Types and arithmetic
C Programming Lesson 2 2:29 English narration · English + 中文 subtitles burned in
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In C, every variable has a type, and you say what it is when you make it.
在 C 里，每个变量都有类型，而且你在创建它时就要说明是什么类型。
An int holds a whole number.
int 装整数。
A double holds a decimal.
double 装小数。
A char holds a single character, in single quotes.
char 装单个字符，写在单引号里。
You state the type once, at the declaration, and from then on that box can never hold anything else — which is why the compiler can catch so much before the program runs.
类型只在声明时说明一次，从那以后这个盒子就再也装不了别的东西—— 正因如此，编译器才能在程序运行之前发现那么多问题。
Now the thing that catches everyone.
现在说那个几乎人人中招的地方。
When both sides are ints, C divides like whole numbers: ten over three is three, not three point three.
当两边都是 int 时，C 按整数来做除法： 10 除以 3 得到 3，而不是 3.33。
Make one side a decimal — write three point zero instead of three — and the whole calculation becomes a decimal one.
把其中一边变成小数——写 3.0 而不是 3——整个计算就变成小数运算。
There is no error and no warning; the fractional part is simply thrown away.
这里没有错误提示，也没有警告；小数部分直接被丢掉。
That is why the average task in this lesson divides by three point zero.
这就是为什么这一课的求平均题要除以 3.0。
To read a value you use scanf, and it needs two things.
要读入一个值，就用 scanf，它需要两样东西。
First, a format telling it what kind to read — percent d for a whole number, the same letters printf uses.
第一，一个格式说明它要读什么类型——%d 表示整数，和 printf 用的字母一样。
Second, and this is new, an ampersand before the variable name.
第二，这一点是新的：变量名前面要加一个 & 符号。
That ampersand means where to put it: you are handing scanf the ADDRESS of your variable, not its value.
这个符号的意思是“放到哪里”：你交给 scanf 的是变量的地址，而不是它的值。
Leave it out and the program will usually crash.
漏掉它，程序通常会崩溃。
Now the lesson's first task, and it is where integer division earns its keep.
现在做课程里的第一道题，而这正是整数除法发挥作用的地方。
A box holds six items, and you are told a count.
一个盒子装 6 件，给你一个总数。
How many full boxes?
有多少个装满的盒子？
Divide, and let the division truncate — that is exactly right, because a part-filled box is not a full one.
做除法，让它截断——这正好是对的，因为装不满的盒子不算装满。
Then percent gives what is left over.
然后用取余算出剩下几件。
Twenty items make three full boxes with two left.
20 件装满 3 盒，还剩 2 件。
The pair, slash and percent, answers both halves of the question.
斜杠和百分号这一对，回答了问题的两半。
Four things to take with you.
带走四点。
One: declare the type once — int, double or char — and it never changes.
第一：类型只声明一次——int、double 或 char——之后不再改变。
Two: an int over an int gives an int, so the decimals are dropped silently.
第二：int 除以 int 得到 int，小数部分会被悄悄丢掉。
Three: percent gives the remainder, and it pairs with the slash.
第三：百分号给出余数，它和斜杠是一对。
Four: scanf needs the ampersand, because it must be told where to put the value.
第四：scanf 需要 & 符号，因为必须告诉它把值放到哪里。
Now do the three tasks; the second one needs three point zero.
现在去做那三道题；第二题需要用 3.0。