C--Conditional Statements

Posted 2021-01-18 jllin

Conditionals

　　Conditional expressions allow your programs to make decisions and take different forks in the road,depending on the value of variables or user input.

　　C provides a few different ways to implement conditional expressions (also know as branches ) in your programs,some of which likely look familiar from Scratch.

　　if (and if-else,and if-else..else)

　　　　Use Boolean expressions to make descisions.

　　　　if (boolean-expression)

　　　　　　{

　　　　　　　　statements;　　　　

　　　　　　}

　　　　if the boolean-expression evaluates to true,all lines of code between the curly braces will execute in order from top-to-bottom.

　　　　if the boolean-expression evaluates to false,those lines of code will not execute.

 

　　　　if (boolean-expression)

　　　　{

　　　　　　statements;

　　　　}

　　　　else

　　　　{

　　　　　　statements;

　　　　}

　　　　if the boolean-expression evaluates to true,all lines of code between the first set of curly braces will execute in order from top-to-bottom.

　　　　if the boolean-expression evaluates to false,all lines of code between the second set of curly braces will execute in order from top-to-bottom.

　　switch statement

　　　　Use discrete cases to make decisions

　　　　int x = GetInt();

　　　　switch(x)

　　　　{

　　　　　　case 1;

　　　　　　　　printf("One! ");

　　　　　　　　break;

　　　　　　case 2:

　　　　　　　　printf("Two! ");

　　　　　　　　break;

　　　　　　case 3:

　　　　　　　　printf("Three! ");

　　　　　　　　break;

　　　　　　default:

　　　　　　　　printf("Sorry! ");

　　　　}

　　　　C‘s switch() statement is a conditional statement that permits enumeration of discrete cases,instead of relying on Boolean expressions.

　　　　　　It‘s important to break;between each case,or you will "fall throug" each case (unless that is desired behavior).

　　The ternary operator (?:)

　　　　Use to replace a very simple if-else to make your code look fancy

　　　　cool but not a good idea

　　　　int x;

　　　　if (expr)

　　　　{

　　　　　　x = 5;

　　　　}

　　　　else

　　　　{

　　　　　　x = 5;

　　　　}

　　　Next

　　　　int x = (expr) ? 5 : 6;

　　　　　　These two snippets of code act identically.　　// 这两段代码片段作用相同

　　　　　　The ternary operator (?:) is mostly a cute trick,but is useful for writing tricially short conditional branches.Be familiar with it,but know that you won‘t need to write it if you don‘t want to.　　// 三元操作符(?:)是一个不错的技巧