[英]C++ program to compute lcm of numbers between 1 to 20 (project euler )
[英]LCM of first 20 natural numbers
我有一些代碼可以讓我獲得前20個自然數的LCM。
這里是。
const bool IsPrime(const unsigned long long number) {
cout<<"checking if "<<number<<" is prime.\n";
//If a number cannot be divided by any number up to
//half of it, then that number is prime.
for (unsigned long long i = 2; i<=number/2; ++i) {
if (number%i==0) {
cout<<number<<" is not prime.\n";
return false;
}
}
//the number seems to be prime.
cout<<number<<" is PRIME.\n";
return true;
}
vector<int> PrimeFactorize(int number) {
cout<<"Prime factorizing "<<number<<"\n";
vector<int> prime_factors;
for (int num = 2; !IsPrime(number); ++num) {
if (number%num==0&&IsPrime(num)) {
cout<<number<<" is divisible by "<<num<<" and "<<num<<" is PRIME.\n";
number /= num;
prime_factors.push_back(num);
num = 1;
}
}
prime_factors.push_back(number);
return prime_factors;
}
void Problem5( ) {
vector<int> prime_factors[19];
int max_prime_powers[19] = {0};
//prime factorize all the numbers.
for (int i = 2; i<=20; ++i) {
prime_factors[i-2] = PrimeFactorize(i);
int temp_max_powers[19] = {0};
for (auto& prime_factor:prime_factors[i-2]) {
++temp_max_powers[prime_factor];
}
//compare powers obtained and update the
//max_prime_factors
for (int u = 0; u<19; ++u) {
if (max_prime_powers[u]<temp_max_powers[u]) {
max_prime_powers[u] = temp_max_powers[u];
}
}
}
//now multiply all the things together to get the lcm.
int LCM=1;
for (int y = 2; y<19; ++y) {
LCM *= (y^(max_prime_powers[y]));
}
cout<<"\n\n\n the answer is "<<LCM<<"\n";
}
我嘗試逐步完成它,它會做應該做的事情,直到到達最后一個循環,在該循環中,我將所有質數乘以冪以得到lcm。 計算結果與預期不符。 即使代碼顯示正確的內容,它也會錯誤地進行計算。
函數返回后,出現運行時錯誤提示
Run-Time Check Failure #2 - Stack around the variable 'temp_max_powers' was corrupted.
有什么事? temp_max_powers如何破壞堆棧?
我正在使用Visual Studio Professional13。這是編譯器錯誤嗎?
更新根據評論,我在那里改正了代碼,而不是
LCM *= (y^(max_prime_powers[y]));
我現在有
LCM *= _Pow_int(y,(max_prime_powers[y]));
這沒有給我正確的答案,並且錯誤仍然彈出。
問題1
LCM *= (y^(max_prime_powers[y]));
在C ++ /中a^b
不用於b
問題2
為LCM選擇20個數字的int可能不是一個好選擇,因為它可能會溢出。
問題3
for (auto& prime_factor:prime_factors[i-2]) {
++temp_max_powers[prime_factor];
}
在這里,您應該通過打印prime_factor
的值進行調試,該值可能為19
或更大,這會導致UB。 (例如,堆棧損壞)
代替int temp_max_powers[19]
,您應該使用std::map<int, int> temp_max_powers
計算LCM的更好算法
如果可以確保乘法過程中沒有整數溢出,則可以使用以下觀察結果來計算LCM。
LCM(a, b) = a * b / GCD(a, b)
//您可以使用歐幾里得方法快速計算GCD LCM(a1, a2, a3, ... an) = LCM(a1, LCM(a2, a3, ... an))
聲明:本站的技術帖子網頁,遵循CC BY-SA 4.0協議,如果您需要轉載,請注明本站網址或者原文地址。任何問題請咨詢:yoyou2525@163.com.