为什么C ++中的vector不会自动调整大小 [英] Why vector in C++ doesn't resize automatically

问题描述

我有一个很长的阶乘程序，它需要找到最多100个阶乘。它可以在多达33个阶乘上运行，但不能从34个阶乘上运行。有人可以帮助您识别问题。

I've a very long factorial program which needs to find factorial up to 100. It works well up to 33 factorial but not from 34. Can someone help in identifying the problem.

#include <iostream>
#include <vector>
#include <utility>


using namespace std;

void bigFactorials(int n)
{
    vector<int> v;//If I specify the size as v(1000) it works fine but I don't 

    //want to specify the size beforehand.
    v.push_back(1);

    // Complete this function
    for(int i=2;i<=n;i++) {
        int carry = 0, mul=0;
        for(auto j=v.rbegin();j!=v.rend();j++) {
            mul=i**j + carry;
            carry=mul/10;
            *j=mul%10;
        }

        if(carry)
            v.insert(v.begin(),carry);    
    }

    for(int i:v)
        cout<<i;
}

int main()
{
    int n;

    cin >> n;
    if( n>0 && n<101 )
        bigFactorials(n);

    return 0;
}

推荐答案

问题是进位> 10 ，然后插入一个整数值而不将其拆分为char，则应按以下方式实现

Problem is when carry > 10, then you insert one integer value without splitting it into chars, it should be implemented as follows

if(carry)
{
    if (carry >= 10)
    {
        while (carry > 0)
        {
            v.insert(v.begin(),carry % 10); // put each char from carry into v
            carry = carry / 10;
        }
    }
    else
        v.insert (v.begin(),carry);
}

使用此功能甚至可以保留 v 作为 vector< char> 并保持50！我们有
30414093201713378043612608166064768844377641568960512000000000000。

with this you can even keep v as vector<char> and for 50! we have 30414093201713378043612608166064768844377641568960512000000000000.

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