Question

I have the following data and nested for loop:

x <- c(12, 27, 21, 16, 12, 21, 18, 16, 20, 23, 21, 10, 15, 26, 21, 22, 22, 19, 26, 26)
y <- c(8, 10,  7,  7,  9,  5,  7,  7, 10,  4, 10,  3,  9,  6,  4,  2,  4,  2,  3,  6)

a <- c(20,25) 
a.sub <- c()
df <- c()
for(j in 1:length(a)){
    a.sub <- which(x >= a[j])   
    for(i in 1:length(a.sub)){
        df[i] <- y[a.sub[i]]    
    }
    print(df)
}

I’d like the loop to return values for df as:

[1] 10  6  3  6  4 10  6  4  2  4  3  6
[1] 10  6  3  6

As I have it, however, the loop returns the same values twice of df for a <- 20 but not a <- 25:

[1] 10  7  5 10  4 10  6  4  2  4  3  6
[1] 10  6  3  6  4 10  6  4  2  4  3  6

Answer 1

for(i in 1:length(a.sub)){
    df[i] <- y[a.sub[i]]    
}

can become

df <- y[a.sub]    

neither a.sub nor df need to be predefined then and thus…

x <- c(12, 27, 21, 16, 12, 21, 18, 16, 20, 23, 21, 10, 15, 26, 21, 22, 22, 19, 26, 26)
y <- c(8, 10,  7,  7,  9,  5,  7,  7, 10,  4, 10,  3,  9,  6,  4,  2,  4,  2,  3,  6)

a <- c(20,25) 
for(j in 1:length(a)){
    a.sub <- which(x >= a[j])   
    df <- y[a.sub]    
    print(df)
}

It could be made shorter. df is unnecessary if you’re just printing the subset of y anyway. Just print it directly. And the selector is so short it wouldn’t make a single line confusing. Furthermore, why use length of a and index.. loop through a directly. So, it could be…

a <- c(20,25) 
for(ax in a){
    print( y[ which(x >= ax) ] )
}