[英]Factorial showing wrong answer in main. Computed correctly in function
Please explain the reason for the following behavior. 请解释以下行为的原因。 Correct answer is after debug "3". 正确答案是在调试“ 3”之后。 I want that to be replicated in main after "4". 我希望将其复制到main中的“ 4”之后。
long int fact[501]={0};
long int factorial(int n)
{
if(n==0 || n==1)
return 1;
if(fact[n]>1)
return fact[n];
if((fact[n]=n*factorial(n-1))>=1000000007)
{ cout<<"2"<<endl<<fact[n]<<endl;
fact[n]=fact[n]%1000000007;
cout<<"3"<<endl<<fact[n]<<endl;
}return fact[n];
}
int main()
{
int n;
cin>>n;
cout<<"1"<<endl;
cout<<factorial(n)<<endl;
cout<<"4"<<endl;
printf("%ld\n",fact[n]);
return 0;
}
The output from the line 该行的输出
cout<<"3"<<endl<<fact[n]<<endl;
does not necessarily correspond to n
from main
. 不一定对应于main
n
。
Try this modified version to see the difference: 尝试使用此修改版本以查看不同之处:
#include <stdio.h>
#include <iostream>
using namespace std;
long int fact[501]={0};
long int factorial(int n)
{
if(n==0 || n==1)
return 1;
if(fact[n]>1)
return fact[n];
if((fact[n]=n*factorial(n-1))>=1000000007)
{ cout<<"2"<<endl<<fact[n]<<endl;
fact[n]=fact[n]%1000000007;
cout<<"3"<<endl<<n<<" "<<fact[n]<<endl;
}return fact[n];
}
int main()
{
int n;
cin>>n;
cout<<"1"<<endl;
cout<<factorial(n)<<endl;
cout<<"4"<<endl;
printf("%ld\n",fact[n]);
return 0;
}
With your compiler/platform, you have long int
with 4 bytes so your value is truncated 在您的编译器/平台上,您有4个字节的long int
,因此您的值被截断了
With n >= 13
, we have 当n >= 13
,我们有
fact[13] = 6227020800
which doesn't fit in a uint32_t
fact[13] = 6227020800
,它不适合uint32_t
when truncated its new value goes to 1932053504 (and then 932053497
after modulo) 截断后,其新值进入1932053504(然后932053497
模后为932053497
)
To fix your implementation you have to use uint64_t
. 要修复您的实现,您必须使用uint64_t
。 ( https://ideone.com/gQrxyW ) ( https://ideone.com/gQrxyW )
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