Why Doesn’t My Perl Map Return Anything?

If you’re working with Perl and find that your map function is returning an empty list, you’re not alone. This issue often confuses developers, especially when they’re trying to manipulate strings within an array. In this blog post, we’ll explore a specific case of filtering out unnecessary characters from a string, and we’ll explain how to resolve this common problem step by step.

The Problem

Let’s look at the code snippet that causes the issue:

@filtered = map {s/ //g} @outdata;

In this line:

• You are trying to filter the contents of @outdata, which is an array of strings containing XML data.
• You specifically want to remove every occurrence of  , which stands for a non-breaking space in HTML.

However, instead of returning the modified array, your code is yielding an empty list. So, what went wrong?

Understanding the Issue

The primary issue lies in how the substitution operator s/// functions in Perl. Here’s a little breakdown:

• The s/// operator modifies the string in place (in this case, the default variable $_), but it does not return the modified string for use in the map function.
• Instead, it returns the number of changes made to the string, which means if no changes were made, the return value could be zero or it might not pass back the expected string for further use.

As a result, using map in this way doesn’t collect the modified strings into @filtered as intended, leading to the empty list.

The Solution

To achieve the desired filtering effect, you need to ensure that the substitution actually returns the modified string after the operation. Here’s how you can accomplish this:

Correct Syntax for the Map Function

Modify your code to include the variable $_ at the end of the map block:

@filtered = map {s/ //g; $_} @outdata;

Explanation of the Corrected Code

1. s/ //g; - This part performs the substitution, removing all instances of   from $_.
2. $_ - This now explicitly returns the value of $_ (the modified string) for each iteration of the map. This ensures that the resultant list (@filtered) contains the altered elements from @outdata.

Additional Tips

• Always remember that when using the substitution operator in map, if you want to return the modified string, you must include it at the end of your block.
• You can also add validation checks to see if @outdata is not empty or well-formed before processing it.

Conclusion

In conclusion, if you find your Perl map statement returning an empty list, make sure to verify your substitution operations. By correctly returning $_ after your substitution, you can effectively process and filter your data as intended. This simple yet crucial adjustment can save you from the frustration of empty outputs, allowing you to focus on more complex tasks at hand.

Now, happy coding with Perl!