# load data
ii<-read.csv('nhanes.csv')

# 1 (multiple solutions)
ii[ii$ridageyr<=24&!is.na(ii$military)&ii$military!='No',
   c('bpxsy1','bpxsy2','bpxsy3','bpxsy4')]
ii[ii$ridageyr<=24&!is.na(ii$military)&ii$military!='No',
   paste('bpxsy',1:4,sep='')]
subset(ii,ridageyr<=24&military!='No',select=c(bpxsy1,bpxsy2,bpxsy3,bpxsy4))

# 2 (multiple solutions)
ii[ii$ridageyr<=24&!is.na(ii$military)&ii$military!='No',seq(10,70,20)]
subset(ii,ridageyr<=24&military!='No',select=seq(10,70,20))

# 3 
ii[nrow(ii),seq(10,70,20)]

# 4
ii[c(100,200,300),nchar(names(ii))==3]

# 5
ii[!is.na(ii$dr1tsodi)&ii$dr1tsodi>10000,c('ridageyr','gender','dr1tsodi')]
subset(ii,dr1tsodi>10000,select=c(ridageyr,gender,dr1tsodi))

# 6
ii[!is.na(ii$dr1tsodi)&ii$dr1tsodi>10000&ii$gender=='Male',
   c('ridageyr','gender','dr1tsodi')]
subset(ii,dr1tsodi>10000&gender=='Male',select=c(ridageyr,gender,dr1tsodi))

# re-order the level
ii$bmicat<-factor(ii$bmicat,levels(ii$bmicat)[c(5,4,1:3)])
table(ii$bmicat,useNA='always')

# 8
ii$bmicat<-with(ii,(bmxbmi>=0)+(bmxbmi>=25)+(bmxbmi>=30)+
                  (bmxbmi>=35)+(bmxbmi>=40))
ii$bmicat<-factor(ii$bmicat,1:5)
levels(ii$bmicat)<-c('Underweight or normal weight','Overweight',
                     'Class I obesity','Class II obesity','Class III obesity')
table(ii$bmicat,useNA='always')

# 9
ii$bmicat<-'Underweight or normal weight'
ii$bmicat[ii$bmxbmi>=25]<-'Overweight'
ii$bmicat[ii$bmxbmi>=30]<-'Class I obesity'
ii$bmicat[ii$bmxbmi>=35]<-'Class II obesity'
ii$bmicat[ii$bmxbmi>=40]<-'Class III obesity'
ii$bmicat<-factor(ii$bmicat,c('Underweight or normal weight','Overweight',
                              'Class I obesity','Class II obesity','Class III obesity'))
table(ii$bmicat,useNA='always')

# 10
ii$bmicat<-ifelse(ii$bmxbmi>=25,'Overweight','Underweight or normal weight')
ii$bmicat<-ifelse(ii$bmxbmi>=30,'Class I obesity',ii$bmicat)
ii$bmicat<-ifelse(ii$bmxbmi>=35,'Class II obesity',ii$bmicat)
ii$bmicat<-ifelse(ii$bmxbmi>=40,'Class III obesity',ii$bmicat)
ii$bmicat<-factor(ii$bmicat,c('Underweight or normal weight','Overweight',
                              'Class I obesity','Class II obesity','Class III obesity'))
table(ii$bmicat,useNA='always')