20180908 class note

# class 조건 걸어 추출

if("dplyr" %in% installed.packages("dplyr") == FALSE)install.packages("dplyr")
library(dplyr)


exam <- read.csv("https://www.dropbox.com/s/zn24y4yccnm2g05/csv_exam.csv?dl=1")
head(exam)
class(exam)
exam %>% dplyr::filter(class==1)
# not equal 
exam %>% dplyr::filter(class!=1)
# and 
exam %>% dplyr::filter(class==1 & math > 50)
# or 
exam %>% dplyr::filter(class==1 | math > 50)
exam %>% dplyr::filter(class==1|class==3|class==5)
# or 구문 중첩하여 사용할 경우 아래와 같이 %in% 구문으로 대체 가능
exam %>% dplyr::filter(class %in% c(1,3,5))

class1 <- exam %>% dplyr::filter(class == 1)
mean(class1$math)
# math 전체를 가져옴
exam %>% select(math)
# math가 아닌 것들만 가져옴
exam %>% select(-math)

exam %>% 
  filter(class==1) %>% 
  select(english) %>% 
  head

# 1반의 수학점수를 내림차림으로 정렬하시오.
# 오름차순 arrange()
# 내림차순 arrange(des())
exam %>% 
  filter(class==1) %>% 
  select(math) %>% 
  arrange(desc(math))

exam %>% 
  arrange(class, desc(math)) %>% 
  mutate(total=math+english+science) %>% 
  head

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