Exercise 4 of 10 · Conditionals
Check the Temperature
What you will make
A short program that checks a temperature and prints whether it counts as a hot day.
The one new idea: if / else branching
Programs constantly need to make decisions based on data — if/else is how you tell R to run one block of code or another depending on a condition, which is the backbone of almost any interesting program.
Go straight to the code ↓The concept
An if statement lets a program choose between two paths. You write if (condition) { ... } else { ... }, and R runs the first block when condition evaluates to TRUE, or the second block when it's FALSE. Conditions are usually built with comparison operators: >, <, >=, <=, == (equals), and != (not equals). In R, it's standard style to write the else keyword on the same line as the if block's closing brace — } else { — rather than starting a new line with else, which avoids a well-known parsing pitfall.
Worked example
stock <- 4
if (stock < 5) {
cat("Reorder soon.\n")
} else {
cat("Stock is fine.\n")
}Reorder soon.Here, stock holds 4, and the condition stock < 5 checks whether that's true. Since 4 is indeed less than 5, the condition is TRUE, so only the if block's cat() call runs, printing "Reorder soon." The else block is skipped entirely — it only would have run if stock had been 5 or more.
Your turn
The starter code is meant to print "It's a hot day." whenever the temperature is above 30, but the condition currently compares against a different number entirely. Find that number in the if statement and change it to match the cutoff described in the comment just above it, so a temperature of 33 correctly triggers the hot-day message.
If something goes wrong
If the program prints "It's a mild day." when you expected the hot-day message, the condition is evaluating to FALSE when it should be TRUE — double-check the exact number and comparison operator being used, and mentally plug in temperature's actual value (33) to see which branch it should take. If you get a syntax error instead, make sure the else keyword is still on the same line as the closing } of the if block, and that both blocks' curly braces are properly matched.
Write your code
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Output will appear here after you run your code.The runtime is starting in the background. You can type now — it will be ready before you are.
The answer appears here once you have run your code at least once.
Things that often go wrong here
- Putting else on its own line after the closing brace of the if block, with nothing before it.
- In R, when the if block's closing brace ends a complete statement on its own, a following else on a new line can cause a syntax error — the safe, standard style is to write `} else {` on the same line, exactly as the starter code already does.
- Using = instead of == to compare values.
- A single = is R's alternate assignment operator, not a comparison — comparing two values for equality requires the double equals sign, ==.
- Editing the message strings instead of the comparison number.
- The text "It's a hot day." and "It's a mild day." are already correct; the bug is which condition decides which message prints, not the wording of the messages themselves.
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