When Using `bc`, `scale` Doesn't Round!

A story where the devil actually was in the details

Ever stumbled upon a weird problem and couldn’t quite figure out where things went wrong?

I’ve been there, and so has a friend of mine while tackling a HackerRank challenge.

The task seemed simple: take a mathematical expression as input and print out the result with three decimal places.

However, getting there wasn’t as straightforward as expected.

Let’s walk through what happened and how we can apply the lessons learned to similar problems.

The Initial Problem

The challenge required reading an expression from the input and then outputting the result to three decimal places. My friend’s first attempt looked like this:

read var
echo $(("scale=3; $var" | bc))

However, one test case failed due to being off by 0.001. This tiny discrepancy was a significant clue that the approach to handling decimal precision and rounding needed reevaluation.

Side Note
Although this is a personal style preference, the usual way I would pipe a string into bc would involve echoing the string or using a heredoc, not directly using the $(()) syntax.

A better version might look like this:

read var
echo "scale=3; $var" | bc

I suggested increasing the scale to 4 (scale=4) to explore the behavior, which led to failing all test cases. It seemed counterintuitive, but the point was to dig deeper into how the bc command processes numbers.

What We Discovered

After running some tests locally (always a good practice before submitting your solution), we noticed something interesting:

echo $input
5+50*3/20 + (19*2)/7
echo "scale=4; $input" | bc
echo "scale=3; $input" | bc
echo "$input" | bc
echo "$input" | bc -l
printf "%.3f\n" `echo "$input" | bc -l`

The exploration revealed a crucial detail: bc truncates rather than rounds numbers when applying the scale. That’s where the printf function becomes handy.

The Practical Solution

The workaround is to use bc -l for the calculation to ensure full precision and then printf "%.3f\n" to format the output correctly, rounding to three decimal places.

Key Takeaways

  1. Precision Matters: The failure of a test case by just 0.001 should immediately signal that the handling of decimal precision and rounding is critical.
  2. Test Locally: Always run your code locally first (unless it’s impossible of course!). It helps catch errors and understand the problem better.
  3. Know Your Tools: Understanding how commands like bc and printf work is crucial. bc sets the number of decimal places but doesn’t round, while printf can format and round the output.
  4. Experiment and Learn: Don’t be afraid to change your approach. Testing with a scale of 4 revealed how bc handles precision.
  5. Pay Attention to Details: Small discrepancies can point to larger issues. A mismatch of 0.001 isn’t just a minor error; it’s an indicator that your method of handling precision needs adjustment.

When you face a coding challenge, keep these insights in mind.

Sometimes, the solution involves more than just writing the code; it’s about understanding the behavior of the tools at your disposal.

Happy coding! Have you ever faced a similiar problem before? Tell me all about it!

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