Summing Instances in a String with Variable Instance Number

In this blog post, we’ll delve into the process of summing instances of numbers within a string, where the number of instances can vary. We’ll explore various approaches to solve this problem, including regular expressions and string manipulation techniques.

Background on Regular Expressions

Regular expressions (regex) are a powerful tool for matching patterns in strings. In regex, we use patterns to match specific sequences of characters. The str_extract_all function in R uses regex under the hood to extract matches from a string.

The general syntax for regex is:

pattern  // pattern to match

In our case, we’re interested in matching numbers ([0-9]+) followed by the string “per hpf”. We can use the (?=) syntax to create an “assertion” that checks if a certain pattern exists.

The Problem with the Original Code

The original code attempts to solve this problem using sapply and rollapply. However, there are two main issues:

1. Pre-specifying the number of numbers to add: The by = 3 argument in rollapply specifies that we want to apply a function over groups of three consecutive matches. However, since we don’t know in advance how many numbers will be present, this approach is flawed.
2. Wrong sign in ‘by’ argument: Even if we were pre-specifying the number of numbers to add, the by argument expects an integer value, not a product function (prod). This would cause an error.

Solution Using str_extract_all

To solve these issues, we can use sapply and str_extract_all instead. Here’s how:

EoEDx$HPF <- sapply(EoEDx$HPF,
                     function(x) {
                         unlist(sapply(str_extract_all(x, "[0-9]+(?= per hpf)"), as.numeric))
                     })

This code uses str_extract_all to extract all matches of the pattern “[0-9]+ per hpf” from each string in the column. It then converts these matches to numeric values using as.numeric. Finally, it uses sapply to apply a function over the extracted numbers.

The unlist function is used to convert the vector returned by str_extract_all (which contains multiple matches) into a single vector.

Summing the Numbers

To sum these numbers, we can use the built-in sum function:

sum(rollapply(unlist(sapply(str_extract_all(EoEDx$HPF, "[0-9]+(?= per hpf)"), as.numeric)), 3, by = 1, prod))

However, this approach still has a flaw: it assumes that there are three numbers in each match. If the number of instances varies, we need to rethink our approach.

Summing Instances with Variable Instance Number

To sum instances with variable instance number, we can use str_extract_all and then apply a function over these matches. Here’s how:

sum(sapply(str_extract_all(EoEDx$HPF, "[0-9]+(?= per hpf)"), as.numeric))

This code uses sapply to apply the same conversion to each match returned by str_extract_all. The result is a vector of numeric values representing the sum of instances.

Extracting the Largest Number

To extract the largest number, we can use the following approach:

as.numeric(sapply(str_extract_all(EoEDx$HPF, "[0-9]+(?= per hpf)"), function(x) x[which.max(as.numeric(x))][1]))

This code uses sapply to apply a function over each match. The function extracts the numeric value from the match and then finds its maximum using which.max. Finally, it returns the first (and in this case, only) element of the resulting vector.

Putting It All Together

Here’s the complete code:

EoEDx$HPF <- sapply(EoEDx$HPF,
                     function(x) {
                         unlist(sapply(str_extract_all(x, "[0-9]+(?= per hpf)"), as.numeric))
                     })

sum_of_instances <- sum(unlist(sapply(str_extract_all(EoEDx$HPF, "[0-9]+(?= per hpf)"), as.numeric)))

largest_number <- sapply(str_extract_all(EoEDx$HPF, "[0-9]+(?= per hpf)"), function(x) x[which.max(as.numeric(x))][1])

This code first extracts all instances of the pattern from each string in the column using str_extract_all. It then sums these instances using sum and applies a function over these matches to extract the largest number.

Conclusion

In this blog post, we explored how to sum instances of numbers within a string with variable instance number. We discussed the limitations of the original code and presented alternative approaches using regular expressions and string manipulation techniques. We also covered how to sum the instances and extract the largest number.

Last modified on 2023-12-06