Posts by Hubert Klein Ikkink

Clojure Goodness: Set Default Value For nil Function Argument With fnil

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Hubert Klein Ikkink

The function fnil can be used to create a new function from an existing function where we define default values for arguments that can be nil. Especially when we want to use a function that we didn’t write ourselves this can be helpful to handle nil values in a consistent way. We can prevent for example a NullPointerException by setting a default value. We can define default values for a maximum of three arguments for the original function. When we invoke the function that is returned from the fnil function any extra arguments after the first arguments are simply passed on to the original function.

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Clojure Goodness: Partition Collection Into Sequences

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Hubert Klein Ikkink

Clojure has the partition, partition-all and partition-by functions to transform a collection into a list of sequences with a (fixed) number of items. We can set the number of items in each sequence by providing a number as the first argument of the partition and partition-all functions. Any remainder elements are not in the resulting list of sequences when we use partition, but are when we use partition-all. We can also specify another collection to use values from to fill up the remainder as the third argument of the partition function.
Optionally we can specify an offset step value as a second argument using both functions. This mean a new partition sequence will start based on stepping through the original collection with the given step value.
Finally we can use a function to define when a new partition must start with the partition-by function. Every time the function returns a new value a new partition will begin.

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Awesome Asciidoctor: Image As Link Reference

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Hubert Klein Ikkink

To make an image linkable in Asciidoctor when formatted to HTML we must use the link attribute when we use the image macro. The value of the link attribute is the address where the user goes when clicking on the image. We can also specify extra link attributes like window to specify the target window for the link to open in.

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Clojure Goodness: Check Substring Is Part Of String

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Hubert Klein Ikkink

Sometimes we want to see if a string is part of another string. Or if the string value starts or ends with a certain string. In Clojure we can use the includes? function from the clojure.string namespace to check if a string is part of another string value. To see if a string starts with a certain value we can use the starts-with? function from the clojure.string namespace. And to check if a string ends with a given value we use ends-with? from the same namespace.

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Clojure Goodness: Getting Results From Multiple Functions With Juxt Function

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Hubert Klein Ikkink

The function juxt can be used to get results from multiple functions on the same argument in one go. We pass the functions we want to invoke as arguments to the juxt function. This results in a new function that will return a vector with the results from each original function that is passed to the juxt function. So the first element in the result vector is the result of the first function, the second element is the result of the second function and so on. The documentation of the juxt function shows this clearly as juxt a b c ⇒ [a(x) b(x) c(x)].

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Clojure Goodness: Reapply Function With Iterate To Create Infinitive Sequence

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Hubert Klein Ikkink

The iterate function create a lazy, infinitive sequence based on function calls. The iterate function takes a function and an initial value as arguments. The first element in the sequence is the initial value, next the function is invoked with the previous element as argument and this continues for each new element. Suppose we have a function #(+ 2 %) that adds 2 to the input argument. Then if we use this function with iterate and start with value 1 the first elements of the sequence will be 1, (+ 2 1), (+ 2 3), (+ 2 5). So first element is the initial value, next element is the invocation of the function with input argument 1. The result of this function is 3, which is then the input for the function to calculate the next element and so on.

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Clojure Goodness: Combine First And Next Functions Multiple Times

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Hubert Klein Ikkink

The first function in Clojure returns the first item of a collection. The next function returns a new sequence with all elements after the first element from a collection. Clojure adds some utility methods to combine first and next with different combinations. We can use the function ffirst which is will return the first element of the first element of a collection and the nfirst function to get the next elements from the first element of a collection. We can use the function fnext to get the first element of the next elements of a collection and the function nnext to get the next elements of the next elements of a collection.

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Java Joy: Using Functions To Replace Values In Strings

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Hubert Klein Ikkink

Since Java 9 we can use a function as argument for the Matcher.replaceAll method. The function is invoked with a single argument of type MatchResult and must return a String value. The MatchResult object contains a found match we can get using the group method. If there are capturing groups in the regular expression used for replacing a value we can use group method with the capturing group index as argument.

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Java Joy: Using Named Capturing Groups In Regular Expressions

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Hubert Klein Ikkink

In Java we can define capturing groups in regular expression. We can refer to these groups (if found) by the index from the group as defined in the regular expression. Instead of relying on the index of the group we can give a capturing group a name and use that name to reference the group. The format of the group name is ?<name> as first element of the group definition. The name of the group can be used with the group method of the Matcher class. Also we can use the name when we want to reference the capturing group for example with the replaceAll method of a Matcher object. The format is ${name} to reference the group by name. Finally we can use a named capturing group also as backreference in a regular expression using the syntax \k<name>.

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Clojure Goodness: Checking Predicate For Every Or Any Element In A Collection

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Hubert Klein Ikkink

In Clojure we can use several functions to see if at least one or all elements in a collection return true or false for a predicate. The function every? only returns true if the predicate function returns true for all elements in the collection. To function not-every? return true if a predicate function return false for all elements in a collection. The some function is a bit different (notice there is no ?) and returns the first logical true value from the predicate function for the elements in the collection. So the return type of the predicate doesn’t have to be a Boolean value and then the return type of some is also not a Boolean. If the predicate returns a Boolean value we can use some like a `any` function (any is not part of Clojure). Clojure provides a not-any? function that returns true if the predicate function returns false for one element in the collection and false otherwise.

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