70,038
社区成员
发帖
与我相关
我的任务
分享为什么原文说total+=*start++和total+=*(start++)是等价的？
《C primer plus》中的一道题目，文中说total+=*start++和total+=*(start++)是一样的，这里我想不通，加了括号后不是应该先算括号里的内容么？怎么会是一样的呢。请高手解答！
#include <stdio.h>
#define SIZE 10
int sump(int *start,int *end);
int main()
{
int marbles[SIZE]={20,10,5,39,4,16,19,26,31,20};
long answer;
answer=sump(marbles,marbles+SIZE);
printf("%ld.\n",answer);
return 0;
}
int sump(int *start,int *end)
{
int total=0;
while(start<end)
total+=*(start++);
return total;
}
...全文
请发表友善的回复…
发表回复
愿你变作彩霞 2016-05-16
- 打赏
- 举报
想知道这张图的出处，顺便primary-expression应该是前++，postfix-expression应该是后++吧
飞行1-5 2015-03-23
- 打赏
- 举报
start++这句还没结束时，start指针地址不变，所以取*不变
fly_dragon_fly 2015-03-23
- 打赏
- 举报
把start++这个表达式看成一个函数，看看它的返回值 ， 它是这样的 tmp=start, ++start, return tmp;
而(start++) 是这样的 ( tmp=start, ++start,return tmp) 比较一下，显然是一样的
一根烂笔头 2015-03-23
- 打赏
- 举报
鱼和熊掌不可兼得，看个人选择！
爱速度的人，还是喜欢压榨时间的，无论编译还是运行！
cfvmario 2015-03-23
- 打赏
- 举报
就算编译角度有差别，现代计算机需要在乎这点？
我强烈建议一律加括号，可读性才是第一位的
左眼看到鬼 2015-03-21
- 打赏
- 举报
写错了。
二楼说的对。
左眼看到鬼 2015-03-21
- 打赏
- 举报
根据运算符优先级来看：
total+=*start++ 的执行顺序是这样的：total += (*start)++
所以等同于下面两个表达式：
total+=(*start)++
total+=*(start++)
在这里把++的作用理解了，这个问题就看明白了。
例如:i++
i++的实现如下：
{
int temp;
temp = i;
i = i+1;
return temp;
}
一根烂笔头 2015-03-20
- 打赏
- 举报
解释一下：
total+=*start++和total+=*(start++)从结果上讲是一样的，从编译角度讲不太一样
*属于unary-operator
++放在表达式之后，构成：postfix-expression ++
total+=*start++
从语法分析上要先分析postfix-expression ++，然后转到unary-expression，过度到cast-expression，整合 * 的unary-operator组合成unary-operator cast-expression，最后转一圈回到赋值表达式
因此先执行++，再执行*
total+=*(start++)
要比上面不加括号的情况下，多一圈语法分析；即start++自身走一圈回到exp，然后再转一圈把*整合进去。
因此从编译角度讲，如果想让代码编译更快点，在知道优先级的情况下，优先选择total+=*start++，因为每加一个括号，就要从上面12点钟方向重新开始一遍语法分析。
不过，编译出来的指令都是一样的，因此运算结果也都是一样的。
一根烂笔头 2015-03-20
- 打赏
- 举报
C99
dooX8086 2015-03-20
- 打赏
- 举报
//只要了解后缀 ++ 就很清楚了
// 后缀 ++ 其实就是豆号表达式组
// 如: 把 start++ 展开就是
int *tmp; //编译器创建的临时变量
(tmp = start, start = start + 1, tmp);
//那么: total+=*(start++); 就是
int *tmp;
total += *( (tmp = start, start = start + 1, tmp) ); // 所以外面这对 () 添加以否都是一样的
// 一目了然了.....
//
赵4老师 2015-03-20
- 打赏
- 举报
//C++ Operators
// Operators specify an evaluation to be performed on one of the following:
// One operand (unary operator)
// Two operands (binary operator)
// Three operands (ternary operator)
// The C++ language includes all C operators and adds several new operators.
// Table 1.1 lists the operators available in Microsoft C++.
// Operators follow a strict precedence which defines the evaluation order of
//expressions containing these operators. Operators associate with either the
//expression on their left or the expression on their right; this is called
//“associativity.” Operators in the same group have equal precedence and are
//evaluated left to right in an expression unless explicitly forced by a pair of
//parentheses, ( ).
// Table 1.1 shows the precedence and associativity of C++ operators
// (from highest to lowest precedence).
//
//Table 1.1 C++ Operator Precedence and Associativity
// The highest precedence level is at the top of the table.
//+------------------+-----------------------------------------+---------------+
//| Operator | Name or Meaning | Associativity |
//+------------------+-----------------------------------------+---------------+
//| :: | Scope resolution | None |
//| :: | Global | None |
//| [ ] | Array subscript | Left to right |
//| ( ) | Function call | Left to right |
//| ( ) | Conversion | None |
//| . | Member selection (object) | Left to right |
//| -> | Member selection (pointer) | Left to right |
//| ++ | Postfix increment | None |
//| -- | Postfix decrement | None |
//| new | Allocate object | None |
//| delete | Deallocate object | None |
//| delete[ ] | Deallocate object | None |
//| ++ | Prefix increment | None |
//| -- | Prefix decrement | None |
//| * | Dereference | None |
//| & | Address-of | None |
//| + | Unary plus | None |
//| - | Arithmetic negation (unary) | None |
//| ! | Logical NOT | None |
//| ~ | Bitwise complement | None |
//| sizeof | Size of object | None |
//| sizeof ( ) | Size of type | None |
//| typeid( ) | type name | None |
//| (type) | Type cast (conversion) | Right to left |
//| const_cast | Type cast (conversion) | None |
//| dynamic_cast | Type cast (conversion) | None |
//| reinterpret_cast | Type cast (conversion) | None |
//| static_cast | Type cast (conversion) | None |
//| .* | Apply pointer to class member (objects) | Left to right |
//| ->* | Dereference pointer to class member | Left to right |
//| * | Multiplication | Left to right |
//| / | Division | Left to right |
//| % | Remainder (modulus) | Left to right |
//| + | Addition | Left to right |
//| - | Subtraction | Left to right |
//| << | Left shift | Left to right |
//| >> | Right shift | Left to right |
//| < | Less than | Left to right |
//| > | Greater than | Left to right |
//| <= | Less than or equal to | Left to right |
//| >= | Greater than or equal to | Left to right |
//| == | Equality | Left to right |
//| != | Inequality | Left to right |
//| & | Bitwise AND | Left to right |
//| ^ | Bitwise exclusive OR | Left to right |
//| | | Bitwise OR | Left to right |
//| && | Logical AND | Left to right |
//| || | Logical OR | Left to right |
//| e1?e2:e3 | Conditional | Right to left |
//| = | Assignment | Right to left |
//| *= | Multiplication assignment | Right to left |
//| /= | Division assignment | Right to left |
//| %= | Modulus assignment | Right to left |
//| += | Addition assignment | Right to left |
//| -= | Subtraction assignment | Right to left |
//| <<= | Left-shift assignment | Right to left |
//| >>= | Right-shift assignment | Right to left |
//| &= | Bitwise AND assignment | Right to left |
//| |= | Bitwise inclusive OR assignment | Right to left |
//| ^= | Bitwise exclusive OR assignment | Right to left |
//| , | Comma | Left to right |
//+------------------+-----------------------------------------+---------------+
晓石头 2015-03-20
- 打赏
- 举报
total+=*start++和total+=*(start++)
因为不管是那种情况都是先执行*start，再赋值运算，最后start+1，
除非是*(++start)才先执行start+1，再*start，再赋值运算，
我理解形如i++的操作，是等赋值运算完后再执行的操作
因为不管是那种情况都是先执行*start，再赋值运算，最后start+1，
除非是*(++start)才先执行start+1，再*start，再赋值运算，
我理解形如i++的操作，是等赋值运算完后再执行的操作
zuxi 2015-03-20
- 打赏
- 举报
这里是优先级表：
http://www.slyar.com/blog/c-operator-priority.html
虽然*和++优先级一样，但是结合性是从右到左，也就是说它们在一起时右边的优先级高。
只此冒泡君 2015-03-20
- 打赏
- 举报
因为自增++ 在这里面的优先级最高 加不加括号 都一样！
