# Questions tagged [recursion]

For questions about recursion, the practice of calling a method or function from within itself.

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### Recursive speculative display list engine - computing text length across stack boundaries

Well, I've been hammering away on this for about a week now with no practical progress because I can't find mental fluidity with the concepts I'm trying to wrangle. I'm Officially Stumped, and would ...
136 views

### Iterative Sorts vs. Recursive Sorts

Naive sorts like Bubble Sort and Insertion Sort are inefficient and hence we use more efficient algorithms such as Quicksort and Merge Sort. But then, these two sorts are recursive in nature, and ...
2k views

### Making recursion optional: Bad practice?

Let's say I have a function like this: /* Mode is a bool that determines whether the function uses it's recursive mode or not */ public static int func(int n, boolean mode) { if (!mode) ...
143 views

### Representing a recursive Structure in OOP

I want to represent an Edifact message in an OOP Language. An Edifact message has the following structure: message := [ ( segment | segmentgroup ) ] segmentgroup : = [ ( segment | segmentgroup )...
136 views

### Walking a graph without recursion

I'm working on a class that receives a relational EDM/data model (e.g. a SQL DB or OData API manifest) and performs a couple tasks on them (not necessarily at the same time, could be two separate runs)...
503 views

### If the whole point of recursion is to break the problem into multiple smaller problems, what if those problems were solved in parallel?

So I was studying here and I was thinking if the whole point of recursion is to break the problem into multiple smaller problems, what if those problems were solved in parallel? A quick search lead me ...
190 views

### What happens when loops are compiled or interpreted?

I have came across a concept in recursion which says that when loops are compiled or interpreted then they get converted to recursive functions. If it is true how it takes place ?
623 views

### How to optimize reusing a large std::unordered_map as a temporary in a frequently called function?

Simplified question with a working example: I want to reuse a std::unordered_map (let's call it umap) multiple times, similar to the following dummy code (which does not do anything meaningful). How ...
415 views

### Pass-Through parameters in recursive code

When writing large amounts of recursive code (for valid reasons), I have come across many parameters that are not used in specific functions, but are still needed for a subset of all of the functions. ...
472 views

### True or false about recursion

I am working on a quiz for a computer science course. I would like to check that the following statement is false: A recursive call may never generate more than one recursive call for the ...
107 views

### Express unique recursive task system

I would like to express a task as either itself or a container for other tasks (recursively). The problem is that each task has to be one of two fundamental types: a goal or a routine. And, children ...
850 views

### Patterns for tracking state in recursive Haskell code

A frequent pattern in my Haskell code is element-wise recursion for transformation of a list with some carried state generated using the data in the list. Usually, this looks something like this: ...
89 views

### Generating combinations without getting stuck in recursive calls

This question is about figuring the design of a recursive function that changes the state of a group of elements by processing one of them at a time, with the goal of reaching a desired state. The ...
137 views

### Why does GHC represent recursion in Haskell with recursive binders instead of a fixpoint operator?

GHC's Core data type represents recursion with recursive binders in the Let constructor; as I understand it, all let expressions in Haskell are effectively let rec expressions. Why does GHC use this ...
148 views

### What is best practice for handling mapping arbitrary depth in arrays?

I have a set of data: id | name | parentid ------------------------ 1 | parent | 0 2 | child | 1 3 | child | 1 4 | parent | 0 5 | child | 4 6 | subchild | 5 7 | child | 4 ...
49 views

### Question about memory handling of trees and certain sorts on large data sets.

If data structures such as trees and certain sorts(quick sort, merge sort) work using recursive algorithms and recursive algorithms take up a lot of memory space in the stack and can only work on a ...
105 views

This article states However, this design does run into problems with some of the methods of the MutableSequence ABC. Most notably, insert and __ delitem __ can't operate on position 0, because ...
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### An elegant way to detect the end of an asynchronous recursive file search?

Given a recursive subroutine in single threaded environment which starts numerous asynchronous I/O operations and registers callback functions for each of them. This callbacks will be called on the ...
274 views

### Recursion explanation

I am not able to understand this basic recursion example. When I executed this code. The output was 43210. Should not the output be 44444, as the value of num is updated to 4 to the latest recursive ...
136 views

### Memoization in case of recursive interdefined functions in Haskell/Functional Programming?

I was reading Memoization with recursion which tells how for a recursively defined function fun we can do memoization by: -- Memoization memoize f = (map f [0 ..] !!) -- Base cases g f 0 = 0 g f 1 = ...
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### What are the considerations to determine whether you can use recursion to solve a problem?

Sometimes in interviews, I may use recursion to solve a problem (such as adding 1 to an infinite precision integer), or when the problem presents itself suitable to use recursion. Sometimes, it might ...
880 views

### Eliminating recursion

It is easy to eliminate tail recursion. There are few curious cases where the not-tail recursion can also be eliminated. Exhibit 1: Fibonacci numbers. A naive recursive solution fib(n) if (n &...
2k views

### How do I represent a recursive process down a tree in a sequence diagramm?

I'm working on a sequence diagram for a layered system that has a tree hierarchy. Now I have a process that works itself recursively down the tree. Meaning calling the same function on a child object. ...
2k views

### Efficiency considerations: nested loop vs recursion

I would consider myself an intermediate Python programmer. One of my recent challenges was creating a list of all possible solutions to a given Countdown problem. Without getting into too much detail,...
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### How do I write an algorithm to solve a set of 3 symbolic equations

Replace each symbol (letter) with a number so that the equations hold across all 3 equations. Solution should be able to solve the general case of any 3 equations. Can assume 2 terms summed to an ...
77 views

### Why isn't this combinatorial solution equivalent to the recursive solution for finding the number of “paths”?

Here's the problem: Given an m x n array, get the number of different paths from the top left corner to the bottom right corner, if you can only move down, right, and diagonally down & right. ...
10k views

### Why does recursion return the first call in the stack and not the last?

I cant for the life of me figure out why this returns 0 rather than 5. i keeps getting incremented before it hits the last return statement, however it always returns 0, from the first call in the ...
903 views

### How does K&R's qsort work?

In the recursion section of K&R's ANSI C book, they demonstrate a version of quicksort [that] is not the fastest possible, but it's one of the simplest. --The C Programming Language (ANSI ...
467 views

### How to decide how to build a recursive function

Sometimes when you're coding you're on a problem which can be solved with some recursive method. What is for you the best way to detect when the recursion is a good way to solve a problem and how to ...
2k views

### Designing the recursive solution

I understand the recursion and find it useful and intuitive while solving problems on trees but for many other problems recursion never fails me to leave puzzled. Recently I was solving the following ...
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### Are recursions intrinsically less safe than iterations?

Java doesn't have a predefined recursion depth limit. As a result the recursion below (a dummy method that returns the value) throws java.lang.StackOverflowError after 62844 (with static) and 14002 (...
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### how is stack and heap are assigned to each processes?

How multiples processes are stored in the main memory , i understand every process will be divided into the equal size pages and will be stored in the frames of main memory. if whole main memory is ...
18k views

### Is a while loop intrinsically a recursion?

I wondered whether a while loop is intrinsically a recursion? I think it is because a while loop can be seen as a function that calls itself at the end. If it is not recursion, then what is the ...
1k views

### How to mentally keep track of recursion

I have great trouble doing recursion, especially trying to visualize it. A great example is the aging wine problem described in the top answer of this link: https://www.quora.com/Are-there-any-good-...
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### Recursion in Merge Sort algorithm. How is it obvious to use this type of recursion?

I dont want to put too much code so I'll just put the code that involves recursion. This algorithm is fairly well known so I think everybody knows the basic code. void mergeSort(int array[], int l, ...
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### Reflexive calls of objects in the same hierarchy

I am not sure how to phrase this. I believe this should have been asked somewhere, but I am unable to find it because I don't know the keywords. Basically, I have some types like this: interface Foo ...
2k views

### How does this recursive function exit? [closed]

In the merge_sort part, there are multiple calls to the same function within the recursive. How are the other two executed when the definition holds that till a condition is false, the recursion does ...
2k views

### Implementing map with tail recursion

I am trying to solve this exercise. It is about reimplementing the map function in haskell for learning purpose. I found a solution which doesn't browse all the elements of the list (simple linked ...
139 views

### String Pattern Matching from Lookup Table - Non-Exponential Solution?

Given the problem... Given a String comprising of non-alhpabetical symbols, and a Lookup Table where a subset of those symbols may equate to a single alphabetical character, output all possible ...
155 views

### Understanding recursive solution of some algorithm

Most of the time we need to understand someone else' code for example I am studying Graph Algorithms from Sedgewick's online resources, the particular code example is taken from cycle detection ...
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### Reverse lookup of data based on user input

I have an array in the format: \$array = array( 0 => array( 'name' => 'Item 1', 'level' => 1, 'points' => 10 ), 1 => array( 'name' => '...
274 views

### Achieving permutation using recursion [closed]

Though I like recursions and understand it, I fail to master it! I am stuck with a problem which I know that recursion can solve, but I do not know how. I have an array of strings like: "hundred", "...
237 views

### For loop and recursion for a new shell in C [closed]

I code a new shell in C, that could be done in several ways: Flex/bison, finite state machine, abstract syntax tree or just a tokenizer in C. Now I've written a for-loop that changes the condition of ...
106 views

### Term for the current iteration of a recursive function call

Is there a standard or widely accepted term for referring to a variable within the current call of a recursive function where the last value of the same variable is passed as an argument? I am trying ...
181 views

### Complexity of recursive calls

my brain is a little stuck as I am trying to compile an example for my blog. I am presenting an algorithm that draws a number from an array, adds it to a sum and calls itself recursively. func solve(...
437 views

### Algorithm to generate all sets of m points in n x n x n cubic lattice which are unique under symmetry

I am implementing an algorithm which is going to be quite computationally complex, and want to try to make sure I'm not doing unnecessary work. There is an n x n x n cubic lattice, e.g. if n=2 this ...
264 views

### Finding all possible ways of inserting a pattern into a string

I've been thinking about this problem for a while, and I can only find a recursive solution but I'm feeling that there is a dynamic programming way to do it, I just can't figure it out. Is this a ...
375 views

### Enumerating the primitive recursive functions

How can I enumerate (by expression tree size, for example) all of the primitive recursive functions that map natural numbers to natural numbers in a traditional programming language like C? For ...