要理解FOR循环的作用

上图显示FOR可以转换为WHILE，因为它们最终具有完全相同的汇编代码(至少在gcc中)。所以我们可以把FOR分解成几部分，来理解它的功能。

for (i = 0; i < 5; ++i) {
  DoSomethingA();
  DoSomethingB();
}

等于WHILE版本

i = 0; //first argument (a statement) of for
while (i < 5 /*second argument (a condition) of for*/) {
  DoSomethingA();
  DoSomethingB();
  ++i; //third argument (another statement) of for
}

It means that you can use FOR as a simple version of WHILE: The first argument of FOR (int i) is executed, outside, before the loop. The third argument of FOR (i++ or ++i) is executed, inside, in the last line of the loop. TL:DR: no matter whether i++ or ++i, we know that when they are standalone, they make no difference but +1 on themselves. In school, they usually teach the i++ way, but there are also lots of people prefer the ++i way due to several reasons. NOTE: In the past, i++ has very little impact on the performance, as it does not only plus one by itself, but also keeps the original value in the register. But for now, it makes no difference as the compiler makes the plus one part the same.

对于用户定义类型的i，这些操作符可以(但不应该)在循环索引上下文中具有有意义的不同语义，这可能(但不应该)影响所描述的循环的行为。

此外，在c++中，使用预增量形式(++i)通常是最安全的，因为它更容易优化。(斯科特·朗廷(Scott Langham)先我一步找到了这个花边新闻。诅咒你，斯科特)

下面是一个Java-Sample和字节码，后增量和前增量显示字节码没有区别:

public class PreOrPostIncrement {

    static int somethingToIncrement = 0;

    public static void main(String[] args) {
        final int rounds = 1000;
        postIncrement(rounds);
        preIncrement(rounds);
    }

    private static void postIncrement(final int rounds) {
        for (int i = 0; i < rounds; i++) {
            somethingToIncrement++;
        }
    }

    private static void preIncrement(final int rounds) {
        for (int i = 0; i < rounds; ++i) {
            ++somethingToIncrement;
        }
    }
}

现在对于字节码(javap -private -c PreOrPostIncrement):

public class PreOrPostIncrement extends java.lang.Object{
static int somethingToIncrement;

static {};
Code:
0:  iconst_0
1:  putstatic   #10; //Field somethingToIncrement:I
4:  return

public PreOrPostIncrement();
Code:
0:  aload_0
1:  invokespecial   #15; //Method java/lang/Object."<init>":()V
4:  return

public static void main(java.lang.String[]);
Code:
0:  sipush  1000
3:  istore_1
4:  sipush  1000
7:  invokestatic    #21; //Method postIncrement:(I)V
10: sipush  1000
13: invokestatic    #25; //Method preIncrement:(I)V
16: return

private static void postIncrement(int);
Code:
0:  iconst_0
1:  istore_1
2:  goto    16
5:  getstatic   #10; //Field somethingToIncrement:I
8:  iconst_1
9:  iadd
10: putstatic   #10; //Field somethingToIncrement:I
13: iinc    1, 1
16: iload_1
17: iload_0
18: if_icmplt   5
21: return

private static void preIncrement(int);
Code:
0:  iconst_0
1:  istore_1
2:  goto    16
5:  getstatic   #10; //Field somethingToIncrement:I
8:  iconst_1
9:  iadd
10: putstatic   #10; //Field somethingToIncrement:I
13: iinc    1, 1
16: iload_1
17: iload_0
18: if_icmplt   5
21: return

}

在这两种情况下，'i'将加1。

但是当你在表达式中使用它时，就有区别了，例如:

int i = 1;
int a = ++i;
// i is incremented by one and then assigned to a.
// Both i and a are now 2.
int b = i++;
// i is assigned to b and then incremented by one.
// b is now 2, and i is now 3

在c#中，在for循环中使用没有区别。

for (int i = 0; i < 10; i++) { Console.WriteLine(i); }

输出和

for (int i = 0; i < 10; ++i) { Console.WriteLine(i); }

正如其他人指出的那样，在一般情况下i++和++i有一个微妙但显著的区别:

int i = 0;
Console.WriteLine(i++);   // Prints 0
int j = 0;
Console.WriteLine(++j);   // Prints 1

i++读取I的值，然后增加它。

++i增加i的值，然后读取它。

我+ +;+ +我;两者都是相似的，因为它们不在表达式中使用。

class A {

     public static void main (String []args) {

     int j = 0 ;
     int k = 0 ;
     ++j;
     k++;
    System.out.println(k+" "+j);

}}

prints out :  1 1