What is the difference between i++ & ++i in a for loop? [duplicate]_问答_开发者

This question already has answers here: Difference between pre-increment and post-increment in a loop? (22 answers) Is there a differen开发者_Go百科ce between x++ and ++x in java? (18 answers) Closed 3 years ago. Locked. This question and its answers are locked because the question is off-topic but has historical significance. It is not currently accepting new answers or interactions.

I've just started learning Java and now I'm into for loop statements. I don't understand how ++i and i++ works in a for-loop.

How do they work in mathematics operations like addition and subtraction?

They both increment the number. ++i is equivalent to i = i + 1.

i++ and ++i are very similar but not exactly the same. Both increment the number, but ++i increments the number before the current expression is evaluted, whereas i++ increments the number after the expression is evaluated.

int i = 3;
int a = i++; // a = 3, i = 4
int b = ++a; // b = 4, a = 4

Here's a sample class:

public class Increment
{
    public static void main(String [] args)
    {
        for (int i = 0; i < args.length; ++i)
        {
            System.out.println(args[i]);
        }
    }
}

If I disassemble this class using javap.exe I get this:

Compiled from "Increment.java"
public class Increment extends java.lang.Object{
public Increment();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   return

public static void main(java.lang.String[]);
  Code:
   0:   iconst_0
   1:   istore_1
   2:   iload_1
   3:   aload_0
   4:   arraylength
   5:   if_icmpge       23
   8:   getstatic       #2; //Field java/lang/System.out:Ljava/io/PrintStream;
   11:  aload_0
   12:  iload_1
   13:  aaload
   14:  invokevirtual   #3; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   17:  iinc    1, 1
   20:  goto    2
   23:  return

}

If I change the loop so it uses i++ and disassemble again I get this:

Compiled from "Increment.java"
public class Increment extends java.lang.Object{
public Increment();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   return

public static void main(java.lang.String[]);
  Code:
   0:   iconst_0
   1:   istore_1
   2:   iload_1
   3:   aload_0
   4:   arraylength
   5:   if_icmpge       23
   8:   getstatic       #2; //Field java/lang/System.out:Ljava/io/PrintStream;
   11:  aload_0
   12:  iload_1
   13:  aaload
   14:  invokevirtual   #3; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   17:  iinc    1, 1
   20:  goto    2
   23:  return

}

When I compare the two, TextPad tells me that the two are identical.

What this says is that from the point of view of the generated byte code there's no difference in a loop. In other contexts there is a difference between ++i and i++, but not for loops.

Both of them increase the variable i by one. It's like saying i = i + 1. The difference is subtle. If you're using it in a loop like this, there's no difference:

for (int i = 0; i < 100; i++) {
}

for (int i = 0; i < 100; ++i) {
}

If you want to know the difference, look at this example:

int a = 0;
int b = a++; // b = 0; a = 1

a = 0;
b = ++a: // b = 1; a = 1

The idea is that ++a increments a and returns that value, while a++ returns a's value and then increments a.

The way for loop is processed is as follows

1 First, initialization is performed (i=0)

2 the check is performed (i < n)

3 the code in the loop is executed.

4 the value is incremented

5 Repeat steps 2 - 4

This is the reason why, there is no difference between i++ and ++i in the for loop which has been used.

The difference is that the post-increment operator i++ returns i as it was before incrementing, and the pre-increment operator ++i returns i as it is after incrementing. If you're asking about a typical for loop:

for (i = 0; i < 10; i++)

for (i = 0; i < 10; ++i)

They're exactly the same, since you're not using i++ or ++i as a part of a larger expression.

Both i++ and ++i are short-hand for i = i + 1.

In addition to changing the value of i, they also return the value of i, either before adding one (i++) or after adding one (++i).

In a loop the third component is a piece of code that is executed after each iteration.

for (int i=0; i<10; i++)

The value of that part is not used, so the above is just the same as

for(int i=0; i<10; i = i+1)

for(int i=0; i<10; ++i)

Where it makes a difference (between i++ and ++i )is in these cases

while(i++ < 10)

for (int i=0; i++ < 10; )

JLS§14.14.1, The basic for Statement, makes it clear that the ForUpdate expression(s) are evaluated and the value(s) are discarded. The effect is to make the two forms identical in the context of a for statement.