Numbers and maths
JavaScript Lesson 3 2:08 English narration · English + 中文 subtitles burned in
Chapters
Transcript
JavaScript does arithmetic without being taught.
JavaScript 不用教就会算术。
Plus and minus and star add, subtract, multiply — and the answers come straight back in the console.
加号、减号、星号分别做加、减、乘—— 结果直接出现在控制台里。
Then look at the division: six divided by four gives one point five.
再看那个除法：6 除以 4 得到 1.5。
If you have met another language you may be waiting for a whole-number answer, or for a conversion step.
如果你学过别的语言，可能正等着一个整数答案，或者等着做一次类型转换。
There is none: JavaScript has one kind of number, and it carries decimals whenever they are needed.
这里都没有：JavaScript 只有一种数字类型，需要小数时它就带小数。
Two more operators earn their place.
还有两个运算符很有用。
Percent gives what is left over after a division: twenty-three percent five is three, because twenty-three is four fives with three to spare.
百分号给出除法之后剩下的部分： 23 % 5 等于 3，因为 23 里有 4 个 5，还剩 3。
Double star raises to a power: two double-star five is thirty-two.
双星号表示乘方：2 ** 5 等于 32。
The remainder is the more useful of the two day to day — it is how you test whether a number is even or odd, and how you pick out every third item in a list.
日常用得更多的是取余——它可以用来判断一个数是偶数还是奇数， 也可以用来从一个列表里每隔两个挑一个。
Now the trap in the lesson's third task.
现在说这一课第三题里的陷阱。
You want the average of two numbers, where a is four and b is eight, so the answer should be six.
你要求两个数的平均值，a 是 4，b 是 8， 所以答案应该是 6。
Write a plus b divided by two with no brackets and you get eight — because the division runs before the addition, so it averages b alone and then adds a.
不加括号写成 a + b / 2，你会得到 8—— 因为除法先于加法执行，于是它只对 b 取了一半，然后再加上 a。
Put brackets round the addition and it is six.
把加法用括号括起来，结果就是 6。
This is the same order you learned in maths: powers, then times and divide, then plus and minus.
这和你在数学课上学的顺序是一样的：先乘方，再乘除，最后加减。
Four things to take with you.
带走四点。
One: plus, minus, star and slash do the arithmetic, and a slash can give you decimals.
第一：加号、减号、星号和斜杠负责算术，而斜杠可以给出小数。
Two: percent gives the remainder, which is your even-or-odd test.
第二：百分号给出余数，这就是你判断奇偶的办法。
Three: double star raises to a power.
第三：双星号表示乘方。
Four: brackets decide who goes first — when a line matters, add them even if you are sure.
第四：括号决定谁先算——遇到重要的一行，即使你有把握也把括号加上。
Now do the three tasks; the third one needs exactly that.
现在去做那三道题；第三题正需要这一点。