Is there an alternative for *max_element for finding the value of the maximum element in an array for a given range. I want to avoid loop and STL both.
You can use recursive approach if you want to avoid loop or any STL function.
A roughly drafted code would look like this
#include<iostream>
using std::cout;
int maxInt;
int maxRecur(int* arr, int length)
{
if (length == 0)
return maxInt;
else
if (maxInt < *arr)
maxInt = *arr;
maxRecur(arr+1, length-1);
}
int main()
{
int arr[5] = { 1,4,9,3,2 };
cout << maxRecur(arr, sizeof(arr) / sizeof(arr[0]));
}
Related
I have passed an array of size 10 to a funtion to sort the array reversely, but it's going wrong after rightly sorting first five elements of the array.
I want to sort the array 'std' reversely here,
# include <iostream>
using namespace std;
int reverse(int a[]); //funtion prototype
int main()
{
int std[10] = {0,1,2,3,4,5,6,7,8,9};
reverse(std);
}
int reverse(int a[]) //funtion defination
{
int index = 0;
for (int i = 9; i >= 0; i--)
{
a[index] = a[i]; //swaping values of the array
cout << a[index] << " ";
index++;
}
}
There's basically three things wrong with your code.
You aren't swapping anything
You have to swap the first half of the array with the second half, not swap the whole array. If you do that then everything gets swapped twice, so that nothing changes
You should print the reversed array after you have finished the reverse, not while you are doing the reverse.
Here's some code that fixes all these problems
# include <iostream>
# include <utility>
void reverse(int a[]);
int main()
{
int std[10] = {0,1,2,3,4,5,6,7,8,9};
reverse(std);
// print the array after reversing it
for (int i = 0; i < 10; ++i)
std::cout << std[i] << ' ';
std::cout << '\n';
}
void reverse(int a[])
{
for (int i = 0; i < 5; ++i) // swap the first half of the array with the second half
{
std::swap(a[i], a[9 - i]); // real swap
}
}
Yes you can.
I usually don't use "C" style arrays anymore (they can still be useful, but the don't behave like objects). When passing "C" style arrays to functions you kind of always have to manuall pass the size of the array as well (or make assumptions). Those can lead to bugs. (not to mention pointer decay)
Here is an example :
#include <array>
#include <iostream>
// using namespace std; NO unlearn trhis
template<std::size_t N>
void reverse(std::array<int, N>& values)
{
int index = 0;
// you only should run until the middle of the array (size/2)
// or you start swapping back values.
for (int i = values.size() / 2; i >= 0; i--, index++)
{
// for swapping objects/values C++ has std::swap
// using functions like this shows WHAT you are doing by giving it a name
std::swap(values[index], values[i]);
}
}
int main()
{
std::array<int,10> values{ 0,1,2,3,4,5,6,7,8,9 };
reverse(values);
for (const int value : values)
{
std::cout << value << " ";
}
return 0;
}
I am using this program to check a number if prime or not.
Use algorithm - Sieve :
#include<bits/stdc++.h>
//#define _max 2000000001
#define _max 20000001
using namespace std;
bool sieve[_max];
void init()
{
memset(sieve,true,sizeof(sieve));
sieve[0]=sieve[1]=false;
for(int i=2;i<_max;i+=2)
{
sieve[i]=false;
}
}
void go_sieve(int n)
{
n++;
for(int i=3;i<n;i+=2)
{
if(sieve[i]==false)
continue;
for(int j=2*i;j<n;j+=i)
sieve[j]=false;
}
}
void print(int n)
{
n++;
printf("-------------\n");
for(int i=0;i<n;i++)
{
if(sieve[i])
cout << i << " ";
}
printf("\n-------------\n");
}
int main()
{
init();
int n;
scanf("%d",&n);
while(n--)
{
int x;
scanf("%d",&x);
go_sieve(x);
//print(x);
if(sieve[x])
printf("Prime\n");
else
printf("Not prime\n");
}
return 0;
}
Now it works upto 2e7 and pretty smoothly, but I want to check upto 2e9, if I change my _max to 2000000001 it gives me segmentation error and exits with an error code.
How can I resolve this problem ?
I have tried a new approach with set :
#include<bits/stdc++.h>
//#define _max 200001
//#define _max 20000001
#define _max 2000000001
using namespace std;
set<int>prime;
set<int>nprime;
void init()
{
prime.insert(2);
}
void go_sieve()
{
for(int i=3;i<_max;i+=2)
{
if(prime.find(i)==prime.end() && nprime.find(i)==nprime.end())
{
prime.insert(i);
//cout << i << endl;
for(int j=2*i;j<_max;j+=i)
nprime.insert(j);
}
if(nprime.find(i)!=nprime.end())
nprime.erase(nprime.find(i));
}
}
void print()
{
set<int> ::iterator itt;
printf("-------------\n");
for(itt=prime.begin();itt!=prime.end();itt++)
{
cout << *itt << " ";
}
printf("\n-------------\n");
}
int main()
{
init();
go_sieve();
//print();
int n;
scanf("%d",&n);
while(n--)
{
int x;
scanf("%d",&x);
if(prime.find(x)!=prime.end())
printf("Prime\n");
else
printf("Not prime\n");
}
return 0;
}
Target is to execute it within 512MB~1GB memory.
If you want to enumerate large ranges of prime numbers, you should use a segmented Sieve of Eratosthenes; it will be faster (due to caching effects) and use less memory.
If you only want to determine if one number is prime, or a few numbers, sieving is a horrible way to do it. Sieving should only be used when you are interested in an entire range of numbers. For n up to a billion, trial division is simple and probably fast enough. For larger numbers, a Miller-Rabin test or Baillie-Wagstaff test is probably better.
I can't reproduce this on my system. My guess is that this has to do with a system dependant limitation.
You declare sieve as a global array (static storage duration) and it's huge (i.e. 2000000001 * sizeof(bool) - could be 2-8G depending on sizeof bool). Maybe your system can't handle that.
Instead of a global array, try using dynamic allocation:
// bool sieve[_max]; comment out this
bool* sieve = NULL;
...
...
int main()
{
sieve = (bool*)malloc(_max * sizeof *sieve);
if (sieve == NULL)
{
// out of memory
exit(1);
}
...
That said:
Your code is C++ but your style is more C like.
In C++ you would probably use a std::vector instead. That would make everything much easier.
BTW: Also avoid globals. Instead define the vector (or dynamic array) in main and pass it by-reference to the functions.
You probably hit some memory limit on your system which causes the segmentation fault.
However, you don't need such a big array. Using Sieve of Eratosthenes, you need to calculate numbers up to x. Instead of an array you can use std::vector and increase its size as you calculate more numbers. This should allow you to calculate some numbers, but with large numbers you will hit the memory limit again.
You could also use some algorithm which requires you to store fewer numbers. To determine whether x is prime, you only need to compare against prime numbers that are smaller than the square root of x. You don't have to store numbers that are not primes. With x = 1e10, you would only need to store 5e8 numbers.
Here is some example with vector (probably not optimal):
#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
std::vector<int> primes = {2};
void calculate(int x) {
const int largest_prime = primes.back();
if (largest_prime >= x) {
// Already calculated
return;
}
for (size_t i = largest_prime + 1; i <= x; i++) {
bool not_prime = false;
for (size_t j = 0; j < primes.size(); j++) {
if (i % primes[j] == 0) {
not_prime = true;
break;
}
}
if (!not_prime) {
primes.push_back(i);
}
}
}
bool check(int x) {
calculate(x);
return std::find(primes.begin(), primes.end(), x) != primes.end();
}
int main() {
std::cout << check(15) << std::endl;
std::cout << check(256699) << std::endl;
}
I'm testing a recursive function that returns the number of occurrence of a given number in an array. I get an unexpected result when I run the code.
#include <iostream.h>
int Occurence(int A[], int size, int n)
{
static int occur=0;
if(size == 0)
{
int occur2 = (int) occur;
return occur2;
}
else
{
if ( n == A[size-1])
occur++;
Occurence(A, size-1, n);
}
}
int main()
{
int A[] = {1,3,2,5,1,2, 3, 7,7, 8,8, 4, 6, 9,9, 0};
int size = sizeof(A)/sizeof(A[0]);
int n;
cout<< "Enter Number to Find : ";
cin >>n;
cout<<endl;
cout<<"Number of Occurence of "<< n << " is :"<< Occurence(A, size, n)<<endl;
return 0;
}
You are missing a return at the end of your function. If size is not 0 then the behaviour of your function is undefined. Adding the return should make it work:
int Occurence(int A[], int size, int n)
{
static int occur=0;
if(size == 0)
{
int occur2 = (int) occur;
return occur2;
}
else
{
if ( n == A[size-1])
occur++;
return Occurence(A, size-1, n);
}
}
Recursion is a very strange way to implement this problem so I assume this is some toy example to demonstrate how recursion works. Even if this is the case you really shouldn't be using a static variable in your implementation. Just make each call return the current sum instead:
int Occurence(int A[], int size, int n)
{
if(size == 0)
{
return 0;
}
else
{
return (n == A[size-1] ? 1 : 0) + Occurence(A, size-1, n);
}
}
This version will return the correct result when called multiple times whereas your original would add to the previous count each time.
In real code simply do:
#include <algorithm>
int Occurence(int A[], int size, int n)
{
return std::count(A, A+size, n);
}
There are some compilation problems in your code. First of all, in C++, the standard library files usually don't have an extension in the filename. So, including <iostream.h> is wrong. You should include <iostream>.
Other problem with your code is that you are using cout and cin without specifying their namespaces. So, instead of using cout and cin directly, use std::cout and std::cin or declare use namespace std after your includes.
EDIT: as Thomas Matthews pointed out, prefer using std::cout and std::cin over using namespace std.
I have to find the minimum and maximum value of elements in a array using divide and conquer. I have written a code but it is not working for more then 6 elements in array. I don't know whats the problem
#include<iostream>
using namespace std;
int minimum=999,maximum,mi,ma;
void result(int mi,int ma)
{
if(maximum<ma)
{
maximum=ma;
}
if(minimum>mi)
{
minimum=mi;
}
}
void maxmin(int arr[],int i,int j)
{
cout<<" i ="<<i<<" j= "<<j<<endl;
if(i==j)
{
mi=ma=arr[i];
result(mi,ma);
}
else if(i==j-1)
{
if(arr[i]>arr[j])
{
ma=arr[i];
mi=arr[j];
}
else
{
mi=arr[i];
ma=arr[j];
}
result(mi,ma);
}
else
{
int mid=i+j/2;
maxmin(arr,i,mid);
maxmin(arr,mid+1,j);
}
}
int main()
{
int arr[10],n;
cin>>n;
for(int i=0;i<n;i++)
{
cin>>arr[i];
}
maxmin(arr,0,n-1);
cout<<" max "<<maximum<<" min "<<minimum<<endl;
return 0;
}
Your code has a few mistakes
Your code reads n from the user input, but you provided only 10 sized array, and user can try to input 10+ numbers, so we will have an undefined behavior in that case.
You write it very bad and unreadable. If you want somebody else to read your code, check in the your favourite book or in the internet information about how to write beautiful and readable code.
You implemented that algorithm yourself. It is a bad habit, use the standard library algorithms and you will not encounter such mistake.
.
#include <iostream> // std::cin, std::cout
#include <cstddef> // std::size_t
#include <algorithm> // std::min_element, std::max_element
int main ()
{
std::size_t array_size;
std::cin >> array_size;
int *some_array = new int[array_size]; // Allocate memory dynamically
for(std::size_t i = 0; i < array_size; ++i)
{
std::cin >> some_array[i];
}
/* Standard library operate on iterators, they are special classes
* that have interface that is similar in many cases to pointers (so we can use pointers as iterators).
* std::min/max_element needs one iterator for the sequence beginning
* and one iterator after the end. It returns iterator to a found element.
*/
int min = *std::min_element(some_array, some_array + array_size);
int max = *std::max_element(some_array, some_array + array_size);
delete[] some_array;
std::cout << "Min = " << min << std::endl << "Max = " << max;
std::cout << std::endl;
}
Code isn't well written and first dry run your code, you will find the problem easily.
Change
else
{
int mid=i+j/2;
maxmin(arr,i,mid);
maxmin(arr,mid+1,j);
}
To
else
{
int mid=(i+j)/2; /*** Adding brackets ***/
maxmin(arr,i,mid);
maxmin(arr,mid+1,j);
}
And check the logic for calling the result function (because according to your logic the two subsets are individually calculating MIN and MAX in itself not in whole array)
I trying to pass an array but don't understand why it gives me those errors. The code is also available in ideone.com
#include <iostream>
using namespace std;
class Max
{
int max = 0;
public:
int getMax(int array[], int size)
{
for(int num : array)
{
if(num > max)
max = num;
}
return max;
}
};
int main( )
{
Max m;
int arr[5] = { 5, 3, 2, 7, 6 };
cout << "Max number is: " << m.getMax(arr,5);
return 0;
}
The problem here as has been mentioned is that passing an array to a function it decays to a pointer. The fix that involves the least changes is to pass the array by reference like so:
template <int U>
int getMax(int (&array)[U])
this fix is probably not the most intuitive for a beginner though. The fix that requires a bit more changes and probably makes more sense to a beginner is to use std::vector or std::array:
int getMax(const std::vector<int> &array)
and in main:
std::vector<int> arr = { 5, 3, 2, 7, 6 };
cout << "Max number is: " << m.getMax(arr);
The cause is the for(:) can not get the size of "int array[]".
You have a size argument, but the begin() & end() can not use it. You must wrap the begin() and end() member functions or just simple it to
for(int i = 0; i< size; i++)
{
int num = array[i];
if(num > max)
max = num;
}
size argument needs type specified (proper type is size_t).
array in getMax function is a pointer (not an array). You can't use range-based for loop with it. You have to use regular for loop which will make use of size argument.