> library(quantmod)
> day
[1] "2016-05-27"
> z=yt[[1]]
> head(z)
           Open High Low Close Volume         B
2015-03-17  303  305 303   303 552000 167771000
2015-03-18  303  303 302   303 326000  98694000
2015-03-19  303  303 300   301 548000 165158000
2015-03-20  300  301 300   300 410000 123119000
2015-03-23  301  303 300   303 428000 129098000
2015-03-24  303  304 301   302 537000 162352000

> max(Hi(z))==Hi(z[day])
            High
2016-05-27 FALSE
> z=yt[["6476-T"]]
> tail(z)
           Open High Low Close Volume       B
2016-05-13  929  929 929   929    400  371600
2016-05-16  927  930 927   927   2200 2042100
2016-05-17   NA   NA  NA    NA      0       0
2016-05-18  927  927 927   927    200  185400
2016-05-19  927  927 927   927   2100 1946700
2016-05-20  927  927 927   927   1300 1205100
> max(Hi(z))==Hi(z[day])
     [,1]
> is.null(z[day])
[1] FALSE
> is.na(z[day])
     Open High Low Close Volume B
> is.nan(z[day])
     Open High Low Close Volume B
> which(sapply(yt,function(z)max(Hi(z))==Hi(z[day])))
Error in which(sapply(yt, function(z) max(Hi(z)) == Hi(z[day]))) :
  argument to 'which' is not logical
> length(z[day])
[1] 0
> which(sapply(yt,function(z)max(Hi(z))==ifelse(length(z[day]),Hi(z[day]),0)))
1810-T 1967-T 2266-T 2270-T 2458-T 2590-T 3385-T 3462-T 3463-T 3676-T 3696-T
   288    386    524    529    620    663   1059   1111   1112   1194   1213
3946-T 4218-T 4506-T 4581-T 6005-T 6465-T 7618-T 7717-T 8783-T 9684-T 9766-T
  1355   1433   1529   1581   2041   2332   2875   2904   3442   3787   3839
9919-T 1482-T
  3917   3979