I'm trying to write a c++ program which gets an integer n (n>=1 && n<=100000) from the user and puts the sum of its digits into b. The output needed is the b-th prime number coming after n. I'm an absolute beginner in programming so I don't know what's wrong with the for loop or any other code that it doesn't show the correct output. For example the 3rd prime number after 12 (1+2=3) is 19 but the loop counts the prime numbers from 2 instead of 12, so it prints 7 as result.
#include <iostream>
using namespace std;
bool isPrime(int n)
{
if(n <= 1)
return false;
for(int i = 2; i <= (n/2); i++)
if(n % i == 0)
return false;
return true;
}
int main()
{
long int n;
int b = 0;
cin>>n;
while(n >= 1 && n <= 100000){
b += n % 10;
n /= 10;
}
for(int i = n, counter = b; counter <= 10; i++)
if(isPrime(i)){
counter++;
if(i > n)
cout<<counter<<"th prime number after n is : "<<i<<endl;
}
return 0;
}
So one of the possible solutions to my question, according to #Bob__ answer (and converting it to the code style I've used in the initial code) is as follows:
#include <iostream>
using namespace std;
bool isPrime(long int number)
{
if(number <= 1)
return false;
for(int i = 2; i <= (number / 2); i++)
if(number % i == 0)
return false;
return true;
}
int sumOfDigits(long int number)
{
int sum = 0;
while(number >= 1 && number <= 100000)
{
sum += number % 10;
number /= 10;
}
return sum;
}
long int bthPrimeAfter(int counter, long int number)
{
while(counter)
{
++number;
if(isPrime(number))
--counter;
}
return number;
}
int main()
{
long int number;
cin>>number;
int const counter = sumOfDigits(number);
cout<<bthPrimeAfter(counter, number)<<"\n";
return 0;
}
As dratenik said in their comment:
You have destroyed the value in n to produce b in the while loop. When the for loop comes around, n keeps being zero.
That's a key point to understand, sometimes we need to make a copy of a variable. One way to do that is passing it to a function by value. The function argument will be a local copy which can be changed without affecting the original one.
As an example, the main function could be written like the following:
#include <iostream>
bool is_prime(long int number);
// ^^^^^^^^ So is `n` in the OP's `main`
int sum_of_digits(long int number);
// ^^^^^^^^^^^^^^^ This is a local copy.
long int nth_prime_after(int counter, long int number);
int main()
{
long int number;
// The input validation (check if it's a number and if it's in the valid range,
// deal with errors) is left to the reader as an exercise.
std::cin >> number;
int const counter = sum_of_digits(number);
std::cout << nth_prime_after(counter, number) << '\n';
return 0;
}
The definition of sum_of_digits is straightforward.
int sum_of_digits(long int number)
{
int sum = 0;
while ( number ) // Stops when number is zero. The condition n <= 100000
{ // belongs to input validation, like n >= 0.
sum += number % 10;
number /= 10; // <- This changes only the local copy.
}
return sum;
}
About the last part (finding the nth prime after the chosen number), I'm not sure to understand what the asker is trying to do, but even if n had the correct value, for(int i = n, counter = b; counter <= 10; i++) would be just wrong. For starters, there's no reason for the condition count <= 10 or at least none that I can think of.
I'd write something like this:
long int nth_prime_after(int counter, long int number)
{
while ( counter )
{
++number;
if ( is_prime(number) )
{
--counter; // The primes aren't printed here, not even the nth.
}
}
return number; // Just return it, the printing is another function's
} // responsabilty.
A lot more could be said about the is_prime function and the overall (lack of) efficiency of this algorithm, but IMHO, it's beyond the scope of this answer.
Related
I need to find the maximum remainder for n divided by any integer number from 1 to n, and the denominator which this remainder is found with.
In my implementation fun1 works as expected and returns the max remainder, fun2 is supposed to give 3 but its giving 2 .probably mistake is at break statement.
Sample input: 5
Expected output: 2 3.
My output: 2 2.
#include <iostream>
#include <algorithm>
using namespace std;
int fun2(int a);
int fun1(int n ,int num);
int main(){
int n = 0; int num = 0;;
cin >> n;
int p = fun1(n, num);
cout << p << "\n";
cout << fun2(p);
}
int fun1(int n, int num){
int b = 0;
for(int i = 1; i <= n; i++){
num = n % i;
b = max(num, b);
}
return b;
}
int fun2(int n,int p ){
int num = 0; int c = 0; int d = 0;
for(int i = 1; i <= n; i++){
num = n % i;
c = max(num, c);
if(c == p){
break;
}
d = i;
}
return d;
}
Since you already managed to successfully find the biggest remainder, you may get use of this function and return the number this remainder is found with:
std::pair<int, int> biggestRemDem(int value) {
int dm = 1;
int rm = 0;
for(int i = dm; i <= value; ++i){
const auto tmpRm = value % i;
if (tmpRm > rm) {
rm = tmpRm;
dm = i;
}
}
return { rm, dm };
}
The signature of the function needs to return std::pair however, but you no longer need the std::max, so the headers required to include are also changed:
#include <iostream>
#include <utility>
std::pair<int, int> biggestRemDem(int value);
int main(){
int n{};
std::cin >> n;
const auto result = biggestRemDem(n);
std::cout << result.first << " " << result.second << std::endl;
}
In fun2 you have:
if(c == p){
break;
}
d = i;
When you found the right index so that c == p the break will exit the loop and d == i; is not execute. Therefore d has the value from the previous loop, i.e. one less than you need.
Apart from that the code really smells:
fun1
should not have a second argument sum.
should remember the index where if found the largest remainder and you would be done
fun2
the maximum remainder is p, no need to max(num, c). Actually drop the c alltogether and just use num == p
n % 1 == 0 and n % n == 0. The loop will always break with i < n. Might as well not have a conditional: for(int i = 1; ; i++)
you need d because at the end of the loop i disappears. Why not pull i out of the loop? int i; for(i = 1; ; i++)
and now you can use a different conditional again
int fun2(int n,int p ){
int i;
for(i = 1; n % i != p; i++) { }
return i;
}
or
int fun2(int n,int p ){
int i = 1;
for(; n % i != p; i++) { }
return i;
}
or
int fun2(int n,int p ){
int i = 1;
while(n % i != p) ++i;
return i;
}
I need to find the maximum remainder for n divided by any integer number from 1 to n, and the denominator which this remainder is found with.
It seems that the asker decided to solve this in two steps. They wrote a function fun1 returning the maximum remainder and a function fun2, which fed with the previously calculated remainder, returns the corresponding dividend.
While not an efficient approach, it could work if implemented correctly, but that's not the case.
Other than some (very) bad naming choices, we can find:
In the original version of the posted code, fun2 has a function prototype with a single parameter and it is called passing the value returned by fun1, which is the maximum remainder. The problem is that this way the function has no way to know what was the original value of n and actually declares a local n, initialized to zero, so that the body of the loop for(int i = 1; i <= n; i++) is never executed.
The actual version of this question shows a definition of fun2 with two parameters, that can't compile unless both the prototype and the call site are changed accordingly.
Assuming that the original n and the remainder p were succesfully passed to fun2, there still would be another issue:
int fun2(int n, int p ) {
int c = 0, d = 0;
for(int i = 1; i <= n; i++) {
int num = n % i;
c = max(num, c);
if(c == p){ // So, if we reach the passed remainder...
break; // We break out of the loop, skipping...
}
d = i; // this line, meaning...
}
return d; // That the dividend previous to the correct one is returned!
}
They could just return i; when c == p.
The answer by The Dreams Wind presents a much better approach to this task. I'd like to suggest an O(1) solution, though. Consider these points:
The result of n % i can't be equal or greater than i. It's in the range [0, i).
n / 2 is the greatest number that can divide n other than n itself. It means that all the numbers i in (n/2, n) are such that n % i > 0.
For every number i in (n/2, n), we can actually say that n % i = n - i.
So, when n is greater than 2, the i corresponding to the maximum remainder is just 1 + n/2 and said remainder is n - n/2 - 1.
Consistently comparing digits symmetrically to its middle digit. If first number is bigger than the last , first is wining and I have to display it else I display last and that keep until I reach middle digit(this is if I have odd number of digits), if digit don't have anything to be compared with it wins automatically.
For example number is 13257 the answer is 7 5 2.
Another one 583241 the answer is 5 8 3.
For now I am only trying to catch when number of digits is odd. And got stuck.. This is my code. The problem is that this code don't display any numbers, but it compares them in the if statement(I checked while debugging).
#include <iostream>
using namespace std;
int countDigit(int n) {
int count = 0;
while (n != 0) {
count++;
n /= 10;
}
return count;
}
int main() {
int n;
cin >> n;
int middle;
int count = countDigit(n);
if (count % 2 == 0) {
cout<<"No mid digit exsist!!";
}
else {
int lastDigit = n % 10;
middle = (count + 1) / 2;
for (int i = 0; i < middle; i++) {
for (int j = lastDigit; j<middle; j--) {
if (i > j) {
cout << i <<' ';
}
else {
cout << j;
}
}
}
}
return 0;
}
An easier approach towards this, in my opinion, would be using strings. You can check the size of the string. If there are even number of characters, you can just compare the first half characters, with the last half. If there are odd numbers, then do the same just print the middle character.
Here's what I'd do for odd number of digits:
string n;
cin>>n;
int i,j;
for(i=0,j=n.size()-1;i<n.size()/2,j>=(n.size()+1)/2;i++,j--)
{
if(n[i]>n[j]) cout<<n[i]<<" ";
else cout<<n[j]<<" ";
}
cout<<n[n.size()/2]<<endl;
We analyze the requirements and then come up with a design.
If we have a number, consisting of digits, we want to compare "left" values with "right" values. So, start somehow at the left and the right index of digits in a number.
Look at this number: 123456789
Index: 012345678
Length: 9
in C and C++ indices start with 0.
So, what will we do?
Compare index 0 with index 8
Compare index 1 with index 7
Compare index 2 with index 6
Compare index 3 with index 5
Compare index 4 with index 4
So, the index from the left is running up and the index from the right is running down.
We continue as long as the left index is less than or equal the right index. All this can be done in a for or while loop.
It does not matter, wether the number of digits is odd or even.
Of course we also do need functions that return the length of a number and a digit of the number at a given position. But I see that you know already how to write these functions. So, I will not explain it further here.
I show you 3 different examples.
Ultra simple and very verbose. Very inefficient, because we do not have arrays.
Still simple, but more compressed. Very inefficient, because we do not have arrays.
C++ solution, not allowed in your case
Verbose
#include <iostream>
// Get the length of a number
unsigned int length(unsigned long long number) {
unsigned int length = 0;
while (number != 0) {
number /= 10;
++length;
}
return length;
}
// Get a digit at a given index of a number
unsigned int digitAt(unsigned int index, unsigned long long number) {
index = length(number) - index - 1;
unsigned int result = 0;
unsigned int count = 0;
while ((number != 0) && (count <= index)) {
result = number % 10;
number /= 10;
++count;
}
return result;
}
// Test
int main() {
unsigned long long number;
if (std::cin >> number) {
unsigned int indexLeft = 0;
unsigned int indexRight = length(number) - 1;
while (indexLeft <= indexRight) {
if (digitAt(indexLeft, number) > digitAt(indexRight, number)) {
std::cout << digitAt(indexLeft, number);
}
else {
std::cout << digitAt(indexRight, number);
}
++indexLeft;
--indexRight;
}
}
}
Compressed
#include <iostream>
// Get the length of a number
size_t length(unsigned long long number) {
size_t length{};
for (; number; number /= 10) ++length;
return length;
}
// Get a digit at a given index of a number
unsigned int digitAt(size_t index, unsigned long long number) {
index = length(number) - index - 1;
unsigned int result{}, count{};
for (; number and count <= index; ++count, number /= 10)
result = number % 10;
return result;
}
// Test
int main() {
if (unsigned long long number; std::cin >> number) {
// Iterate from left and right at the same time
for (size_t indexLeft{}, indexRight{ length(number) - 1 }; indexLeft <= indexRight; ++indexLeft, --indexRight)
std::cout << ((digitAt(indexLeft,number) > digitAt(indexRight, number)) ? digitAt(indexLeft, number) : digitAt(indexRight, number));
}
}
More modern C++
#include <iostream>
#include <string>
#include <algorithm>
#include <cctype>
int main() {
if (std::string numberAsString{}; std::getline(std::cin, numberAsString) and not numberAsString.empty() and
std::all_of(numberAsString.begin(), numberAsString.end(), std::isdigit)) {
for (size_t indexLeft{}, indexRight{ numberAsString.length() - 1 }; indexLeft <= indexRight; ++indexLeft, --indexRight)
std::cout << ((numberAsString[indexLeft] > numberAsString[indexRight]) ? numberAsString[indexLeft] : numberAsString[indexRight]);
}
}
You are trying to do something confusing with nested for-cycles. This is obviously wrong, because there is nothing “quadratic” (with respect to the number of digits) in the entire task. Also, your code doesn’t seem to contain anything that would determine the highest-order digit.
I would suggest that you start with something very simple: string’ify the number and then iterate over the digits in the string. This is obviously neither elegant nor particularly fast, but it will be a working solution to start with and you can improve it later.
BTW, the sooner you get out of the bad habit of using namespace std; the better. It is an antipattern, please avoid it.
Side note: There is no need to treat odd and even numbers of digits differently. Just let the algorithm compare the middle digit (if it exists) against itself and select it; no big deal. It is a tiny efficiency drawback in exchange for a big code simplicity benefit.
#include <cstdint>
#include <iostream>
#include <string>
using std::size_t;
using std::uint64_t;
uint64_t extract_digits(uint64_t source) {
const std::string digits{std::to_string(source)};
auto i = digits.begin();
auto j = digits.rbegin();
const auto iend = i + (digits.size() + 1) / 2;
uint64_t result{0};
for (; i < iend; ++i, ++j) {
result *= 10;
result += (*i > *j ? *i : *j) - '0';
}
return result;
}
int main() {
uint64_t n;
std::cin >> n;
std::cout << extract_digits(n) << std::endl;
}
If the task disallows the use of strings and arrays, you could try using pure arithmetics by constructing a “digit-inverted” version of the number and then iterating over both numbers using division and modulo. This will (still) have obvious limitations that stem from the data type size, some numbers cannot be inverted properly etc. (Use GNU MP for unlimited integers.)
#include <cstdint>
#include <iostream>
using std::size_t;
using std::uint64_t;
uint64_t extract_digits(uint64_t source) {
uint64_t inverted{0};
size_t count{0};
for (uint64_t div = source; div; div /= 10) {
inverted *= 10;
inverted += div % 10;
++count;
}
count += 1;
count /= 2;
uint64_t result{0};
if (count) for(;;) {
const uint64_t a{source % 10}, b{inverted % 10};
result *= 10;
result += a > b ? a : b;
if (!--count) break;
source /= 10;
inverted /= 10;
}
return result;
}
int main() {
uint64_t n;
std::cin >> n;
std::cout << extract_digits(n) << std::endl;
}
Last but not least, I would strongly suggest that you ask questions after you have something buildable and runnable. Having homework solved by someone else defeats the homework’s purpose.
I have a program like this: given a sequence of integers, find the biggest prime and its positon.
Example:
input:
9 // how many numbers
19 7 81 33 17 4 19 21 13
output:
19 // the biggest prime
1 7 // and its positon
So first I get the input, store it in an array, make a copy of that array and sort it (because I use a varible to keep track of the higest prime, and insane thing will happen if that was unsorted) work with every number of that array to check if it is prime, loop through it again to have the positon and print the result.
But the time is too slow, can I improve it?
My code:
#include <iostream>
#include <cmath>
#include <algorithm>
using namespace std;
int main()
{
int n;
cin >> n;
int numbersNotSorted[n];
int maxNum{0};
for (int i = 0; i < n; i++)
{
cin >> numbersNotSorted[i];
}
int numbersSorted[n];
for (int i = 0; i < n; i++)
{
numbersSorted[i] = numbersNotSorted[i];
}
sort(numbersSorted, numbersSorted + n);
for (int number = 0; number < n; number++)
{
int countNum{0};
for (int i = 2; i <= sqrt(numbersSorted[number]); i++)
{
if (numbersSorted[number] % i == 0)
countNum++;
}
if (countNum == 0)
{
maxNum = numbersSorted[number];
}
}
cout << maxNum << '\n';
for (int i = 0; i < n; i++)
{
if (numbersNotSorted[i] == maxNum)
cout << i + 1 << ' ';
}
}
If you need the biggest prime, sorting the array brings you no benefit, you'll need to check all the values stored in the array anyway.
Even if you implemented a fast sorting algorithm, the best averages you can hope for are O(N + k), so just sorting the array is actually more costly than looking for the largest prime in an unsorted array.
The process is pretty straight forward, check if the next value is larger than the current largest prime, and if so check if it's also prime, store the positions and/or value if it is, if not, check the next value, repeat until the end of the array.
θ(N) time compexity will be the best optimization possible given the conditions.
Start with a basic "for each number entered" loop:
#include <iostream>
#include <cmath>
#include <algorithm>
using namespace std;
int main() {
int n;
int newNumber;
cin >> n;
for (int i = 0; i < n; i++) {
cin >> newNumber;
}
}
If the new number is smaller than the current largest prime, then it can be ignored.
int main() {
int n;
int newNumber;
int highestPrime;
cin >> n;
for (int i = 0; i < n; i++) {
cin >> newNumber;
if(newNumber >= highestPrime) {
}
}
}
If the new number is equal to the highest prime, then you just need to store its position somewhere. I'm lazy, so:
int main() {
int n;
int newNumber;
int highestPrime;
int maxPositions = 1234;
int positionList[maxPositions];
int nextPosition;
int currentPosition = 0;
cin >> n;
for (int i = 0; i < n; i++) {
cin >> newNumber;
currentPosition++;
if(newNumber >= highestPrime) {
if(newNumber == highestPrime) {
if(nextPosition+1 >= maxPositions) {
// List of positions is too small (should've used malloc/realloc instead of being lazy)!
} else {
positionList[nextPosition++] = currentPosition;
}
}
}
}
}
If the new number is larger than the current largest prime, then you need to figure out if it is a prime number, and if it is you need to reset the list and store its position, etc:
int main() {
int n;
int newNumber;
int highestPrime = 0;
int maxPositions = 1234;
int positionList[maxPositions];
int nextPosition;
int currentPosition = 0;
cin >> n;
for (int i = 0; i < n; i++) {
cin >> newNumber;
currentPosition++;
if(newNumber >= highestPrime) {
if(newNumber == highestPrime) {
if(nextPosition+1 >= maxPositions) {
// List of positions is too small (should've used malloc/realloc instead of being lazy)!
} else {
positionList[nextPosition++] = currentPosition;
}
} else { // newNumber > highestPrime
if(isPrime(newNumber)) {
nextPosition = 0; // Reset the list
highestPrime = newNumber;
positionList[nextPosition++] = currentPosition;
}
}
}
}
}
You'll also want something to display the results:
if(highestPrime > 0) {
for(nextPosition= 0; nextPosition < currentPosition; nextPosition++) {
cout << positionList[nextPosition];
}
}
Now; the only thing you're missing is an isPrime(int n) function. The fastest way to do that is to pre-calculate a "is/isn't prime" bitfield. It might look something like:
bool isPrime(int n) {
if(n & 1 != 0) {
n >>= 1;
if( primeNumberBitfield[n / 32] & (1 << (n % 32)) != 0) {
return true;
}
}
return false;
}
The problem here is that (for positive values in a 32-bit signed integer) you'll need 1 billion bits (or 128 MiB).
To avoid that you can use a much smaller bitfield for numbers up to sqrt(1 << 31) (which is only about 4 KiB); then if the number is too large for the bitfield you can use the bitfield to find prime numbers and check (with modulo) if they divide the original number evenly.
Note that Sieve of Eratosthenes ( https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes ) is an efficient way to generate that smaller bitfield (but is not efficient to use for a sparse population of larger numbers).
If you do it right, you'll probably create the illusion that it's instantaneous because almost all of the work will be done while a human is slowly typing the numbers in (and not left until after all of the numbers have been entered). For a very fast typist you'll have ~2 milliseconds between numbers, and (after the last number is entered) humans can't notice delays smaller than about 10 milliseconds.
But the time is too slow, can I improve it?
Below loop suffers from:
Why check smallest values first? Makes more sense to check largest values first to find the largest prime. Exit the for (... number..) loop early once a prime is found. This takes advantage of the work done by sort().
Once a candidate value is not a prime, quit testing for prime-ness.
.
// (1) Start for other end rather than as below
for (int number = 0; number < n; number++) {
int countNum {0};
for (int i = 2; i <= sqrt(numbersSorted[number]); i++) {
if (numbersSorted[number] % i == 0)
// (2) No point in continuing prime testing, Value is composite.
countNum++;
}
if (countNum == 0) {
maxNum = numbersSorted[number];
}
}
Corrections left for OP to implement.
Advanced: Prime testing is a deep subject and many optimizations (trivial and complex) exist that are better than OP's approach. Yet I suspect the above 2 improvement will suffice for OP.
Brittleness: Code does not well handle the case of no primes in the list or n <= 0.
i <= sqrt(numbersSorted[number]) is prone to FP issues leading to an incorrect results. Recommend i <= numbersSorted[number]/i).
Sorting is O(n * log n). Prime testing, as done here, is O(n * sqrt(n[i])). Sorting does not increase O() of the overall code when the square root of the max value is less than log of n. Sorting is worth doing if the result of the sort is used well.
Code fails if the largest value was 1 as prime test incorrectly identifies 1 as a prime.
Code fails if numbersSorted[number] < 0 due to sqrt().
Simply full-range int prime test:
bool isprime(int num) {
if (num % 2 == 0) return num == 2;
for (int divisor = 3; divisor <= num / divisor; divisor += 2) {
if (num % divisor == 0) return false;
}
return num > 1;
}
If you want to find the prime, don't go for sorting. You'll have to check for all the numbers present in the array then.
You can try this approach to do the same thing, but all within a lesser amount of time:
Step-1: Create a global function for detecting a prime number. Here's how you can approach this-
bool prime(int n)
{
int i, p=1;
for(i=2;i<=sqrt(n);i++) //note that I've iterated till the square root of n, to cut down on the computational time
{
if(n%i==0)
{
p=0;
break;
}
}
if(p==0)
return false;
else
return true;
}
Step-2: Now your main function starts. You take input from the user:
int main()
{
int n, i, MAX;
cout<<"Enter the number of elements: ";
cin>>n;
int arr[n];
cout<<"Enter the array elements: ";
for(i=0;i<n;i++)
cin>>arr[i];
Step-3: Note that I've declared a counter variable MAX. I initialize this variable as the first element of the array: MAX=arr[0];
Step-4: Now the loop for iterating the array. What I did was, I iterated through the array and at each element, I checked if the value is greater than or equal to the previous MAX. This will ensure, that the program does not check the values which are less than MAX, thus eliminating a part of the array and cutting down the time. I then nested another if statement, to check if the value is a prime or not. If both of these are satisfied, I set the value of MAX to the current value of the array:
for(i=0;i<n;i++)
{
if(arr[i]>=MAX) //this will check if the number is greater than the previous MAX number or not
{
if(prime(arr[i])) //if the previous condition satisfies, then only this block of code will run and check if it's a prime or not
MAX=arr[i];
}
}
What happens is this- The value of MAX changes to the max prime number of the array after every single loop.
Step-5: Then, after finally traversing the array, when the program finally comes out of the loop, MAX will have the largest prime number of the array stored in it. Print this value of MAX. Now for getting the positions where MAX happens, just iterate over the whole loop and check for the values that match MAX and print their positions:
for(i=0;i<n;i++)
{
if(arr[i]==MAX)
cout<<i+1<<" ";
}
I ran this code in Dev C++ 5.11 and the compilation time was 0.72s.
So I created a c++ code that counts "special" numbers in a given range, which is defined as a number where the total of its number of 1-bits in its binary representation is less than our equal to 3 (i.e. 1(1), 2(10), 3(11), 4(100), 5(101), 6(110), 7(111), 8(1000).
I successfully created the code, however there is one problem. At high number ranges, it outputs segmentation fault.
Any idea as to why this happens?
#include <iostream>
#include <cmath>
using namespace std;
long special(long x, long y){
long sArray[y];
long output = 0;
sArray[0]=0;
sArray[1]=1;
for(long i = 2; i<=y; i++){
long j = floor(i/2);
sArray[i] = sArray[j]+(i%2);
if (i>=x && sArray[i]<=3){
output++;
}
}
return output;
}
int main()
{
cout<<special(5,2717261);
return 0;
}
The segmentation fault occurs because you try to declare an array that's too large and extends outside the current memory segment.
Frankly, you don't need this array, and can just count the number of special numbers in the given range:
boolean isSpecial(long num) {
int bits = 0;
while (num > 0) {
if (num % 2 > 0) {
++bits;
if (bits >= 3) {
return true;
}
}
num /= 2;
}
return false;
}
long special(long x, long y) {
long output = 0;
for(long i = x; i <= y; ++i) {
if (isSpecial(i)) {
++output;
}
}
return output;
}
I wrote the following dp code for finding the prime factors of a number.
#include <bits/stdc++.h>
#define max 1000001
using namespace std;
vector <int> prime;
vector<bool> isprime(max,true);
vector<bool> visited(max,false);
vector<int> data(max,-1);
void dp(int n,int last)
{
if(n >= max || visited[n])
return;
visited[n] = true;
for(int i = last;i<prime.size();i++)
{
if(n*prime[i] >= max || data[n*prime[i]] != -1)
return;
data[n*prime[i]] = prime[i];
dp(n*prime[i],i);
}
}
int main()
{
isprime[1] = false;
data[1] = 1;
for(int i = 4;i<max;i += 2)
isprime[i] = false;
for(int i = 3; i*i< max;i += 2)
{
for(int j = i*i; j < max;j += i)
isprime[j] = false;
}
prime.push_back(2);
data[2] = 2;
for(int i =3;i<max;i += 2)
if(isprime[i])
{
prime.push_back(i);
data[i] = i;
}
for(int i = 0;i<prime.size();i++)
{
dp(prime[i],i);
}
cout<<"...1\n";
for(int i = 2;i<=8000;i++)
{
cout<<i<<" :- ";
int temp = i;
while(temp!= 1)
{
cout<<data[temp]<<" ";
temp = temp/data[temp];
}
cout<<endl;
}
return 0;
}
Here, last is the last index of prime number n.
But I am getting segmentation fault for this, when I change max to 10001, it runs perfectly. I'm not getting why is this happening since the data-structures used are 1-d vectors which can hold values up to 10^6 easily.
I checked your program out using GDB. The segfault is taking place at this line:
if(n*prime[i] >= max || data[n*prime[i]] != -1)
In your first ever call to DP in your for loop, where you call dp(2,0), the recursive calls eventually generate this call: dp(92692,2585).
92692 * 2585 = 239608820
This number is larger than a 32 bit integer can hold, so the r-value generated by the integer multiplication of those two numbers overflows and becomes negative. nprime[i] becomes negative, so your first condition of the above loop fails, and the second is checked. data[n * prime[i]] is accessed, and since n*prime[i] is negative, your program accesses invalid memory and segfaults. To fix this, simply change n to a long long in your parameter list and you should be fine.
void dp(long long n, int last)