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stocks <- read.delim("stocks.txt") 
the.tickers <-unique(stocks$ticker)
lows <- c() 
highs <- c()
for(ticker in the.tickers) {   
      look.at <-stocks$ticker == ticker  ## i do not know why the code wrote as this. Any one know? 
      lows <- append(lows,min(stocks$low[look.at],na.rm=TRUE)) ## to find lowest in the 'low' column for each ticker  
      highs <-append(highs,max(stocks$high[look.at],na.rm=TRUE)) 
}  ## to find the highest in the 'high' column for each ticker
the.tickers

My question is, variable look.at is a boolean vector, which contains FALSE. Because there is no character showing as ticker. How can stocks$low return a numerical value even though look.at is all FALSE?

Here is a brief summary of stocks.txt data structure:

str(stocks)
'data.frame':   70061 obs. of  8 variables:
 $ ticker  : Factor w/ 23 levels "AAPL","AMGN",..: 12 12 12 12 12 12 12 12 12 12 ...
 $ industry: Factor w/ 7 levels "Banks","Biotechnology",..: 7 7 7 7 7 7 7 7 7 7 ...
 $ date    : Factor w/ 3086 levels "1-Apr-02","1-Apr-03",..: 1291 1086 985 683 580 478 373 269 2948 2843 ...
 $ open    : num  817 811 805 818 827 ...
 $ high    : num  818 819 813 820 827 ...
 $ low     : num  811 806 801 813 817 ...
 $ close   : num  815 811 808 814 822 ...
 $ volume  : num  1464122 2098176 1838552 3099791 1651111 ...

As you can see, look.at is a boolean object. And it is all False

head(look.at,10)
 [1] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE

This is the return value:

head(stocks$low[look.at],10)
 [1] 14.94 15.06 15.04 15.12 15.20 15.25 14.95 14.41 14.50 14.58

It does not make sense why Stocks$low[look.at] can return value while look.at is all FALSE.

Variable low is a numeric variable that only contains numbers and NA values. 

head(stocks$low, 10)
 [1] 811.44 806.45 801.47 813.34 817.39 822.31 823.67 831.50 825.05 829.58

sample of data

> head(stocks,10)
   ticker   industry      date   open   high    low  close  volume
1    GOOG Technology 20-Mar-13 816.83 817.51 811.44 814.71 1464122
2    GOOG Technology 19-Mar-13 811.24 819.25 806.45 811.32 2098176
3    GOOG Technology 18-Mar-13 805.00 812.76 801.47 807.79 1838552
4    GOOG Technology 15-Mar-13 818.50 820.30 813.34 814.30 3099791
5    GOOG Technology 14-Mar-13 826.99 826.99 817.39 821.54 1651111
6    GOOG Technology 13-Mar-13 827.90 830.69 822.31 825.31 1641413
7    GOOG Technology 12-Mar-13 830.71 831.89 823.67 827.61 2008979
8    GOOG Technology 11-Mar-13 831.69 839.70 831.50 834.82 1595678
9    GOOG Technology  8-Mar-13 834.50 834.92 825.05 831.52 2912283
10   GOOG Technology  7-Mar-13 834.06 836.62 829.58 832.60 2054238

The.tickers as follows. 

> the.tickers <- unique(stocks$ticker)
> the.tickers
 [1] GOOG AAPL Msft C    KEY  WFC  JPM  SO   DUK  D    HE   EIX  LUV  AMGN GILD CELG BIIB CAT  DE   IMO  MRO  HES  YPF 
Levels: AAPL AMGN BIIB C CAT CELG D DE DUK EIX GILD GOOG HE HES IMO JPM KEY LUV MRO Msft SO WFC YPF
cljsibyl123
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  • It would be great if you could supply a minimal reproducible example to go along with your question. Something we can work from and use to show you how it might be possible to answer your question. That way others can also befit form your question, and the accompanying answer, in the future. You can have a look at [this SO post](https://stackoverflow.com/questions/5963269/how-to-make-a-great-r-reproducible-example) on how to make a great reproducible example in R. – Eric Fail Jan 03 '16 at 20:31

2 Answers2

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My question is, variable look.at is a boolean value, how can stocks$low pick up a boolean value, and return a numerical value?

look.at is a boolean vector, not a value. The bracket operator applies each value of the boolean vector to each value of the numeric vector and returns a new vector containing the values of stocks$low that correspond to TRUE values within look.at. Example:

> stockprice <- c(1,2,3)
> look.at <- c(T,F,T)
> stockprice[look.at]
[1] 1 3

Mirosław Zalewski makes the great point in a comment that (unlike what I thought), the bracket operation will work even if look.at is longer or shorter than the stockprice vector:

stockprice <- c(1,2,3,4,5)
look.at <- c(T,F,T,T)
stockprice[look.at]
[1] 1 3 4 5

> stockprice <- c(1,2,3)
> look.at <- c(T,F,T,T)
> stockprice[look.at]
[1]  1  3 NA
Community
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patrickmdnet
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  • "note that the operation only works if both vectors are the same length" - not true. Logical vector values will be repeated until they match length of substituted vector. If logical vector is longer than substituted vector, all TRUE will return NA (which kind of makes sense, because there is no value at specified position). But you do have a point that when vectors are not the same length, results might be unpredictable. – Mirek Długosz Jan 03 '16 at 20:46
  • My case is like > stockprice <- c(1,2,3) look.at <- c(F,F,F), stockprice[look.at] still gives me value 1,2,3. It does not make sense that, why the stockprice[look.at] can pick up value even look.at is all equal to False?? Thanks. – cljsibyl123 Jan 04 '16 at 04:11
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If you are trying to get low and high price per ticker, here is how I would do it with the dplyr package:

library(dplyr)
stocks %>% group_by(ticker) %>% summarise(low = min(low, na.rm = TRUE), high = max(high, na.rm = TRUE))
Gopala
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