write a function that return frequency of integers that have last integer of j in integers 1 to n

[vegstro]

I want to write a function called beta, such that beta(j)n will give the number of integers that have the last digit j, where j is an integer from 1-9, from integers 1 to n. e.g. beta(2)4=1, beta(1)10=2
Help me pls, I am struggling as I have no prior knowledge of r.

[technocrat]

@vegstro

no prior knowledge of r

That can be an advantage. Every R problem can be thought of with advantage as the interaction of three objects— an existing object, x , a desired object,y , and a function, f, that will return a value of y given x as an argument. In other words, school algebra— f(x) = y. Any of the objects can be composites.

In this problem the information at hand consists of integers in two pieces: j is a single integer and n is a sequence of integers; taken together they are the x of the equation. y is the length of a vector of the integers in n that satisfy the condition that the last digit is the same as j.

Everything in R is an object; objects have properties depending on the class; some are containers for data and others perform functions. Functions are said to be first-class objects—that just means that they can be given as arguments to other functions, just like f(g(x) = y.

Let's tackle the simplest parts of the problem, first.

Our goal is a function f that will take j and n and do something. It will look like

the_function <- function(x,y) # do something next

Call it with j and n is just

the_function(2,1:10)

for example. The 2 represents the integer we want to match and 1:10 is the sequence of integers from one to 10. : expands to `c(1,2,3,4,5,6,7,8,9,10).

Assume we have a vector as a result of the_function, and we want to know: How many?

length(my_vector)

That's simple enough.

For the rest of the work, we're going to call on the {stringr} package, which is good to deal with patterns in strings. Why strings and not integers? Because it's much easier to convert the numbers to characters and do a simple search.

For that we first need another simple, builtin— as.character, which converts an integer to a character.

Then we need to describe how to recognize a last character in a string.

last_one <- ".$"

which translates to . = a single character and $ the end of the string.

So, now we are in a position to identify the final characters (string representations of the integers, "1", etc. The final piece is which, which allows us to gather up all of the last integers that match the target.

suppressPackageStartupMessages({
  library(stringr)
})

target <- ".$"
get_last <- function(j,n) which(str_extract(as.character(n),target) == as.character(j))

get_last(2,1:100)
#>  [1]  2 12 22 32 42 52 62 72 82 92
length(get_last(2,1:100)) 
#> [1] 10

[vegstro]

Can the output be the number of integers that fulfill the condition, such that desired output can be generated?

[vegstro]

I mean the function get_last(j,1:n)

[technocrat]

@technocrat

length(get_last(2,1:100))

or

get_last <- function(j,n) length(which(str_extract(as.character(n),target) == as.character(j)))

[vegstro]

something is wrong, what is missing?

[technocrat]

Remove and > symbols at beginning of lines