"Learning Notes", "C language" logical operators

Sometimes, we need to set up a number of conditions at the same time to execute a piece of code, such as: the user only input QQ and password, to execute the login code, if only entered QQ or only entered a password, you can not execute the login code. In this case, we are going to use the logical operators provided by the C language.

The result of a logical operation is only 2: "True" is 1, "false" is 0

&& Logic and

1> using formats

"Condition a && condition B"

2> operation Results

Only if both condition A and condition B are established, the result is 1, which is true, and the rest is 0, or false. Therefore, if a condition A or condition B is not tenable, the result is 0, that is, "false"

3> Operation Process

Always first determine if condition A is established

L If condition A is established, then determine if condition B is established: If condition B is established, the result of "condition a && condition B" is 1, i.e. "true", if condition B is not established, the result is 0, i.e. "false"

If condition A is not established, it will not be judged whether the condition B is established: Since condition A is not established, the result of condition a && condition B must be 0, that is, "false", regardless of condition B.

4> Example

The combination of logic and direction is "from left to right". such as Expressions (a>3) && (a<5)

If the value of a is 4: First Judge A>3, set up, and then Judge A<5, also established. So the result is 1.

If the value of a is 2: First Judge A>3, not set up, stop judging. So the result is 0.

Therefore, if the value of a is within the range of (3, 5), the result is 1; otherwise, the result is 0.

5> Note

If you want to determine if the value of a is within the range of (3, 5), you must not write 3<a<5, because the relationship operator is "left to right". For example A is 2, it will first calculate 3<a, that is, 3<2, the condition is not set, the result is 0. Compared with 5, namely 0<5, the condition is set up, the result is 1. Therefore, the result of 3<a<5 is 1, the condition is established, that is, when a is a value of 2 o'clock, the value of a is within the range of (3, 5). This is obviously wrong. The correct way to judge is: (a>3) && (a<5)

LC Language Provisions: Any non-0 value is "true", only 0 is "false". Therefore logic and also apply to numerical values. For example, 5 && 4 result is 1, "true", -6 && 0 result is 0, "false"

2.| | Logical OR

1> using formats

"Condition A | | Condition B "

2> operation Results

When condition A or condition B is set up (also including condition A and condition B), the result is 1, which is true, and only if condition A and condition B are not set, the result is 0, which is "false".

3> Operation Process

Always first determine if condition A is established

If condition A is established, it will not be judged whether the condition B is established: Because condition A has been established, regardless of condition B, "condition A | | The result of condition B must be 1, which is "true."

If condition A is not established, then determine if condition B is true: If condition B is established, "condition A | | Condition B "results in 1, that is," true ", if condition B is not set, the result is 0, i.e." false "

4> Example

The logical or binding direction is "from left to right". such as expressions (a<3) | | (a>5)

If the value of a is 4: First Judge A<3, not set up, and then Judge A>5, also does not establish. So the result is 0.

If the value of a is 2: First Judge A<3, set up, stop judging. So the result is 1.

Therefore, if the value of a is within the range (-∞, 3) or (5, +∞), the result is 1; otherwise, the result is 0.

5> Note

C language provisions: Any non-0 value is "true", only 0 is "false". So logic or also applies to numeric values. such as 5 | | 4 The result is 1, for "true";-6 | | 0 The result is 1, for "true"; 0 | | The result of 0 is 0, which is "false"

3.! Logical Non-

1> using formats

“! Condition a "

2> operation Results

In fact, the condition A is reversed: if condition A is established, the result is 0, that is, "false"; if condition A is not established, the result is 1, that is, "true". That is to say: true false, false become true.

3> Example

The logical non-union direction is "from right to left". Like an expression! (a>5)

If the value of a is 6: First Judge A>5, set up, and then reverse the result is 0

If the value of a is 2: First Judge A>3, not set, and then take the result of the reverse is 1

Therefore, if the value of a is greater than 5, the result is 0; otherwise, the result is 1.

4> Note

Logical non-operators can be used multiple times:! (4>2) The result is 0, is "false",!! (4>2) The result is 1, is "true",!!! (4>2) The result is 0, which is "false"

C language provisions: Any non-0 value is "true", only 0 is "false". Therefore, a non-0 value is not logical! The result of the operation is 0, the value of 0 is logical non! The result of the operation is 1.! 5,!6.7 、!-9 results are 0,!0 results of 1

4. Priority level

The order of precedence for logical operators is: parentheses () > minus sign->! > Arithmetic operators > Relational operators > && > | |

An expression! (3>5) | | (2<4) && (6<1): Calculate first! (3>5), (2<4), (6<1), the result is 1, the equation becomes 1 | | 1 && 0, then 1 && 0, the formula becomes 1 | | 0, the final result is 1.

Expression 3+2<5| | 6>3 equivalent to ((3+2) < 5) | | (6>3) with a result of 1

Expression 4>3 &&!-5>2 equivalent to (4>3) && ((! ( -5)) > 2) with a result of 0

1#include <stdio.h>2 3 intMain ()4 {5     //logic and Conditions 1 && conditions 26     7     //int a = 10>3 && 7<6;8     9     //int a = 0 &&;Ten      One     //printf ("a=%d\n", a); A      -     /* - int a = ten; the int b = ten; -      - //int C = (a>5) && (++b>=11); -      + int c = (a<5) && (++b>=11); -      + //a = ten A //b = Ten at //c = 0 - printf ("a=%d, b=%d, c=%d\n", A, B, c); -     */ -      -      -     //logic or Condition 1 | | condition 2 in      -     //int a = 0 | | to     /* + int a = ten; - int b = ten; the      * int c = (a<5) | | (b++-ten); $     Panax Notoginseng //a = ten - //b = One the //c = 0 + printf ("a=%d, b=%d, c=%d\n", A, B, c); A     */ the      +     //logical non! condition -     //returns 0 if the condition is true, and returns 1 if the condition is not true $      $     //int a =! (10>8); -      -     //int a =!-10; the      -     //int a =!10>8;Wuyi      the     /* - int a =!! Ten; Wu      - printf ("a=%d\n", a); About     */ $     return 0; -}

"Learning Notes", "C language" logical operators