英文:
Why 0 is in the output?
问题
在输出中为什么会出现零？
输出应该没有零。
在第二个 push_back 函数中，如果我使用 b[j]，那么零就不会出现。
英文:
#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;
int main() {
vector<vector<int> >a(2);
for(int i=0;i<2;i++)
{
int m;
cout<<"Enter number of elements on the row:"<<endl;
cin>>m;
vector<int> b(m);
cout<<"Element:"<<endl;
for(int j=0;j<m;j++)
{
int k;
cin>>k;
b.push_back(k)//if i use b[j]=k; here the zeros don't appear.
//cout<<b[j];
}
a.push_back(b);
}
for(unsigned int i=0;i<a.size();i++)
{
for(unsigned int j=0;j<a[i].size();j++)
cout<<a[i][j]<<" ";
cout<<endl;
}
return 0;
}
Why does zero appear in the output?
The output should be without zeros.
In the second push_back function, if I use b[j] then zeros don't appear.
答案1
得分: 5
你正在用两个空的vector初始化a，然后将两个填充了数据的vector推入其中。
而对于b，你正在用m个默认的零值初始化，然后将m个用户输入的整数推入其中。
你不应该在构造函数中为任何一个vector初始化数值。这会将默认值推入vector，从而影响你的结果。在这种情况下，只需删除构造函数中的值，让push_back() 来处理添加操作。
如果你想要在不添加值的情况下预分配vector的内存，可以使用它的reserve() 方法，例如：
vector<vector<int>> a;
a.reserve(2);
...
vector<int> b;
b.reserve(m);
否则，如果你确实使用构造函数来填充vector 的默认值，那么你需要修改循环以使用vector::operator[] 而不是vector::push_back()，例如：
vector<vector<int>> a(2);
for(int i=0; i<2; i++)
{
...
vector<int> b(m);
for(int j=0; j<m; j++)
{
...
b[j] = k;
}
a[i] = b;
}
英文:
You are initializing a with 2 blank vectors in it and then pushing 2 populated vectors into it.
And you are initializing b with m number of default zeros and then pushing m user-input integers into it.
You should not be initializing either vector with a value in its constructor. That pushes default values into the vector, which is throwing off your results. In this case, simply remove the constructor values and let push_back() handle the adds for you.
If you want to preallocate a vector's memory without adding values to it, use its reserve() method instead, eg:
vector<vector<int> >a;
a.reserve(2);
...
vector<int> b;
b.reserve(m);
Otherwise, if you do use the constructor to pre-fill the vectors with defaults, then
you need to change your loops to use vector::operator[] instead of vector::push_back(), eg:
vector<vector<int> >a(2);
for(int i=0;i<2;i++)
{
...
vector<int> b(m);
for(int j=0;j<m;j++)
{
...
b[j] = k;
}
a[i] = b;
}
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