练习30: Else 和 If–笨方法学ruby在线

前一练习中你写了一些「if 语句 (if-statements)」，并且试图猜出它们是什么，以及实现的是什么功能。在你继续学习之前，我给你解释一下上一节的加分练习的答案。上一节的加分练习你做过了吧，有没有？

1. 你认为 if 对于它下一行的代码做了什么？if 语句为代码创建了一个所谓的「分支(branch)」，就跟 RPG游戏中的情节分支一样。if 语句告诉你的脚本：「如果这个布尔表示式为真，就执行接下来的代码，否则就跳过这一段。」
2. 把练习29中的其它布尔表示式放到 if 语句中会不会也可以执行呢？试一下。可以。而且不管多复杂都可以，虽然写复杂的东西通常是一种不好的写作风格。
3. 如果把变数 people、cats和 dogs 的初始值改掉，会发生什么事情？因为你比较的对象是数字，如果你把这些数字改掉的话，某些位置的 if 语句会被演绎为 True，而它下面的代码块将被运行。你可以试着修改这些数字，然后在头脑里假想一下那一段代码会被运行。

把我的答案和你的答案比较一下，确认自己真正懂得代码「块(block)」的含义。这点对于你下一节的练习习很重要，因为你将会写很多的if 语句。

把这一段写下来，并让它运行起来：

<span class="nv">people</span> <span class="o">=</span> 30 <span class="nv">cars</span> <span class="o">=</span> 40 <span class="nv">buses</span> <span class="o">=</span> 15 <span class="k">if</span> cars > people puts <span class="s2">"We should take the cars."</span> elsif cars < people puts <span class="s2">"We should not take the cars."</span> <span class="k">else</span> puts <span class="s2">"We can't decide."</span> end <span class="k">if</span> buses > cars puts <span class="s2">"That's too many buses."</span> elsif buses < cars puts <span class="s2">"Maybe we could take the buses."</span> <span class="k">else</span> puts <span class="s2">"We still can't decide."</span> end <span class="k">if</span> people > buses puts <span class="s2">"Alright, let's just take the buses."</span> <span class="k">else</span> puts <span class="s2">"Fine, let's stay home then."</span> end

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<span class="nv">people</span> <span class="o">=</span> 30 <span class="nv">cars</span> <span class="o">=</span> 40 <span class="nv">buses</span> <span class="o">=</span> 15   <span class="k">if</span> cars > people   puts <span class="s2">"We should take the cars."</span> elsif cars < people   puts <span class="s2">"We should not take the cars."</span> <span class="k">else</span>   puts <span class="s2">"We can't decide."</span> end   <span class="k">if</span> buses > cars   puts <span class="s2">"That's too many buses."</span> elsif buses < cars   puts <span class="s2">"Maybe we could take the buses."</span> <span class="k">else</span>   puts <span class="s2">"We still can't decide."</span> end   <span class="k">if</span> people > buses   puts <span class="s2">"Alright, let's just take the buses."</span> <span class="k">else</span>   puts <span class="s2">"Fine, let's stay home then."</span> end

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