(I originally asked this on stackoverflow. However, Excel formulas definitely straddle the line between "programming" and, um, "superusing", so I believe this cross-post is appropriate.)

Many of Excel's built-in functions can take array arguments. Sometimes the result is documented in the help and sometimes not. So:

```
=IF({1,0,1}, 42, 99)
```

will return {42, 99, 42}. The help for 'IF' covers array arguments. But:

```
=INDEX({2,3,5,7,11}, {2,4})
```

will return {3, 7}. This is intuitive, but I can't find a Microsoft source that documents it. And:

```
=INDEX({1,2,3,4;5,6,7,8;9,10,11,12;13,14,15,16}, {1,3},{2,4})
```

returns {2, 12}, which is not intuitive at all.

Is there any source that covers these less-common array usages? It seems like there has to be one, but I'm not finding it in a web search because it requires using "Excel", "function", and "array"...

(Also, can someone tag this question with 'array-formula' since I lack the power to create tags?)

I'm comfortable with using array formulas and use them all the time, but I hate having to figure out what will happen by trial and error. This particular question arose when I was doing the (simplified equivalent of):

```
=IF(ISNA(udf()), {1,2,3,4}, {5,6,7,8})
```

where 'udf' was returning, say, {1,2,3}, and the formula was evaluating to {5,6,7,#N/A}. This surprised me, even though I can figure out what is going on and work around it. It just bugs me that I can't find an authoritative source that lays it all out.

Link to question on stackoverflow (there are several answers and some comments, but nothing that satisfies the question right now) here: http://stackoverflow.com/questions/3998632/is-there-any-documentation-of-the-behavior-of-built-in-excel-functions-called-wit

EDIT: I made a large edit in the stackoverflow version of this question, and incorporated part of Neal's answer there. The addition is very much on the "programming" side of Excel formulas, though, so I'm not going to repeat it here.