[R] Comparison conditionals when extracting from data.frame not working reliably

[Matti Viljamaa]

I’m getting strange behaviour when trying to extract rows from a two-column data.frame with double values.

My data looks like:

       mom_iq kid_score
1   121.11750        65
2    89.36188        98
3   115.44320        85
4    99.44964        83
…

and I’m testing extracting rows that have mom_iq at some interval, so e.g.

kidmomiq[kidmomiq$mom_iq > 80.0 && kidmomiq$mom_iq < 130.0,]

correctly returns rows that have mom_iq \in ]80.0, 130.0[.

However, if I adjust this to:

kidmomiq[kidmomiq$mom_iq > 80.0 && kidmomiq$mom_iq < 120.0,]

(and there are clearly values that also fall \in ]80.0, 120.0[ !)

I get:

[1] mom_iq    kid_score
<0 rows> (or 0-length row.names)

Why does the extraction fail on some values, but not some others?