The real difference between i++ and ++i

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One 、i++ and ++i Essential difference

  The core difference is ：i++ There are reasons for intermediate preservation .

Two 、 Test drill

#include <stdio.h>

int add1()
{
    int i=0;
#if 1
    printf("add1 = %d\n", (i++ + i++));
    printf("i =%d\n", i);
    i =0 ;
    printf("add2 = %d\n", (i++ + ++i));
    printf("i =%d\n", i);
#endif 
    i =0;
    printf("add3 = %d\n", (++i + ++i));
    printf("i=%d\n", i);
    
         
    return 0;
}

int main(void)
{
    int i = add1();

    printf("main i= %d\n", (i++));

    return 0;
}

 

[email protected]:/mnt/hgfs/project/visual_gateway/software/src_intra_20220618/test/v1/test/xx3/test$ ./a.out 
add1 = 1
i =2
add2 = 2
i =2
add3 = 4
i=2
main i= 0

analysis ：

Combined with the first part ： give an example i++ Return to temp, ++i return i, Just to distinguish

（1）printf("add1 = %d\n", (i++ + i++));

       

• First, execute the... On the left i++, here i=0, The result returned temp1 = 0, here i The value of is 1.
• Next, execute the... On the right i++, here i=1, The result returned temp2=1,    here i The value of is 2.
• Finally, execute the middle addition , by （temp1 + temp2） 

（2）printf("add2 = %d\n", (i++ + ++i));

•   First, execute the... On the left i++, here i=0, The result returned is temp=0, here i The value of is 1.
•   Next, execute the... On the right ++i, here i=1, The result returned is i =2, here i The value of is 2.
• Finally, execute the middle addition , by （temp + i）

（3）printf("add3 = %d\n", (++i + ++i));

• First, execute the... On the left ++i, here i=0, The result returned is i=1, here i The value of is 1.
• Next, execute the... On the right ++1, here i=1, The result returned is i=2, here i The value of is 2.
• Both left and right return i, At this time i All equal to 2
• Finally, execute the middle addition , by （i+i）= 4( The main thing that needs to be done here is ： First time back i It was equal to 1, Now it has become 2)

（4）printf("main i= %d\n", (i++));

The previous return value i=0;

here , The result is also 0.

3、 ... and 、 Other tests

1.demo

#include <stdio.h>

int add1()
{
    int i=0;
    printf("add1 = %d\n", (i++ + i++)); // Print equals 1, i=2
    printf("i =%d\n", i);
    printf("add2 = %d\n", (i++ + ++i)); // Print equals 6, i=4
    printf("i =%d\n", i);
    printf("add3 = %d\n", (++i + ++i)); // Print equals 12, i=6
    printf("i=%d\n", i);
	
    return 0;
}

int main(void)
{
    int i = add1();
    printf("main i= %d\n", (i++));
    return 0;
}