打印第二列时的二维字符向量打印空间 [英] 2d char vector printing spaces when printing second column

问题描述

这里我有一个二维字符向量 -

Here I have a 2d vector of char -

std::vector<std::vector<char>> solution = {
 {"O","1"}, 
 {"T","0"}, 
 {"W","9"}, 
 {"E","5"}, 
 {"N","4"}
};

从第一列打印任何内容 - 打印正常.

Printing anything from first column - prints fine.

cout << "In main - " << solution [ 1 ] [ 0 ]; // Prints T

但是当我尝试访问第二列的元素时.

But when I try to access element of second column.

cout << "In main - " << solution [ 1 ] [ 1 ]; // Prints blank space - I can't seem to understand why's the case.

经过安静的搜索后，我尝试在每个元素周围加上单引号.

After quiet amount of searching I tried by putting single quotes around every element.

std::vector<std::vector<char>> solution = {
  {'O','1'}, 
  {'T','0'}, 
  {'W','9'}, 
  {'E','5'}, 
  {'N','4'}
};

在这种情况下它工作正常.

It works fine in this case.

cout << "In main - " << solution [ 1 ] [ 1 ]; // Gives me 0 in this case.

现在为什么在访问 "" 中的第二列时出现空格?双引号 场景.

Now why is it that I'm getting blank spaces when accessing second column in "" double quotes scene.

推荐答案

在您的第一个示例中，对于 solution 的每个元素，您正在尝试构造一个 vector 来自 2 个字符串文字.这将使用带有 2 个迭代器的 vector 构造函数(因为字符串文字可以转换为指针).由于指针未表示有效范围，因此会调用未定义行为 (UB).

In your first example, for each element of solution, you are trying to construct a vector<char> from 2 string literals. This will use the constructor of vector that takes 2 iterators (since string literals can be converted to pointers). Since the pointers are not denoting a valid range, this invokes undefined behavior (UB).

在第二个示例中，对于 solution 的每个元素，您尝试从 2 个 char 构建一个 vector，这非常好，并为您提供预期的结果.

In the second example, for each element of solution, you are trying to construct a vector<char> from 2 chars, which is perfectly fine, and gives you the result you expected.

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