Type Conversion Basics

Part of the Fundamentals section of Coddy's R journey — lesson 30 of 78.

As mentioned in the previous lesson, readline() always returns a character string. If a user types "25", you get the text "25", not the number 25. This matters because you can't do math with text.

To convert a character string to a number, use as.numeric():

input <- "42"
number <- as.numeric(input)

cat(number + 8, "\n")  # Output: 50

This is essential when working with user input. Here's a common pattern you'll use:

age_text <- readline()
age <- as.numeric(age_text)

cat("In 10 years, you will be", age + 10, "\n")

You can also convert in the opposite direction using as.character():

number <- 100
text <- as.character(number)

cat("The value is:", text, "\n")

Converting numbers to characters is useful when you need to combine them with other text or when a function specifically requires character input. These conversion functions are your tools for moving between data types whenever your program needs it.

Challenge

Easy

Read two numeric values as input, perform a calculation, and display the result with a label.

You will receive two inputs:

A price (as a string)
A quantity (as a string)

Convert both inputs to numeric values using as.numeric(), then calculate the total cost by multiplying price by quantity.

Display the output in the following format:

Total: [result]

Use cat() for the output and include a newline character \n at the end.

For example, if the inputs are 15.50 and 4, the output should be:

Total: 62

Cheat sheet

Use as.numeric() to convert a character string to a number:

input <- "42"
number <- as.numeric(input)

cat(number + 8, "\n")  # Output: 50

Common pattern for converting user input:

age_text <- readline()
age <- as.numeric(age_text)

cat("In 10 years, you will be", age + 10, "\n")

Use as.character() to convert a number to a character string:

number <- 100
text <- as.character(number)

cat("The value is:", text, "\n")

Try it yourself

# Read input
con <- file("stdin", "r")
price_str <- suppressWarnings(readLines(con, n = 1))
quantity_str <- suppressWarnings(readLines(con, n = 1))
close(con)

# TODO: Write your code below
# 1. Convert price_str and quantity_str to numeric values using as.numeric()
# 2. Calculate the total cost (price * quantity)
# 3. Display the result using cat() with the format "Total: [result]\n"