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Conditional judgment statement 1. Conditional operator

In the learning record of the third day, among the other operators, there is a conditional operator exp?x:y for conditional judgment. If emp determines that the result is true, then the result will return x, otherwise it will return y.

Int a = 2nint b = 3X string str = a > b? "an is bigger than b": "b is bigger than a"

The conditional operator here is more or less the same as the if-else statement that will be introduced below.

2. Judgment sentence

(1) if statement

In a programming language, an if statement is followed by a Boolean expression and one or more program statements:

If (emp) {/ / statement 1 / / statement 2 / /.}

The program statement in the if statement is executed only if the result of the Boolean expression emp is true, otherwise the if statement is skipped.

(2) if-else statement

In general judgment, two or more results are expected. For example, the conditional operator mentioned above, compare the size of two values, how to output a result.

Therefore, the if statement outputs the contents when the Boolean expression is true, while the else statement outputs the contents when the Boolean expression is false.

Int a = 3 int b = 2 if (a > b) {Console.WriteLine ("an is greater than b");} else {Console.WriteLine ("a less than b");}

It's easy to see that there's something wrong with the logic of this code. If an and b are the same value, either output is wrong. At this time, multiple judgment statements may be required for an entire conditional statement. If you have multiple judgment statements, you need to use the if-else if-else statement.

If (a = b) {Console.WriteLine ("an equals b");} else if (a > b) {Console.WriteLine ("an is greater than b");} else {Console.WriteLine ("a less than b");}

In this way, the whole logic becomes clear and clear.

(3) nested judgment

Under the condition that one premise is satisfied, judge whether another premise is satisfied. At this point, you need to use nested statements to make multiple judgments.

For example, if you want to go to an Internet bar to play games, the first condition is that you should be 18 years old, and the second condition is that you have enough money in your wallet. Otherwise, you can't go in and play games.

If (age > = 18) {if (money > = 10) {Console.WriteLine ("I choose Yasso!") ;}}

If you have met two conditions, then you can have a happy Hasa in the game!

(4) switch statement

If there are more than ten or twenty conditions to judge, then if you use else if all the time, it will make the code look bloated and jumbled.

The switch statement allows you to test when a variable is equal to multiple values.

/ / Syntax switch (expression) {case exp1: statement1; break; case exp2: statement2; break; / /. Case expn: statementn; break; default: break; state; break;}

The expression after the switch statement must be an integer or enumerated type, or a class type.

There can be any case statement in a switch statement, and the value after case must be of the same type as expression and must be constant.

The default statement is optional. When all case statements are not satisfied, the default statement is executed if it exists; if it does not exist, a null value is output.

C # does not support running from one case to another case statement. Therefore, break keywords are a must. When you encounter the first break, you end the entire switch statement.

(4) nested switch statements

Switch statements can also be nested.

Switch (num1) {case 1: Console.WriteLine ("this 1 belongs to external"); switch (num2) {case 1: Console.WriteLine ("this 1 belongs to internal"); break;} break;}

One switch statement is nested with another switch statement, even if the external and internal case constant values contain the same value, there will be no contradiction.

It is worth noting that the internal case is followed by the break statement, and the external case must not forget the break keyword.

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