Questions tagged [recursion]
Recursion is a kind of function call in which a function calls itself. Such functions are also called recursive functions. Structural recursion is a method of problem solving where the solution to a problem depends on solutions to smaller instances of the same problem.
recursion
47,082
questions
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What is tail recursion?
Whilst starting to learn lisp, I've come across the term tail-recursive. What does it mean exactly?
1103
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answers
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What is tail call optimization?
Very simply, what is tail-call optimization?
More specifically, what are some small code snippets where it could be applied, and where not, with an explanation of why?
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What is the maximum recursion depth, and how to increase it?
I have this tail recursive function here:
def recursive_function(n, sum):
if n < 1:
return sum
else:
return recursive_function(n-1, sum+n)
c = 998
print(recursive_function(...
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votes
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answers
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Recursively look for files with a specific extension
I'm trying to find all files with a specific extension in a directory and its subdirectories with my Bash (Ubuntu 10.04 LTS (Lucid Lynx) release).
This is what's written in a script file:
#!/bin/bash
...
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votes
15
answers
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What is the most efficient/elegant way to parse a flat table into a tree?
Assume you have a flat table that stores an ordered tree hierarchy:
Id Name ParentId Order
1 'Node 1' 0 10
2 'Node 1.1' 1 10
3 'Node 2' 0 ...
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votes
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answers
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How to recursively find and list the latest modified files in a directory with subdirectories and times
Operating system: Linux
Filesystem type: ext3
Preferred solution: Bash (script/one-liner), Ruby, or Python
I have several directories with several subdirectories and files in them. I need to make a ...
- 5,763
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votes
21
answers
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views
Way to go from recursion to iteration
I've used recursion quite a lot during my many years of programming to solve simple problems, but I'm fully aware that sometimes you need iteration due to memory/speed problems.
So, sometime in the ...
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votes
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answers
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How to search a string in multiple files and return the names of files in Powershell?
I have started learning powershell a couple of days ago, and I couldn't find anything on google that does what I need so please bear with my question.
I have been asked to replace some text strings ...
- 6,190
417
votes
7
answers
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views
Determining complexity for recursive functions (Big O notation)
I have a Computer Science Midterm tomorrow and I need help determining the complexity of these recursive functions. I know how to solve simple cases, but I am still trying to learn how to solve these ...
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13
answers
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Is recursion ever faster than looping?
I know that recursion is sometimes a lot cleaner than looping, and I'm not asking anything about when I should use recursion over iteration, I know there are lots of questions about that already.
...
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348
votes
30
answers
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Recursively list files in Java
How do I recursively list all files under a directory in Java? Does the framework provide any utility?
I saw a lot of hacky implementations. But none from the framework or nio
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votes
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answers
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Try-finally block prevents StackOverflowError
Take a look at the following two methods:
public static void foo() {
try {
foo();
} finally {
foo();
}
}
public static void bar() {
bar();
}
Running bar() clearly ...
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votes
8
answers
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views
Does Python optimize tail recursion?
I have the following piece of code which fails with the following error:
RuntimeError: maximum recursion depth exceeded
I attempted to rewrite this to allow for tail recursion optimization (TCO). ...
- 8,413
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votes
10
answers
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Is log(n!) = Θ(n·log(n))?
I am to show that log(n!) = Θ(n·log(n)).
A hint was given that I should show the upper bound with nn and show the lower bound with (n/2)(n/2). This does not seem all that intuitive to me. Why would ...
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votes
31
answers
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Recursion or Iteration?
Is there a performance hit if we use a loop instead of recursion or vice versa in algorithms where both can serve the same purpose? Eg: Check if the given string is a palindrome.
I have seen many ...
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votes
13
answers
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How to do a recursive sub-folder search and return files in a list?
I am working on a script to recursively go through subfolders in a mainfolder and build a list off a certain file type. I am having an issue with the script. It's currently set as follows:
for root, ...
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votes
8
answers
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views
What exactly is a reentrant function?
Most of the times, the definition of reentrance is quoted from Wikipedia:
A computer program or routine is
described as reentrant if it can be
safely called again before its
previous invocation has ...
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254
votes
14
answers
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views
List files recursively in Linux CLI with path relative to the current directory
This is similar to this question, but I want to include the path relative to the current directory in unix. If I do the following:
ls -LR | grep .txt
It doesn't include the full paths. For example, ...
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votes
6
answers
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views
Anonymous recursive PHP functions
Is it possible to have a PHP function that is both recursive and anonymous? This is my attempt to get it to work, but it doesn't pass in the function name.
$factorial = function( $n ) use ( $...
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votes
20
answers
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Understanding recursion [closed]
I'm having major trouble understanding recursion at school. Whenever the professor is talking about it, I seem to get it but as soon as I try it on my own it completely blows my brains.
I was trying ...
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votes
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answers
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Can every recursion be converted into iteration?
A reddit thread brought up an apparently interesting question:
Tail recursive functions can trivially be converted into iterative functions. Other ones, can be transformed by using an explicit ...
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votes
4
answers
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How can I create nonexistent subdirectories recursively using Bash?
I am creating a quick backup script that will dump some databases into a nice/neat directory structure and I realized that I need to test to make sure that the directories exist before I create them. ...
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221
votes
15
answers
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views
How to make a recursive lambda
I am writing the following recursive lambda function:
#include <iostream>
#include <functional>
auto term = [](int a)->int {
return a*a;
};
auto next = [](int a)->int {
return +...
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votes
12
answers
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views
Nested defaultdict of defaultdict
Is there a way to make a defaultdict also be the default for the defaultdict? (i.e. infinite-level recursive defaultdict?)
I want to be able to do:
x = defaultdict(...stuff...)
x[0][1][0]
{}
So, I ...
- 12k
218
votes
4
answers
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views
Breadth First Vs Depth First
When Traversing a Tree/Graph what is the difference between Breadth First and Depth first? Any coding or pseudocode examples would be great.
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votes
9
answers
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Recursion in Angular directives
There are a couple of popular recursive angular directive Q&A's out there, which all come down to one of the following solutions:
manually incrementally 'compile' HTML based on runtime scope ...
- 11.3k
176
votes
18
answers
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views
Loop through all nested dictionary values?
I'm trying to loop through a dictionary and print out all key value pairs where the value is not a nested dictionary. If the value is a dictionary I want to go into it and print out its key value ...
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votes
7
answers
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Implications of foldr vs. foldl (or foldl')
Firstly, Real World Haskell, which I am reading, says to never use foldl and instead use foldl'. So I trust it.
But I'm hazy on when to use foldr vs. foldl'. Though I can see the structure of how ...
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votes
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answers
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How to search by key=>value in a multidimensional array in PHP
Is there any fast way to get all subarrays where a key value pair was found in a multidimensional array? I can't say how deep the array will be.
Simple example array:
$arr = array(0 => array(id=&...
dan
168
votes
37
answers
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views
Java recursive Fibonacci sequence
Please explain this simple code:
public int fibonacci(int n) {
if(n == 0)
return 0;
else if(n == 1)
return 1;
else
return fibonacci(n - 1) + fibonacci(n - 2);
}
I'm ...
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votes
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answers
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views
How do I recursively delete a directory and its entire contents (files + sub dirs) in PHP? [duplicate]
How do I delete a directory and its entire contents (files and subdirectories) in PHP?
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votes
9
answers
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self referential struct definition?
I haven't been writing C for very long, and so I'm not sure about how I should go about doing these sorts of recursive things... I would like each cell to contain another cell, but I get an error ...
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votes
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answers
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Solution for "Fatal error: Maximum function nesting level of '100' reached, aborting!" in PHP
I have made a function that finds all the URLs within an html file and repeats the same process for each html content linked to the discovered URLs. The function is recursive and can go on endlessly. ...
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votes
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answers
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javascript: recursive anonymous function?
Let's say I have a basic recursive function:
function recur(data) {
data = data+1;
var nothing = function() {
recur(data);
}
nothing();
}
How could I do this if I have an ...
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votes
6
answers
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views
GDB corrupted stack frame - How to debug?
I have the following stack trace. Is it possible to make out anything useful from this for debugging?
Program received signal SIGSEGV, Segmentation fault.
0x00000002 in ?? ()
(gdb) bt
#0 0x00000002 ...
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votes
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answers
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recursion versus iteration
Is it correct to say that everywhere recursion is used a for loop could be used? And if recursion is usually slower what is the technical reason for ever using it over for loop iteration?
And if it ...
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votes
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answers
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How exactly does tail recursion work?
I almost understand how tail recursion works and the difference between it and a normal recursion. I only don't understand why it doesn't require stack to remember its return address.
// tail ...
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votes
12
answers
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views
How to [recursively] Zip a directory in PHP? [duplicate]
Directory is something like:
home/
file1.html
file2.html
Another_Dir/
file8.html
Sub_Dir/
file19.html
I am using the same PHP Zip class used in PHPMyAdmin http://trac....
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121
votes
20
answers
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views
List all the files and folders in a Directory with PHP recursive function
I'm trying to go through all of the files in a directory, and if there is a directory, go through all of its files and so on until there are no more directories to go to. Each and every processed item ...
- 1,415
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votes
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answers
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views
What is recursion and when should I use it?
One of the topics that seems to come up regularly on mailing lists and online discussions is the merits (or lack thereof) of doing a Computer Science Degree. An argument that seems to come up time and ...
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votes
14
answers
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Find all occurrences of a key in nested dictionaries and lists
I have a dictionary like this:
{
"id": "abcde",
"key1": "blah",
"key2": "blah blah",
"nestedlist": [
{
...
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answers
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Simplest way to do a recursive self-join?
What is the simplest way of doing a recursive self-join in SQL Server?
PersonID | Initials | ParentID
1 CJ NULL
2 EB 1
3 MB 1
4 SW 2
...
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votes
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answers
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Understanding how recursive functions work
As the title explains I have a very fundamental programming question which I have just not been able to grok yet. Filtering out all of the (extremely clever) "In order to understand recursion, you ...
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votes
37
answers
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views
How to find all combinations of coins when given some dollar value [closed]
I found a piece of code that I was writing for interview prep few months ago.
According to the comment I had, it was trying to solve this problem:
Given some dollar value in cents (e.g. 200 = 2 ...
116
votes
55
answers
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views
Real-world examples of recursion [closed]
What are real-world problems where a recursive approach is the natural solution besides depth-first search (DFS)?
(I don't consider Tower of Hanoi, Fibonacci number, or factorial real-world problems....
116
votes
9
answers
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views
Is recursion a feature in and of itself?
...or is it just a practice?
I'm asking this because of an argument with my professor: I lost credit for calling a function recursively on the basis that we did not cover recursion in class, and my ...
user1948076
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votes
6
answers
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views
Why are functions in OCaml/F# not recursive by default?
Why is it that functions in F# and OCaml (and possibly other languages) are not by default recursive?
In other words, why did the language designers decide it was a good idea to explicitly make you ...
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Can generators be recursive?
I naively tried to create a recursive generator. Didn't work. This is what I did:
def recursive_generator(lis):
yield lis[0]
recursive_generator(lis[1:])
for k in recursive_generator([6,3,9,...
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votes
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Why does my recursive function return None?
I have this function that calls itself:
def get_input():
my_var = input('Enter "a" or "b": ')
if my_var != "a" and my_var != "b":
print('You didn\'t type "a" or "b". Try again.')
...
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votes
9
answers
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One-liner to recursively list directories in Ruby?
What is the fastest, most optimized, one-liner way to get an array of the directories (excluding files) in Ruby?
How about including files?
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