警告C4018：'<' ：签名/未签名不匹配ON VECTORS [英] warning C4018: '<' : signed/unsigned mismatch ON VECTORS

问题描述

当我在for循环中运行我的程序时，我收到一条警告，上面写着：



警告C4018：'<'：签名/未签名不匹配

Hi,

when i run my program at the for loop i receive a warning which says:

warning C4018: '<' : signed/unsigned mismatch

void insertion_sort(vector<int> &items)
{
	cout<<" insertion_sort... ";
	int key = 0;
	int i=0;
	for( int j=2; j<items.size(); j++){//HERE
		key = items[j];
		i = j-1;
		while (i>0 && items[i]>key){
			items[i+1] = items[i];
			i=i-1;
		}
		items[i+1] = key;
	}
}

似乎我不能使用vector.size（）。

将signed或unsigned int与vector.size（）进行比较，vector.max_size（）将不起作用。我甚至不能：

it seems like i cannot use vector.size() .
comparing a signed or unsigned int with a vector.size(), vector.max_size() won't work. i can't even:

cout<<items.size();
//or
cout<<items.max_size();
//or
signed int a = items.size();
cout<<a;
//or
unsigned int a = items.size();
cout<<a;
//or
int a = items.size();
cout<<a;

为什么会这样？

如何解决？

谢谢，

Why does it happen?
How can i fix it?
Thanks,

推荐答案

那是因为您将j声明为 int ，但 items.size（）返回无符号值。将i和j声明为 unsigned int 应该让它消失。

That's because you declared j as an int, but items.size() returns an unsigned value. Declaring i and j as unsigned int should make it go away.

items.size（）返回 std :: size_t ，通常是64位无符号整数，而 j 是一个签名int。编译器会警告您，因为您将 j 与 items.size（）进行比较。使 j 和 i 无符号长整数（无符号64位整数）。

items.size() returns a std::size_t, usually a 64-bit unsigned int, and j is a signed int. The compiler warns you for that, because you compare j with items.size(). Make j and i an unsigned long (an unsigned 64-bit int).

unsigned long i=0;
for(unsigned long j=2; j<items.size(); j++){

#pragma warning ( disable : 4018 )

或

or

void insertion_sort(vector<int> &items)
{
	cout<<" insertion_sort... ";
	int key = 0;
	int i=0;
	for( int j=2; j<(int)items.size(); j++){//HERE
		key = items[j];
		i = j-1;
		while (i>0 && items[i]>key){
			items[i+1] = items[i];
			i=i-1;
		}
		items[i+1] = key;
	}
}</int>

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