# C + + short-circuit evaluation (logic and, logic, or) instance _c language

Source: Internet
Author: User

This article describes the C + + short-circuit evaluation (logic and, logic or), share for everyone to reference. The specific methods are analyzed as follows:

1, logic or short circuit

First look at the following code:

```#include <iostream>
using namespace std;
int main ()
{
int a = 1;
cout << "a =" << a <<endl;
true | |　　(a=0);
cout << "a =" << a <<endl;
}

```

The results of the operation are as follows:

```A = 1

a = 1

```

The logical or expressive form is as follows:

expression1 | | Exexpression2

This uses logic or, because of a logical or short-circuit, expression1 is true, then the expression2 (i.e. (a=0)) is no longer evaluated, and the entire expression evaluates to True, so the value of a is still 1, unchanged.

2. Logic and Short Circuit

First look at the following code:

```#include <iostream>
using namespace std;
int main ()
{
int a = 1;
cout << "a =" << a <<endl;
False && (a=3);
cout << "a =" << a <<endl;
}

```

The results of the operation are as follows:

```A=1

a=1

```

The logical and expressive forms are as follows:

Expression1 && Exexpression2

The above code uses logic and, due to logic and short-circuit, expression1 is false, then the expression2 is no longer evaluated, the entire result is false, so the value of a has not changed.

3, Application examples

Here is an example of a post on CSDN (http://topic.csdn.net/u/20121011/10/c7e0a805-b4e2-44db-9d71-455f5f851240.html):

Without the If statement, no assembly, how to make the product of two is always less than or equal to 255?

When you look at a post, you will find that there are many ways, such as the simplest conditional expression:

```result = ((a*b) > 255)? 255:a*b;

```

This is the first to be proposed, but as if the landlord does not agree, then try the following two ways:

To short-circuit with logic or:

```BOOL TMP = (result = A*b) < 255) | | (result=255);

```

Short Circuit with logic:

```BOOL TMP = (result = a*b) >= 255) && (result=255);

```

The complete code is as follows:

```#include <iostream>
using namespace std;
int main ()
{
int a,b,result;
while (true)
{
cin>>a>>b;
Result    = ((a*b) > 255)? 255:a*b;    BOOL TMP = (result = A*b) < 255) | | (result=255);
BOOL TMP = (result = a*b) >= 255) && (result=255);
cout<<result<<endl;
}
}

```

The operation effect is as follows:

Related Keywords:
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