# Conditional variable calculation

Dear Community,

I’m very fresh to Survey Solution designing, though I work with an already established questionnaire that needs some additional variables for manual validation. Therefore, this question might seem very simple to you.

The specific problem I’m having is that I want to calculate the standardized area of a plot in ha, based on different units a farmer can provide.
Thus when ar_1_1 is the raw area and ar_1_1_un the applied unit (suppose m²). Then ar_1_1_ha should be ar_1_1 / 10000. Though the difficulty I’m facing is to get several different calculations based on the differing units.
I tried if - else if - … - else conditions like in C# but they do not seem to work and throw a circular reference error.

Code looks like this:

if (ar_1_1_un == 1)
{
(ar_1_1_ha = ar_1_1);
}
else if (ar_1_1_un == 2)
{
(ar_1_1_ha = ar_1_1/2.47105);
}
else
{
(ar_1_1_ha = ar_1_1*ar_1_1_un_ha);
}

Help would be greatly appreciated.

Best,
Daniel

Hello @DOTTO , your intuition is correct, but you can’t write a program, the program has been already written for you:

`double AREA=______________;`

and all you have to do is to fill in the gap. That’s why it is an expression, not a program that you are writing, and you can’t use if…else in this context.

Programmers would want to use a ternary operator in this context. See here:

``````double AREA = (ar_1_1_un==1)?(ar_1_1):( ....)
``````

Statisticians would probably prefer:

``````COEF=new[]{989898, 1, 2.47105}[ar_1_1_un.Value]
AREA=ar_1_1/COEF
``````

With 2 or 3 units I’d write an expression with the ternary operator, but if the number of units is getting large, or you have several questions like that in the questionnaire - use lookup tables.

Best, Sergiy

PS: did you get it why you need 989898 in the list of coefficients?

Dear @sergiy,
thank you for the answer that seems very to the point and suited for my case! Thank you for taking the time!
Guessing on your post scriptum: If the multi-select question for the unit was not filled it evaluates to 0 as an index and therefore you’ll see an astronomically high area as a check? Though I’m not aware from were C# starts to count indexes

Best,
Daniel

No. If the question is not answered its value is null (not zero). You will get an exception at extracting an array element with such an index. Correspondingly the area is going t to have the null value, meaning ‘not calculated’.

This is closer to the point. C# starts indexing in arrays from zero (0), but your options in ar_1_1_un start from 1, so we need a fake element to align the array with the options in your units question. Any value will do (it will never be applied since 0 is not among the choices, so element with index 0 will never be addressed).

We could have avoided that by subtracting 1:

``````COEF=new[]{1, 2.47105}[ar_1_1_un.Value - 1]
``````

or a different offset if the options numbering starts with a different value, such as 101 (of course if the options are numbered with 101, I wouldn’t place 100 fake numbers in front of the usable ones).

Best, Sergiy

1 Like