最近阿莎在學習Johns Hopkins University在Coursera.org 提供的R Programming課程
老師解說的很清楚，但是內容帶的很快, 有些函式完全沒有解說，因為簡單的函式自已搜尋就可以了。 然而阿莎發現台灣的教學或分享的內容都很簡易，花了好多時間在找文章，因此在此分享阿莎筆記，可以節省很多重覆的搜尋時間。
Quiz 的題目會有少許變化，阿莎的題目和答案為下
Question 1
R was developed by statisticians working at
Answer
The University of Auckland
Question 2
The definition of free software consists of four freedoms (freedoms 0 through 3). Which of the following is NOT one of the freedoms that are part of the definition?
Answer
任何目的(any purpose), restriait (限制公開)，undesired purpose (不被接受的)的3個選擇是答案
Question 3
In R the following are all atomic data types EXCEPT
Answer
Atomic data type 是data的最小單位。因此以下非Atomic data type:
matrix，data frame, table, array
Question 4
If I execute the expression x <- 4 in R, what is the class of the object 'x' as determined by the 'class()' function?
Answer
單一整數(x <- 4)為numeric, 多整數(y<-1:4)為integer. numeric可以包含小數，但integer只代表整數，因此integer為numeric的subset. 答案為numeric
Question 5
What is the class of the object defined by the expression x <- c(4, 4,"a", TRUE)?
Answer
vector中的class都必須一樣，不一樣的情況下系統會自動強制變更。
變更順序為character, complex, numeric, integer, logical。有character的話變更成character，再者為complex, 以此類推。
答案為character
Question 6
If I have two vectors x <- c(1,3, 5) and y <- c(3, 2, 10), what is produced by the expression cbind(x, y)?
Answer
a numeric matrix with 3 rows and 2 columns
> x <- c(1,3, 5)
> y <- c(3, 2, 10)
> cbind(x,y)
x y
[1,] 1 3
[2,] 3 2
[3,] 5 10
Question 7
A key property of vectors in R is that
Answer
elements of a vector all must be of the same class
Question 8
Suppose I have a list defined as x <- list(2, "a", "b", TRUE). What does x[[1]] give me?
Answer
a numeric vector containing the element 2.
Question 9
Suppose I have a vector x <- 1:4 and y <- 2:3. What is produced by the expression x + y?
Answer
an integer vector with the values 3, 5, 5, 7.
> x <- 1:4
> y <- 2:3
> x+y
[1] 3 5 5 7
Question 10
Suppose I have a vector x <- c(17, 14, 4, 5, 13, 12, 10) and I want to set all elements of this vector that are greater than 10 to be equal to 4. What R code achieves this?
Answer
x[x > 10] <- 4
Question 11
In the dataset provided for this Quiz, what are the column names of the dataset? 1, 2, 3, 4, 5, 6
Answer
Ozone, Solar.R, Wind, Temp, Month, Day
Explanation
> data<-read.table('H:/Desktop/Data Specialist/R programming/Quiz1/rprog_data_quiz1_data/hw1_data.csv',header=TRUE,sep=",")
> head(data)
Ozone Solar.R Wind Temp Month Day
Question 12
Extract the first 2 rows of the data frame and print them to the console. What does the output look like?
Answer
Ozone Solar.R Wind Temp Month Day
1 41 190 7.4 67 5 1
2 36 118 8.0 72 5 2
Explanation
> head(data,2)
Ozone Solar.R Wind Temp Month Day
1 41 190 7.4 67 5 1
2 36 118 8.0 72 5 2
Question 13
How many observations (i.e. rows) are in this data frame?
Answer
153
Explanation
> nrow(data)
[1] 153
Question 14
Extract the last 2 rows of the data frame and print them to the console. What does the output look like?
Answer
Ozone Solar.R Wind Temp Month Day
152 18 131 8.0 76 9 29
153 20 223 11.5 68 9 30
Explanation
> tail(data,2)
Ozone Solar.R Wind Temp Month Day
152 18 131 8.0 76 9 29
153 20 223 11.5 68 9 30
Question 15
What is the value of Ozone in the 47th row?
Answer
21
Explanation
> data[47,]
Ozone Solar.R Wind Temp Month Day
47 21 191 14.9 77 6 16
Question 16
How many missing values are in the Ozone column of this data frame?
Answer
37
Explanation
#Calculate how many missing data
> length(which(is.na(data)))
[1] 44
#Calculate how many missing date with column name of Ozone
> datana<-subset(data,is.na(Ozone))
> nrow(datana)
[1] 37
Question 17
What is the mean of the Ozone column in this dataset? Exclude missing values (coded as NA) from this calculation.
Answer
42.1
Explanation
> datanotna<-subset(data,!is.na(Ozone))
> apply(datanotna,2,mean)
Ozone Solar.R Wind Temp Month Day
42.129310 NA 9.862069 77.870690 7.198276 15.534483
Question 18
Extract the subset of rows of the data frame where Ozone values are above 31 and Temp values are above 90. What is the mean of Solar.R in this subset?
Answer
212.8
Explanation
> datasub18<-subset(data,data$Ozone>31 & data$Temp>90,select=Solar.R)
> apply(datasub18,2,mean)
Solar.R
212.8
Question 19
What is the mean of "Temp" when "Month" is equal to 6?
Answer
79.1
Explanation
> datasub19<-subset(data,data$Month==6,select=Temp)
> apply(datasub19,2,mean)
Temp
79.1
Question 20
What was the maximum ozone value in the month of May (i.e. Month = 5)?
Answer
115
Explanation
> datasub20<-subset(data,!is.na(Ozone)&data$Month==5,select=Ozone)
> apply(datasub20,2,max)
Ozone
115 資料來源:https://github.com/cynthia0611/Coursera-R-Programming/blob/master/Week-1/Quiz%201.md
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