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<>1、for循环条件中i++与++i的区别 //for循环体语法 for(语句1;语句2;语句3) { 循环体执行代码块 }
语句1是在循环体开始前执行
语句2是定义运行循环体的条件
语句3在循环体 已经被执行之后来执行
根据for循环的语法定义
,在语句3执行时++i和i++的结果是一样的,都需要等代码块执行完毕之后才能运行语3。++i:执行完代码块后执行i=i+1,此时i的值增加了1;i++:先引用i=i,再执行i=i+1,也相当于执行代码块后实现了i自增1。
性能区别:在大量数据的时候++i的性能要比i++性能要好。i++由于是需要在使用当前值之后再+1,所以会需要一个临时的变量来进行转存。而++i则是直接进行+1的操作,省去了对内存的操作环节,所以能够提高性能。
<>2、数组中i++与++i的区别 // 有整型数组如下 array[i++] array[++i]
//array[i++]执行次序:先取array[i],后i=i+1 //array++i执行次序:先i=i+1,再取array[i]
区别:array[i++]中i++是后自增,必须先使用数组当前元素的值,再i自增1。因此返回的结果是i对应的数组中的值
array[++i]中++i是前自增,先i自增,再取i自增后的数组中的值。因此返回结果是i自增后对应的数组中的值
<>3、指针中i++与++i的区别
(1)第一种
*(i++)与*i++ //*i++ //等价于*(i++) //分2步执行:*i; i=i+1; //*(i++) //分2步执行:*(i); i=i+1;
因此这两种情况等价:先输出指针i在自增前对应的值,然后指针i自增,指针指向下一个地址。返回的结果是i指针自增前对应的值
(2)第二种
*(++i)与*++i //*(++i) //分两步执行:i=i+1;*(i) //*++i //等价与*(++i) //分两步执行:i=i+1;*i
因此这两种情况等价:都先将i自增1,然后与*结合,再取对应地址的值。返回的结果是原i指针指向地址的下一个地址的值
(3)第三种
++*i 与 ++(*i) //++*i //等价与++(*i) //分两步执行:*i;然后对其值自增1 //++(*i)
//分两步执行:*i;然后对其值自增1
因此这两种情况等价:先取i指针对应的值,然后对值自增1。返回的结果应该是自增后的值
(4)第四种
(*i)++ //分两步执行:先取i对应地址的值,再将其值整体自增1
因此这种情况:因为括号结合率最高,因此先取i对应地址的值,再将其值自增1。返回的结果应该是i对应地址的值
注:后缀++与*优先级相同结合率从右至左