R for Everyone: Advanced Analytics and Graphics (Addison-Wesley Data & Analytics Series) by Jared P. Lander

Author:Jared P. Lander [Lander, Jared P.]
Language: eng
Format: azw3
Publisher: Pearson Education
Published: 2017-06-14T04:00:00+00:00

Click here to view code image

> tipsByDay <- ddply(tips, "day", plyr::summarize,

+ tip.mean=mean(tip), tip.sd=sd(tip),

+ Length=NROW(tip),

+ tfrac=qt(p=.90, df=Length-1),

+ Lower=tip.mean - tfrac*tip.sd/sqrt(Length),

+ Upper=tip.mean + tfrac*tip.sd/sqrt(Length)

+ )

>

> ggplot(tipsByDay, aes(x=tip.mean, y=day)) + geom_point() +

+ geom_errorbarh(aes(xmin=Lower, xmax=Upper), height=.3)

The use of NROW instead of nrow is to guarantee computation. Where nrow works only on data.frames and matrices, NROW returns the length of objects that have only one dimension.

> nrow(tips)

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