C--Conditional Statements

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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

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