You need to generate sequneces, for which all lower bounds equals to 1, and upper bounds are defined by a given vector sizes:

sizes <- c(3, 5, 7, 10)

# we need to generate:
# 1 2 3
# 1 2 3 4 5 
# 1 2 3 4 5 6 7
# 1 2 3 4 5 6 7 8 9 10

The function seq is not vectorized, i.e. one cannot pass vectors of length greater than 1 to from and to argument.

sizes <- c(3, 5, 7, 10)
seq(from = c(1, 1, 1, 1), to = sizes)

## Error in seq.default(from = c(1, 1, 1, 1), to = sizes) : 
##  'from' must be of length 1

A crude solution:

sizes <- c(3, 5, 7, 10)

seq(from = 1, to = sizes[1])
## [1] 1 2 3
seq(from = 1, to = sizes[2])
## [1] 1 2 3 4 5
seq(from = 1, to = sizes[3])
## [1] 1 2 3 4 5 6 7
seq(from = 1, to = sizes[4])
## [1]  1  2  3  4  5  6  7  8  9 10

Basically, the same line is repeated 4 times. We need a tool that will execute the same code chunk a certain number of times using each element from sizes.

Typically, good developers have an allergy to manual labor :)

We have this tool and it is called for loop:

for(element in vector) {
    expression
}

for loop executes expression the same number of times (iterations) as a length of vector. At the same time iterator variable element is assigned to successive values from this vector. Typically expression does something to this element.

sizes <- c(3, 5, 7, 10)

for(element in sizes) {
    print(seq(from = 1, to = element))
}

## [1] 1 2 3
## [1] 1 2 3 4 5
## [1] 1 2 3 4 5 6 7
## [1] 1  2  3  4  5  6  7  8  9 10

Note: there is nothing specific about the name element, it could be any valid variable name in R.

for loops work with all types of vectors, i.e. with atomic vectors…

colors_set <- c("red", "black", "green", "yellow")
for(color in colors_set) {
    print(color)
}
## [1] "red"
## [1] "black"
## [1] "green"
## [1] "yellow"

…and lists:

my_list <- list("string", 2:3, TRUE)
for(element in my_list) {
    print(element)
}
## [1] "string"
## [1] 2 3
## [1] TRUE

Nevertheless, it is possible to iterate over indices:

colors_set <- c("red", "black", "green", "yellow")
for(i in 1:length(colors_set)) { # or use seq_along(colors_set)
    print(colors_set[i])
}
## [1] "red"
## [1] "black"
## [1] "green"
## [1] "yellow"

Get mean of each element in the list:

grades <- list(
    jack = c(6.0, 5.5, 3.5, 5.5),
    bob = c(2.0, 5.5, 4.5, 3.5),
    stacy = c(5.5, 5.5, 5.0, 5.5, 5.5)
)

for(person in grades) {
    print(mean(person))
}

## [1] 5.125
## [1] 3.875
## [1] 5.4

Application: multiply each numeric element of the list by 10:

messy_list <- list("some", 3.14, TRUE)

for(i in 1:length(messy_list)) {
    if(is.numeric(messy_list[[i]]))
        messy_list[[i]] <- messy_list[[i]] * 10
}
messy_list

## [[1]]
## [1] "some"
##
## [[2]]
## [1] 31.4
## 
## [[3]]
## [1] TRUE

for loops in R can be nested:

matrix_1999 <- matrix(data = 1:4, nrow = 2, ncol = 2, byrow = TRUE)

for(i in 1:nrow(matrix_1999)) {
    for(j in 1:ncol(matrix_1999)) {
        print(matrix_1999[i,j])
    }
}
## [1] 1
## [1] 2
## [1] 3
## [1] 4

Next time lapply?

Problem: coerce character messy prices to clean numeric.

prices <- c("CHF 850", "CHF 790", "CHF 1,390", "CHF 1,560",
            "CHF 1,950", "CHF 670", "CHF 1,850", "CHF 1,326",
            "CHF 1,430")

We need to remove first four character (i.e., "CHF "); remove "," from each element; explicetly coerce to numeric.

prices <- c("CHF 850", "CHF 790", "CHF 1,390", "CHF 1,560",
            "CHF 1,950", "CHF 670", "CHF 1,850", "CHF 1,326",
            "CHF 1,430")

prices_without_CHF <- substring(text = prices, first = 5)
prices_without_coma <- gsub(",", "", prices_without_CHF)
prices_numeric <- as.numeric(prices_without_coma)

prices_numeric

## [1]  850  790 1390 1560 1950  670 1850 1326 1430

Too many intermediate variables, too easy to do a mistake.

prices <- c("CHF 850", "CHF 790", "CHF 1,390", "CHF 1,560",
            "CHF 1,950", "CHF 670", "CHF 1,850", "CHF 1,326",
            "CHF 1,430")

as.numeric(
    gsub(
        x = substring(
            text = prices,
            first = 5
        ),
        pattern = ",",
        replacement = ""
    )
)

## [1]  850  790 1390 1560 1950  670 1850 1326 1430

Too long, too ugly, too confusing, read inside out, etc.

Pipe %>% takes the output of one statement and makes it the input of the next statement. It allows you to write a sequence of function calls from left to right:

library(magrittr)

prices <- c("CHF 850", "CHF 790", "CHF 1,390", "CHF 1,560",
            "CHF 1,950", "CHF 670", "CHF 1,850", "CHF 1,326",
            "CHF 1,430")

prices %>% 
    substring(first = 5) %>%
    gsub(pattern = ",", replacement = "") %>%
    as.numeric()

prices

## [1]  850  790 1390 1560 1950  670 1850 1326 1430

A simpler example:

library(magrittr)

f(g(x))

g(x) %>% f()