阶乘函数只返回C++中输入的答案

Factorial function only returning inputted answer in C++?

本文关键字：输入 答案 C++ 函数 返回 阶乘      更新时间：2023-10-16

我必须创建一个程序来计算任何数字的阶乘，问题是如果我输入20以上的任何数字，它只返回该数字。在我的else-if语句中，可能是什么导致了这种情况，有更好的方法来解决吗？(此函数在main中被调用，并且在num<=20时工作(

void factorial() {
//User input for number
long long num;
std::cout << "Input any positive integer to find its factorial: ";
std::cin >> num;
unsigned long long numFact = 1;
if (num <= 20) {
while (num > 0) {
numFact = numFact * num;
num = num - 1;
}
std::cout << numFact;
}
else if (num > 20) {
std::vector<int> multFactorial;
//stores num as seperate elements in vector multFactorial
while (num > 0) {
int remain = num % 10;
num = num / 10;
multFactorial.insert(multFactorial.begin(), remain);
}
std::vector<int> answer;
std::vector<int> answerFinal;
//Manually multiplies elements in multFactorial
//Then adds new vectors created by multiplying to get final answer
//Repeats until factorial is solved
//Ex: 21 * 20; 0 * 1 and 0 * 2 stored as {0 , 0}
//2*1 and 2*2 stored as {4, 2, 0}
//Vectors will be addes to get {4, 2, 0} and then that will be multiplied 
by 19 until num = 1
while (num > 1) {
for (int i = multFactorial.size() - 1; i >= 0; i--) {
int remain1 = ((num - 1) % 10) * multFactorial[i];
answer.insert(answer.begin(), remain1);
int remain2 = (((num - 1) / 10) * multFactorial[i]);
answerFinal.insert(answerFinal.begin(), remain2);
}
answerFinal.insert(answerFinal.begin(), 0);
//Adds vectors to get final value seperate as digits
for (int i = multFactorial.size() - 1; i >= 0; i--) {
multFactorial[i] = answer[i] + answerFinal[i];
}
num = num - 1;
}
//Prints what should be the factorial of the number input
for (size_t i = 0; i < multFactorial.size(); i++) {
std::cout << multFactorial[i];
}
}
}

大数的阶乘会产生大数。这可以在C、C++等语言中通过将结果放入任意长度的字符串来实现。

这里有一个类似于你的算法。

https://www.geeksforgeeks.org/factorial-large-number/

最好的建议是对照这个检查你的代码。如果有调试器，请使用调试器，并逐行执行代码。如果未打印出中间结果，并与预期结果进行比较。

编辑：根据评论，上面引用的代码与下面的代码相似，以防将来链接断开。

// C++ program to compute factorial of big numbers
#include<iostream>
using namespace std;
// Maximum number of digits in output
#define MAX 100   // change to whatever value you need
int multiply(int x, int res[], int res_size);
// Calculate factorial of large number
void factorial(int n)
{
int res[MAX];
// Initialize result
res[0] = 1;
int res_size = 1;
// Apply factorial formula
for (int x=2; x<=n; x++)
res_size = multiply(x, res, res_size);
// print out the result 
cout << "Factorial is n";
for (int i=res_size-1; i>=0; i--)
cout << res[i];
}
// Multiplies x with the number represented by res[].
// res_size is size of res[] or number of digits in the 
// number represented by res[]. 
int multiply(int x, int res[], int res_size)
{
int carry = 0;  // Initialize carry
// One by one multiply n with individual digits of res[]
for (int i=0; i<res_size; i++)
{
int prod = res[i] * x + carry;
// Store last digit of 'prod' in res[]  
res[i] = prod % 10;  
// Put rest in carry
carry  = prod/10;    
}
// Put carry in res and increase result size
while (carry)
{
res[res_size] = carry%10;
carry = carry/10;
res_size++;
}
return res_size;
}
// Main program
int main()
{
//put code here to read a number
factorial(50); // take 50 for example
return 0;
}
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