number = number + 1;
赋值运算符右侧的表达式给 number 加 1,然后将结果赋值给 number,替换先前存储的值。实际上,这个声明给 number 增加了 1。同样的方式,以下语句从 number 中减去 5:number = number - 5;
如果之前从未看过这种类型的语句,则它可能会导致一些初学者理解上的困惑,因为相同的变量名称出现在赋值运算符的两边。表 1 显示了以这种方式编写的语句的其他示例:语 句 | *** 作 | 在执行语句之后 x 的值 |
---|---|---|
x = x + 4; | 给 x 加 4 | 10 |
x = x-3; | 从 x 减去 3 | 3 |
x = x * 10; | 使 x 乘以 10 | 60 |
x==x/2; | 使 x 乘以 2 | 3 |
x = x % 4 | 求 x/4 的余数 | 2 |
由于这些类型的运算在编程中很常见,因此 C++ 提供了一组专门为这些任务而设计的 运算符。表 2 显示了复合赋值运算符,也称为复合运算符。
运算符 | 用法示例 | 等价表达式 |
---|---|---|
+= | x += 5; | x = x + 5; |
-= | y -= 2; | y = y - 2; |
*= | z *= 10; | z = z * 10; |
/= | a /= b; | a = a / b; |
%= | c %= 3; | c = c % 3; |
表 2 中的用法示例说明,复合赋值运算符不需要程序员键入变量名称两次。
下面的程序使用了多变量赋值语句和复合赋值运算符:
//This program tracks the inventory of two Widget stores.// It illustrates the use of multiple and combined assignment.#include <iostream>using namespace std;int main(){ int beglnv,// Beginning inventory for both stores sold,// Number of Widgets sold store1,// Store 1's inventory store2; // Store 2's inventory // Get the beginning inventory for the two stores cout << "One week ago,2 new Widget stores opened\n"; cout << "at the same time with the same beginning\n"; cout << "inventory. What was the beginning inventory? "; cin >> beglnv; // Set each store1s inventory store1 = store2 = beglnv; // Get the number of Widgets sold at each store cout << "How many Widgets has store 1 sold? "; cin >> sold; store1 -= sold; // Adjust store 1's inventory cout << " How many Widgets has store 2 sold? "; cin >> sold; store2 -= sold; // Adjust store 2's inventory //display each store1s current inventory cout << " \nThe current inventory of each store: \n"; cout << "Store 1: " << store1 << endl; cout << "Store 2: " << store2 << endl; return 0;}程序输出结果:
One week ago,2 new Widget stores opened at the same time with the same beginning inventory. What was the beginning inventory? 100
How many Widgets has store 1 sold? 25
How many Widgets has store 2 sold? 15
The current inventory of each store: Store 1: 75
Store 2: 85
result * = a + 5;
在该语句中,和 result 相乘的是 a+5 的和。请注意,复合赋值运算符的优先级低于常规算术运算符的优先级。上述语句和以下语句是等效的:result = result *(a + 5);
表 3 显示了使用复合赋值运算符的其他示例。示例用法 | 等价表达式 |
---|---|
x += b + 5; | x = x + (b + 5); |
y -= a * 2; | y = y - (a * 2); |
z *= 10 - c; | z = z * (10 - c); |
a /= b + c; | a = a / (b + c); |
c %= d - 3; | c = c % (d - 3); |
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