way it will still round to 0. I'm just OCD with math. :p – Drew Kennedy Jul 09 '15 at 17:11
1

@DrewKennedy Sorry, finger slipped. – Michael Dunlap Jul 09 '15 at 17:30

score 4 · Answer 2 · answered Jul 09 '15 at 17:09

4

You are casting to double after doing integer division.

answered Jul 09 '15 at 17:09

Matt Eckert

1,946
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16

score 3 · Accepted Answer · answered Jul 09 '15 at 17:10

3

The following will evaluate to ( 120000+1 )/167428 ) to 0 as they are int's and do not keep decimal precision, try this

Convert.ToInt32(Math.Round(((double)(120000+1 ) / (double)167428 ) * 100))

answered Jul 09 '15 at 17:10

Marko

2,734
24
24

sorry... have to wait 10 minutes to accept now ... o.0. When did SO implement that ? – Kraang Prime Jul 09 '15 at 17:13

Jorgesys · Answer 4 · 2015-07-09T17:21:26.183

2

this is related to implicit conversion

https://msdn.microsoft.com/en-us/library/ms173105.aspx

the operation: (120000+1)/167428 returns an integer as result, 0. you are casting double after doing this integer division.

like other users suggest try casting the values:

  (double)(( (float)120000f+1 )/(float)167428f )*100

edited Jul 09 '15 at 17:21

answered Jul 09 '15 at 17:10

Jorgesys

124,308
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Really, referencing a java doc for a C# question? – leppie Jul 09 '15 at 17:17
Why not the concept is the same, probably your knowledge is only C# or .net technologies. – Jorgesys Jul 09 '15 at 17:21
And? There has already been 5 other answers saying that. Misleading redirection is foul. But I see you at least fixed it. Thanks. – leppie Jul 09 '15 at 17:24
"hasta Lo que no se tragan les hace daño", ok so merg a chuparselas si come rahat! :) thak you. – Jorgesys Jul 09 '15 at 17:37
1

XD .. nice. I upvoted for the research. Also noticed you changed to MSDN link which is cool. Just like to point out, if using `f`, then don't need to cast using `(float)`. – Kraang Prime Jul 09 '15 at 17:39

score 0 · Answer 5 · answered Jul 09 '15 at 17:09

(( 120000+1 )/167428 ) will evaluate to 0

167428 does not go into 120001. what you can try is this:

Convert.ToInt32( 
       Math.Round( 
                  (((double)120000f+(double)1f )/(double)167428f )*(double)100f
                  , 0 )
            )

score 0 · Answer 6 · answered Jul 09 '15 at 17:11

If r.Count > i+1, then (i+1) / r.Count will give you a zero. Both are integers and so anything after the decimal point gets cut off.

You probably mean to cast either i+1 or r.Count to double, then you'll get the answer you expect.

Eg:

(i+1) / (double)r.Count

score 0 · Answer 7 · answered Jul 09 '15 at 17:14

0

Try this:

You have to convert one of the two values to double before you divide.

Convert.ToInt32( 
           Math.Round( 
                      (((double)( 120000+1 ))/167428 )*100
                      , 0 )
                )

answered Jul 09 '15 at 17:14

whymatter

755
8
18

Why is this returning 0?

7 Answers7