Storing a nested for loop into a single vector in R

Question

Suppose I have the following code

b = 1:3
m = 5

for(j in 1:2){
  for(i in 1:5){
    print((1-i/m)* b[j] + (i/m)* b[j+1])
  }
}

If i print this i get the following output

[1] 1.2
[1] 1.4
[1] 1.6
[1] 1.8
[1] 2
[1] 2.2
[1] 2.4
[1] 2.6
[1] 2.8
[1] 3

However, now I would like to store this data into a single column vector. When i substitute print for an empty vector or list z[i] <- this ofcourse does not work. Does anyone know how to get the for loop value into a single column vector?

Answer 1

Base R, sapply

b = 1:3
m = 5

df <- sapply(1:2,function(j){
  sapply(1:5,function(i){
    out <- (1-i/m)* b[j] + (i/m)* b[j+1]
    return(out)
  })
})

result <- c(df[c(1:5),c(1:2)]) 

Result

[1] 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0

Speed test - sapply vs for loop

b = 1:10001
m = 5

> system.time({ 
+ df <- sapply(1:10000,function(j){
+   sapply(1:5,function(i){
+     out <- (1-i/m)* b[j] + (i/m)* b[j+1]
+     return(out)
+   })
+ })
+ })
   user  system elapsed 
   0.19    0.00    0.19 
 
> system.time({ 
+ out <- c()
+ for(j in 1:10000){
+   for(i in 1:5){
+     out <- c(out, (1-i/m)* b[j] + (i/m)* b[j+1])
+   }
+ }
+ })
   user  system elapsed 
   3.00    0.02    3.02