All Questions
12,615
questions
1910
votes
23
answers
241k
views
Image Processing: Algorithm Improvement for 'Coca-Cola Can' Recognition
One of the most interesting projects I've worked on in the past couple of years was a project about image processing. The goal was to develop a system to be able to recognize Coca-Cola 'cans' (note ...
- 41.2k
661
votes
32
answers
653k
views
How do you compare float and double while accounting for precision loss?
What would be the most efficient way to compare two double or two float values?
Simply doing this is not correct:
bool CompareDoubles1 (double A, double B)
{
return A == B;
}
But something like:
...
- 6,651
634
votes
19
answers
746k
views
How to replace all occurrences of a character in string?
What is the effective way to replace all occurrences of a character with another character in std::string?
- 6,972
408
votes
21
answers
350k
views
Determine if two rectangles overlap each other?
I am trying to write a C++ program that takes the following inputs from the user to construct rectangles (between 2 and 5): height, width, x-pos, y-pos. All of these rectangles will exist parallel to ...
- 5,445
358
votes
11
answers
255k
views
Fast ceiling of an integer division in C / C++
Given integer values x and y, C and C++ both return as the quotient q = x/y the floor of the floating point equivalent. I'm interested in a method of returning the ceiling instead. For example, ceil(...
- 17.3k
347
votes
2
answers
36k
views
How to implement classic sorting algorithms in modern C++?
The std::sort algorithm (and its cousins std::partial_sort and std::nth_element) from the C++ Standard Library is in most implementations a complicated and hybrid amalgamation of more elementary ...
- 69.9k
244
votes
29
answers
151k
views
Detecting endianness programmatically in a C++ program
Is there a programmatic way to detect whether or not you are on a big-endian or little-endian architecture? I need to be able to write code that will execute on an Intel or PPC system and use exactly ...
- 2,449
227
votes
19
answers
398k
views
Which is the fastest algorithm to find prime numbers?
Which is the fastest algorithm to find out prime numbers using C++? I have used sieve's algorithm but I still want it to be faster!
- 3,201
214
votes
32
answers
195k
views
Rounding up to the nearest multiple of a number
OK - I'm almost embarrassed posting this here (and I will delete if anyone votes to close) as it seems like a basic question.
Is this the correct way to round up to a multiple of a number in C++?
I ...
- 8,662
203
votes
6
answers
273k
views
Rotating a point about another point (2D)
I'm trying to make a card game where the cards fan out. Right now to display it Im using the Allegro API which has a function:
al_draw_rotated_bitmap(OBJECT_TO_ROTATE,CENTER_X,CENTER_Y,X
,Y,...
- 53.4k
202
votes
12
answers
13k
views
Throwing the fattest people off of an overloaded airplane.
Let's say you've got an airplane, and it is low on fuel. Unless the plane drops 3000 pounds of passenger weight, it will not be able to reach the next airport. To save the maximum number of lives, ...
- 2,219
180
votes
6
answers
466k
views
Is it more efficient to copy a vector by reserving and copying, or by creating and swapping? [duplicate]
I am trying to efficiently make a copy of a vector. I see two possible approaches:
std::vector<int> copyVecFast1(const std::vector<int>& original)
{
std::vector<int> newVec;
...
Sasha
151
votes
37
answers
19k
views
Need for predictable random generator
I'm a web-game developer and I got a problem with random numbers. Let's say that a player has 20% chance to get a critical hit with his sword. That means, 1 out of 5 hits should be critical. The ...
130
votes
13
answers
33k
views
Finding duplicates in O(n) time and O(1) space
Input: Given an array of n elements which contains elements from 0 to n-1, with any of these numbers appearing any number of times.
Goal : To find these repeating numbers in O(n) and using only ...
- 1,325
128
votes
2
answers
48k
views
std::back_inserter for a std::set?
I guess this is a simple question. I need to do something like this:
std::set<int> s1, s2;
s1 = getAnExcitingSet();
std::transform(s1.begin(), s1.end(), std::back_inserter(s2), ...
- 66.7k
122
votes
13
answers
267k
views
Algorithm to convert RGB to HSV and HSV to RGB in range 0-255 for both
I am looking for color space converter from RGB to HSV, specifically for the range 0 to 255 for both color spaces.
- 53.4k
122
votes
9
answers
47k
views
Where can I get a "useful" C++ binary search algorithm?
I need a binary search algorithm that is compatible with the C++ STL containers, something like std::binary_search in the standard library's <algorithm> header, but I need it to return the ...
- 69.3k
116
votes
16
answers
92k
views
How can I test whether a number is a power of 2?
I need a function like this:
// return true if 'n' is a power of 2, e.g.
// is_power_of_2(16) => true
// is_power_of_2(3) => false
bool is_power_of_2(int n);
Can anyone suggest how I could ...
- 1,171
116
votes
3
answers
30k
views
Magic number in boost::hash_combine
The boost::hash_combine template function takes a reference to a hash (called seed) and an object v. According to the docs, it combines seed with the hash of v by
seed ^= hash_value(v) + 0x9e3779b9 + (...
- 360k
112
votes
19
answers
29k
views
What is the fastest way to compute sin and cos together?
I would like to compute both the sine and co-sine of a value together (for example to create a rotation matrix). Of course I could compute them separately one after another like a = cos(x); b = sin(x);...
- 22.6k
105
votes
10
answers
206k
views
Calculate mean and standard deviation from a vector of samples in C++ using Boost
Is there a way to calculate mean and standard deviation for a vector containing samples using Boost?
Or do I have to create an accumulator and feed the vector into it?
- 1,605
103
votes
6
answers
181k
views
Finding the position of the maximum element
Is there a standard function that returns the position (not value) of the maximum element of an array of values?
For example:
Suppose I have an array like this:
sampleArray = [1, 5, 2, 9, 4, 6, 3]
I ...
- 11.6k
102
votes
4
answers
40k
views
Difference between std::set and std::priority_queue
Since both std::priority_queue and std::set (and std::multiset) are data containers that store elements and allow you to access them in an ordered fashion, and have same insertion complexity O(log n), ...
- 8,369
98
votes
26
answers
74k
views
Searching in a sorted and rotated array
While preparing for an interview I stumbled upon this interesting question:
You've been given an array that is sorted and then rotated.
For example:
Let arr = [1,2,3,4,5], which is ...
- 983
95
votes
12
answers
143k
views
What is the fastest way to transpose a matrix in C++?
I have a matrix (relatively big) that I need to transpose. For example assume that my matrix is
a b c d e f
g h i j k l
m n o p q r
I want the result be as follows:
a g m
b h n
c I o
d j p
e k q
f ...
- 17.6k
94
votes
11
answers
93k
views
Inverting a 4x4 matrix
I am looking for a sample code implementation on how to invert a 4x4 matrix. I know there is Gaussian eleminiation, LU decomposition, etc., but instead of looking at them in detail I am really just ...
- 33.5k
90
votes
14
answers
81k
views
LRU cache design
Least Recently Used (LRU) Cache is to discard the least recently used items first
How do you design and implement such a cache class? The design requirements are as follows:
1) find the item as fast ...
- 7,873
89
votes
6
answers
75k
views
Difference between priority queue and a heap
It seems that a priority queue is just a heap with normal queue operations like insert, delete, top, etc. Is this the correct way to interpret a priority queue? I know you can build priority queues in ...
- 4,827
86
votes
9
answers
7k
views
How to make a for loop variable const with the exception of the increment statement?
Consider a standard for loop:
for (int i = 0; i < 10; ++i)
{
// do something with i
}
I want to prevent the variable i from being modified in the body of the for loop.
However, I cannot ...
- 4,431
86
votes
5
answers
51k
views
Easiest way of using min priority queue with key update in C++
Sometimes during programming contests etc., we need a simple working implementation of min priority queue with decrease-key to implement Dijkstra algorithm etc.. I often use set< pair<key_value, ...
- 1,091
84
votes
16
answers
132k
views
Generating combinations in C++
I have been searching for a source code for generating combinations using C++. I found some advanced codes for this but that is good for only specific number predefined data. Can anyone give me some ...
- 973
83
votes
15
answers
197k
views
Splitting a string by a character
I know this is a quite easy problem but I just want to solve it for myself once and for all
I would simply like to split a string into an array using a character as the split delimiter. (Much like ...
- 5,398
82
votes
14
answers
189k
views
Permutation of array
For example I have this array:
int a[] = new int[]{3,4,6,2,1};
I need list of all permutations such that if one is like this, {3,2,1,4,6}, others must not be the same. I know that if the length of ...
user466534
81
votes
10
answers
9k
views
Does the range-based 'for' loop deprecate many simple algorithms?
Algorithm solution:
std::generate(numbers.begin(), numbers.end(), rand);
Range-based for-loop solution:
for (int& x : numbers) x = rand();
Why would I want to use the more verbose std::...
- 260k
79
votes
18
answers
73k
views
Circular lock-free buffer
I'm in the process of designing a system which connects to one or more stream of data feeds and do some analysis on the data than trigger events based on the result. In a typical multi-threaded ...
- 1,606
77
votes
13
answers
70k
views
Fastest way to get the integer part of sqrt(n)?
As we know if n is not a perfect square, then sqrt(n) would not be an integer. Since I need only the integer part, I feel that calling sqrt(n) wouldn't be that fast, as it takes time to calculate the ...
- 358k
72
votes
23
answers
86k
views
How do I get the intersection between two arrays as a new array?
I faced this problem many times during various situations. It is generic to all programming languages although I am comfortable with C or Java.
Let us consider two arrays (or collections):
char[] A =...
- 1,575
71
votes
14
answers
210k
views
Creating all possible k combinations of n items in C++
There are n people numbered from 1 to n. I have to write a code which produces and print all different combinations of k people from these n. Please explain the algorithm used for that.
- 1,555
71
votes
3
answers
51k
views
std::transform() and toupper(), no matching function
I tried the code from this question C++ std::transform() and toupper() ..why does this fail?
#include <iostream>
#include <algorithm>
int main() {
std::string s="hello";
std::string ...
- 835
70
votes
9
answers
97k
views
How to get a random element from a C++ container?
What is a good way to get a [pseudo-]random element from an STL range?
The best I can come up with is to do std::random_shuffle(c.begin(), c.end()) and then take my random element from c.begin().
...
- 8,421
70
votes
8
answers
16k
views
Emulate "double" using 2 "float"s
I am writing a program for an embedded hardware that only supports 32-bit single-precision floating-point arithmetic. The algorithm I am implementing, however, requires a 64-bit double-precision ...
user210870
70
votes
14
answers
105k
views
Fast String Hashing Algorithm with low collision rates with 32 bit integer [closed]
I have lots of unrelated named things that I'd like to do quick searches against. An "aardvark" is always an "aardvark" everywhere, so hashing the string and reusing the integer would work well to ...
- 885
69
votes
9
answers
12k
views
Performance issue: Java vs C++
I have always heard that C++ was way more efficient than Java (and that is why most games are developed in C++).
I wrote a small algorithm to solve the "Eight queens puzzle" in both Java and C++, ...
- 2,171
69
votes
7
answers
7k
views
How is vector<vector<int>> "heavier" than vector<pair<int,int>>?
During a recent interview, I suggested using vector<pair<int,int>> over vector<vector<int>> since we only wanted to store two values for every entry in the vector. I said ...
- 857
66
votes
10
answers
106k
views
Most efficient/elegant way to clip a number?
Given a real (n), a maximum value this real can be (upper), and a minimum value this real can be (lower), how can we most efficiently clip n, such that it remains between lower and upper?
Of course, ...
- 2,560
66
votes
16
answers
136k
views
Efficiently getting all divisors of a given number
According to this post, we can get all divisors of a number through the following codes.
for (int i = 1; i <= num; ++i){
if (num % i == 0)
cout << i << endl;
}
For example,...
- 46.2k
66
votes
3
answers
3k
views
Is there a nice way to assign std::minmax(a, b) to std::tie(a, b)?
std::tie(a, b) = std::minmax(a, b);
I think this is intuitive code. Clean and understandable. Too bad it doesn't work as intended, as std::minmax templates for const&. If therefore the values are ...
- 7,974
66
votes
4
answers
31k
views
what is the difference between set and unordered_set in C++?
I came across this good question, which is similar but not at all same since it talks about Java, which has different implementation of hash-tables, by virtue of having synchronized accessor /mutators:...
- 8,492
64
votes
11
answers
189k
views
Use of for_each on map elements
I have a map where I'd like to perform a call on every data type object member function. I yet know how to do this on any sequence but, is it possible to do it on an associative container?
The ...
- 6,832
64
votes
10
answers
50k
views
What is the right approach when using STL container for median calculation?
Let's say I need to retrieve the median from a sequence of 1000000 random numeric values.
If using anything but std::list, I have no (built-in) way to sort sequence for median calculation.
If using ...
- 12.3k