Hello, I have a big table with 3 columns and 103918 rows. This is the example,

           time             species                 dbh
5 1  4.9377297 
575 1  11.64127213 
575 1  109.8182438 
575 1  8.029809521 
5 1  24.32501874 
575 1  4.895992119 
575 1  11.40567637 
575 1  2.795090562 
575 1  21.79281837 
575 1  52.57476174 
575 1  27.7290919 
575 1  3.23262083 
575 2  19.30612651 
575 1  2.956672964 
575 1  111.690689 
575 1  11.82499086 
575 1  63.86200585 
575 1  111.8312759 
575 1  49.23078501 
25 1  2.810866156 
575 1  10.93097209 
575 1  23.7930745 
575 1  21.68010008 
575 1  13.32423271 
575 1  23.10306499 
575 1  59.646657 
1000 2  20.47707761 
575 1  3.255755538 
575 1  29.3392412 
575 1  2.578551542 
575 1  52.71564453 
575 1  119.8069955 
575 1  83.45738555 
575 1  7.763555744 
725 1  2.578551542 
I would like to calculate the dbh mean but only for species 1, time 575. I have tried this

> names (exp1)
[1] "time"    "species" "dbh"    
> time = c(1)
> species=c(2)
> dbh=c(3)
> dbhmean <- mean (exp1 [time==575 & species ==1])
> dbhmean
named list()

I am not sure why it appears the message "named list()"...

Can anybody give me some hints on how to do this correctly?

Thanks.
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