为什么在加scanf格式串前导空格建议？ [英] Why is adding a leading space in a scanf format string recommended?

问题描述

我目前正在读这本书C11编程的初学者，并在scanf（）的章这样说的：

I am currently reading this book "C11 Programming for beginners" and in the scanf() chapter it says:

总是第一个控制串字符前加一个空格，以确保准确的文字输入。如：

"Always add a leading space before the first control string character to ensure accurate character input." As in:

scanf(" %s", whatever);

控制字符串％S，收到了空间。
我知道这听起​​来pretty不言自明，但我真的不明白有什么可以去错了，这个空间如何确保准确的文字输入。

The control string %s, has a space before it. I know it sounds pretty self-explanatory, but I don't really get what could go wrong, and how this space ensures accurate character input.

先谢谢了。

推荐答案

@WhozCraig也指出缺点这个建议。它自带无理性的，也不详细。而且它并不适用于％S为％S消耗了领先的空白带或不带领先的空间。

@WhozCraig well stated shortcomings to this advice. Its comes without rational nor detail. Further it does not apply to "%s" as "%s" consumes leading white-space with or without a leading space.

一个领先的空白，无论是，'\\ T'，的'\\ n'等方面都做同样的事情：直接 scanf（）的来消费，而不是存储可选领先的空白字符。这是从previous scanf（）的不消耗用户的的'\\ n'典型用法有用在<大骨节病>输入

A leading white-space, be it ' ', '\t', '\n', etc. all do the same thing: direct scanf() to consume and not store optional leading white-space char. This is useful as typical usage of previous scanf() does not consume the user's '\n' from the Enter

scanf("%d", &some_int);
scanf("%c", &some_char);  // some_char is bound to get the value '\n'

所有 scanf（）的输入符忽略前导空格，除了3：％C，％N，％[]。

All scanf() input specifiers ignore leading spaces, except 3: "%c", "%n", "%[]".

简单指令的做的使用如以下前导空格受益。 previous遗留的空白之前消耗的$。

Simply directive do benefit with the leading space as in the following. Previous left-over white-space is consumed before '$'.

int Money;
scanf(" $%d", &Money);

通常情况下，虽然不总是前的前导空格％C是读书时是有益的一个字符的用户输入。

Often, though not always a leading space before "%c" is beneficial as when reading a single char of user input.

char ch;
scanf(" %c", &ch);

什么是最错误的建议是，1当使用％S，提供一个宽度参数）是稳健的code必要的，2）返回值应检查

What is most wrong with the advice is that 1) when using "%s", supplying a width parameter is essential to robust code and 2) the return value should be checked.

char buf[30];
int cnt = scanf("%29s", buf);
if (cnt != 1) Handle_NoInput_or_EOF_IOError();

最后，建议使用与fgets（）在 scanf（）的作为一个可以 - 这是通常的案例。

Lastly, recommend to use fgets() over scanf() as one can - which is the usually the case.

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