This is an excerpt from the Scala Cookbook partially modified for the internet. This is Recipe 3. You want to iterate over the elements in a Scala collection, either to operate on each element in the collection, or to create a new collection from the existing collection. There are many ways to loop over Scala collections, including for loops, while loops, and collection methods like foreachmapflatMapand more. This solution focuses primarily on the for loop and foreach method.
When your algorithm requires multiple lines, use the same for loop syntax, and perform your work in a block:.
Notice that an input Array yields an Array and not something else, like a Vector. When your algorithm requires multiple lines of code, perform the work in a block after the yield keyword:. If you need access to a counter inside a for loop, use one of the following approaches. First, you can access array elements with a counter like this:. Scala collections also offer a zipWithIndex method that you can use to create a loop counter:. See Recipe On a related note, the following example shows how to use a Range to execute a loop three times:.
Using a Range like this is known as using a generator. The next recipe demonstrates how to use this technique to create multiple loop counters. Recipe 3. When iterating over keys and values in a Map, I find this to be the most concise and readable for loop:.
When you have an algorithm you want to run on each element in the collection, just use the anonymous function syntax:. The Scala Language Specification provides details on precisely how a for loop is translated under various conditions. I encourage you to read the Specification for details on the rules, but a simplification of those rules can be stated as follows:. Again, the Specification is more detailed than this, but those statements will help get you started in the right direction.
These statements can be demonstrated with a series of examples. Each of the following examples starts with a for loop, and the code in each example will be compiled with the following scalac command:.Vrp server files fivem
This command provides some initial output about how the Scala compiler translates the for loops into other code. This code is intentionally small and trivial so you can see how the for loop is translated by the compiler.
When you compile this code with the scalac -Xprint:parse command, the full output looks like this:. For this example, the important part of the output is the area that shows the for loop was translated by the compiler into the following code:. As you can see, the Scala compiler translates a simple for loop over a collection into a foreach method call on the collection.
The code continues to get more and more detailed as the compiler phases continue, but for this demonstration, only the first step in the translation process is necessary. Note that although I use a Range in these examples, the compiler behaves similarly for other collections.
For example, if I replace the Range in the previous example with a Listlike this:. Given this introduction, the following series of examples demonstrates how various for loops are translated by the Scala 2. As shown, a simple, single guard is translated into a withFilter method call on the collection, followed by a foreach call. The same for loop with two guards is translated into two withFilter calls:.
The -Xprint:parse option shows a small amount of compiler output, while the -Xprint:all option produces hundreds of lines of output for some of these examples, showing all the steps in the compilation process. For more details, see the Scala Language Specification for exact rules on the for loop translation process.Learning Scala? Learn the Fundamentals First!
The details are currently in Section 6. By Alvin Alexander.There are a number of ways to iterate over a Scala List using the foreach method which is available to Scala sequences like ListArrayArrayBufferVectorSeqetc. A common way to iterate over a Scala List is with the foreach method. Here's a quote about foreach from the book Programming in Scala :. It simply applies the procedure to each List element.
The result of the operation is again Unit ; no list of results is assembled. Note that this is a relatively common way to use the foreach method. The Scala for comprehension is not specific to lists, but is an extremely powerful way to operate on a List and other sequences.
So far, so good. Now let's add a simple if clause to the for comprehension to print only the elements we want to print:. If you already know about the for comprehension, you know that you can add multiple if clauses, and much more functionality.
I could easily write an entire tutorial on the Scala for comprehension, so to keep this tutorial short, I'll stop here for now. Before leaving, I will add these notes however, from the book Programming in Scala :. Scala provides the for comprehension, which provides syntactically pleasing nesting of mapflatMapand filter The for comprehension is not a looping construct, but is a syntactic construct the compiler reduces to mapflatMapand filter.
I apologize that these examples are not as detailed as I prefer. I hope this short tutorial on how to iterate over a Scala List and other sequences using the foreach method and for comprehension have been helpful. As you can tell from these examples, there's much more power available to you with both approaches, which is one of the great things about the Scala programming language.
How to iterate over Scala Lists with foreach and for. By Alvin Alexander. Last updated: November 13, Here's a quote about foreach from the book Programming in Scala : foreach takes a procedure — a function with a result type Unit — as the right operand.
Before leaving, I will add these notes however, from the book Programming in Scala : Scala provides the for comprehension, which provides syntactically pleasing nesting of mapflatMapand filter More detailed examples I apologize that these examples are not as detailed as I prefer.
Scala - for Loops
Scala: How to use zipWithIndex or zip to create loop counters. If you want to understand all the Buddhas The most difficult yoga asana is Anchorage, Alaska: Cruise ships canceled. A dementia story. Cousins the movie.In scala, for loop is known as for-comprehensions. It can be used to iterate, filter and return an iterated collection. The for-comprehension looks a bit like a for-loop in imperative languages, except that it constructs a list of the results of all iterations.
In the above syntax, range is a value which has start and end point. You can pass range by using to or until keyword. In the below example, until is used instead of to. The major difference between until and to is, to includes start and end value given in the range, while until excludes last value of the range.
So, the below example will print only 1 to 9. It is helpful to apply until keyword when you are iterating string or array, because array range is 0 to n So, your code will not complain of upper range. You can use for to filter your data.
In the below example, we are filtering our data by passing a conditional expression. This program prints only even values in the given range. In the above example, we have used yield keyword which returns a result after completing of loop iterations. The for use buffer internally to store iterated result and after finishing all iterations it yields the final result from that buffer.
It does not work like imperative loop.Film festivals worth submitting to
In scala, you can iterate collections like list, sequence etc, either by using for each loop or for-comprehensions. In the below code we have use three approaches of for-each loop. You can implement any of them according to your need. In the above example, we have used by keyword. The by keyword is used to skip the iteration.
When you code like: by 2 it means, this loop will skip all even iterations of loop. JavaTpoint offers too many high quality services. Mail us on hr javatpoint. Please mail your requirement at hr javatpoint. Duration: 1 week to 2 week. Scala Tutorial. Scala Functions Higher Order Functions. Scala Trait Scala Trait Mixins. Scala Access Modifiers. Scala Array Multidimensional Array. Scala Tuples Scala Tuple Example. Next Topic Scala Break. Spring Boot.
Selenium Py. Verbal A. Angular 7. Compiler D. Software E. Web Tech.In Scala, for loop is also known as for-comprehensions. A for loop is a repetition control structure which allows us to write a loop that is executed a specific number of times. The loop enables us to perform n number of steps together in one line. The range can be represented by using either i to j or i until j.
In for loop, We can use to when We want to print the values from 0 to n. Or in other words, when We use to with for loop it includes both start and end value like as shown in the below program, it prints from 0 to 10 not print from 0 to 9 like in until.P2279 zafira b
In for loop, We can use until when We want to print the value from 0 to n Or in other words, until with for loop it excludes the end value like as shown in the below program, it prints only from 0 to 9 not print from 0 to We can also use multiple ranges in single for-loop. These ranges are separated by a semi-colon. Let us discuss with the help of an example. In the below example, we use two different ranges into a single loop, i. In Scala, We can use for-loop with collections like List etc.
It provides an efficient way to iterate over the collections. In Scala, for-loop allows you to filter some elements from the given collection using one or more if statements in for-loop.
Explanation: In the above example, the for loop use two filters to filter the given collection. These filters eliminate those ranks which are less than 7 and greater than 2. In Scala, the return value of the for loop is stored in a variable or may return through a function. To do this you should use yield keyword to prefix the body of for loop.
Explanation: In the above example, the output is a variable where all the values of rank are stored in the form of a collection.A common use case is to use a for loop with yield to create a new data structure from an existing data structure.See result 2075 date
Beware: zipWithIndex creates a new sequence from the existing sequence, you may want to call view before invoking zipWithIndex :. The zipWithIndex counter starts at 0 ; zip lets you control where the counter starts from:.
Note that these examples work as shown because zip and zipWithIndex both return a sequence of Tuple2 elements. Here are some examples of how to use a for loop with a Scala Map mutable or immutable :. You can use a Scala Future with a for comprehension, but you have to make sure you create the future s before the comprehension, like this:. For more details on this, see my How to use multiple Scala Futures in a for loop example.
See the Map examples above for examples of using foreach with a Map. As a quick summary, if you wanted to see some Scala for loop examples in a concise format, I hope this is helpful.
For more details on the for comprehension, see these links:. By Alvin Alexander. Last updated: January 6, How to iterate over Scala Lists with foreach and for. Scala: How to use zipWithIndex or zip to create loop counters.
If you know both arrays are the same length and you don't need to worry how fast it is, the easiest and most canonical is to use zip inside a for-comprehension:. The zip method creates a new single array, however.
To avoid that overhead you can use zipped on a tuple which will present the elements in pairs to methods like foreach and map :. Faster still is to index into the arrays, especially if the arrays contain primitives like Intsince the generic code above has to box them.
There is a handy method indices that you can use:. Finally, if you need to go as fast as you possibly can, you can fall back to manual while loops or recursion, like so:. If you are computing some value, rather than having it be a side effect, it's sometimes faster with recursion if you pass it along:. Since you can drop a method definition in anywhere, you can use recursion without restriction. You can add an annotation.
Taking all these different approaches to calculate a dot product on length vectors, we can compare these to a reference implementation in Java:.Suture reaction dog
This is particularly bad, of course, with primitives! You get similarly huge jumps in time taken if you try to use ArrayList s of Integer instead of Array of int in Java.
Note in particular that zipped is quite a reasonable choice if you have objects stored. Do beware of premature optimization, though! There are advantages to in clarity and safety to functional forms like zip. If you always write while loops because you think "every little bit helps", you're probably making a mistake because it takes more time to write and debug, and you could be using that time optimizing some more important part of your program.
But, assuming your arrays are the same length is dangerous. Are you sure? How much effort will you make to be sure? Maybe you shouldn't make that assumption? If you don't need it to be fast, just correct, then you have to choose what to do if the two arrays are not the same length. If you want to do something with all the elements up to the length of the shorter, then zip is still what you use:.
In any of these cases, you can use yield with for or map instead of foreach to generate a collection.Scala provides a data structure, the arraywhich stores a fixed-size sequential collection of elements of the same type. An array is used to store a collection of data, but it is often more useful to think of an array as a collection of variables of the same type.
Instead of declaring individual variables, such as number0, number1, This tutorial introduces how to declare array variables, create arrays, and process arrays using indexed variables. The index of the first element of an array is the number zero and the index of the last element is the total number of elements minus one. To use an array in a program, you must declare a variable to reference the array and you must specify the type of array the variable can reference.
Here, z is declared as an array of Strings that may hold up to three elements. Here, the last example shows that in general the index can be any expression that yields a whole number. Following picture represents an array myList. Here, myList holds ten double values and the indices are from 0 to 9. When processing array elements, we often use loop contol structures because all of the elements in an array are of the same type and the size of the array is known.
Save the above program in Demo. The following commands are used to compile and execute this program. Scala does not directly support various array operations and provides various methods to process arrays in any dimension.
If you want to use the different methods then it is required to import Array. There are many situations where you would need to define and use multi-dimensional arrays i. For example, matrices and tables are examples of structures that can be realized as two-dimensional arrays. This is an array that has three elements each being an array of integers that has three elements.
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Try the following example which makes use of concat method to concatenate two arrays. You can pass more than one array as arguments to concat method. Use of range method to generate an array containing a sequence of increasing integers in a given range. You can use final argument as step to create the sequence; if you do not use final argument, then step would be assumed as 1. Let us take an example of creating an array of range 10, 20, 2 : It means creating an array with elements between 10 and 20 and range difference 2.
Elements in the array are 10, 12, 14, 16, and Another example: range 10, Here range difference is not given so by default it assumes 1 element. It create an array with the elements in between 10 and 20 with range difference 1.
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