if

一. if语句

书P39例3-3：

#include <iostream>
using namespace std;
void main()
{
    int a, b, max;
    cout << "Please input two numbers: " << endl; 
    cin >> a >> b;
    max = a;
    if (max < b) max = b;
    cout << "max=" << max << endl;
}

本project遇到的问题：

1.必须写成“if（<表达式>）语句”的形式吗？

if   <表达式>

语句

的形式也可以,例：

#include <iostream>
using namespace std;
void main()
{
    int a, b, max;
    cout << "Please input two numbers: " << endl; 
    cin >> a >> b;
    max = a;
    if (max < b)  
        max = b;
    cout << "max=" << max << endl;
}

2.cin输入时,保持输入>>,Don't suddenly become input halfway through“,”

这样就会报错：变量未初始化.例：

#include <iostream>
using namespace std;
int main()
{
    int a, b, max;
    cout << "Please input two numbers: " << endl; 
    cin >> a , b;
    max = a;
    if (max < b)  max = b;
    cout << "max=" << max << endl;
}

 3.cin输入时,Enter multiple variables,用“enter（换行键）”Input distinguishes between different variables,而不是用“,”来区分,例：

 At this point regardless of the variablebWhat value to enter,The results are only outputa

（因为使用“,”is ignored by default“,”后的b或默认“,”后为0）

4.VISUAL BASIC中的“if...then...”语句在C++中不成立

（ifStatements cannot be entered afterthen）,例：

#include <iostream>
using namespace std;
int main()
{
    int a, b, max;
    cout << "Please input two numbers: " << endl; 
    cin >> a >> b;
    max = a;
    if (max < b) then max = b;
    cout << "max=" << max << endl;
}

二.  if...else语句

 书P39例3-4：

#include <iostream>
using namespace std;
int main()
{
    int a, b;
    cout << "Please input two numbers:" << endl; 
    cin >> a >> b;
    if (a > b)
        cout << "max=" << a << endl; 
    else
        cout << "max=" << b << endl;
}

三.  if...else...if语句

 书P40例3-5：

#include <iostream>
using namespace std;
int main()
{
    char c;
    cout << "input a character: " << endl; 
    cin >> c;
    if (c < 32)
        cout << "This is a control character." <<endl;
    else if (c >= '0' && c <= '9')
        cout << "This is a digit." <<endl;
    else if (c >= 'A' && c <= 'Z')
        cout << "This is an upper case letter." << endl; 
        else if (c >= 'a' && c <= 'z')
        cout << "This is a lower case letter." << endl; 
        else
        cout << "This is an other character." << endl;
}

注意：

The input string is treated as uppercase as the output/lowercase letters,例：

 个人认为,这个 if...else...if语句,Actually it is not if...else...if,而是 if...else if

因为 if...else...if 语句的效果（as follows with the bookP40）The flowcharts drawn are not the same：

例：

#include <iostream>
using namespace std;
int main()
{
    int a;
    cin >> a ;
    if (a==1)
        cout << 1<< endl; 
    else
        cout<< 2 << endl;
    if (a == 2)
        cout <<3 << endl;
}

结果：

所以应该是：

if...else if语句

Here we are discussingC++The function is not included in the function”{ }“的情况,

CSome rules in the language and internal use”{ }“的情况,详情可参考：

C语言：if、if...else、if...else if ...else、if...if...if 语句的区别_斯文～的博客-CSDN博客_if if