输出的差异取决于 for 循环中省略花括号 (c++)

Question

我一直在按照本网站参考指南的建议阅读 c++ 入门书，我注意到作者省略了花括号，因为 loop.I 检查了其他网站，大括号应该放在在通常。放置大括号并省略 it.The 代码在下方时会有不同的输出

int sum = 0;
for (int val = 1; val <= 10; ++val)
    sum += val;  
    std::cout << "Sum of 1 to 10 inclusive is " << sum << std::endl;

// This pair of code prints the std::cout once
for (int val = 50; val <=100;++val)
    sum += val;
    std::cout << "Sum of 50 to 100 inclusive is " << sum << std::endl;

// -------------------------------------------------------------------------
for (int val = 1; val <= 10; ++val) {
    sum += val;  
    std::cout << "Sum of 1 to 10 inclusive is " << sum << std::endl;
}

// This pair of code prints the std::cout multiple times
for (int val = 50; val <=100;++val) {
    sum += val;
    std::cout << "Sum of 50 to 100 inclusive is " << sum << std::endl;
}

如果有人能解释输出的差异，我将不胜感激。提前致谢！

Answer 1

对于大括号，大括号中的所有内容都由 for 循环执行

for (...;...;...)
{
     // I get executed in the for loop!
     // I get executed in the for loop too!
}
// I don't get executed in the for loop!

然而，如果没有大括号，它只会直接执行后面的语句：

for (...;...;...)
     // I get executed in the for loop!
// I don't get executed in the for loop!
// I don't get executed in the for loop either!

Answer 2

for语句具体定义如下

for ( for-init-statement conditionopt; expressionopt) statement
                                                      ^^^^^^^^^

其中语句可以是任何语句，包括复合语句

compound-statement:
    { statement-seqopt}

因此在这个 for 语句的例子中

for (int val = 1; val <= 10; ++val)
sum += val;

声明是

sum += val;

而在这个 for 语句的例子中

for (int val = 1; val <= 10; ++val) {
sum += val;  
std::cout << "Sum of 1 to 10 inclusive is " << sum << std::endl;
}

声明是

{
sum += val;  
std::cout << "Sum of 1 to 10 inclusive is " << sum << std::endl;
}

输出的差异取决于 for 循环中省略花括号 (c++)

Diffrientiation in outputs depending on the ommition of curly braces in for loops (c++)

c++

curly-braces

curly-brackets