为什么在格式字符串末尾有换行符时scanf要求两次输入? [英] Why does scanf ask twice for input when there's a newline at the end of the format string?

问题描述

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char *method1(void)
{
    static char a[4];
    scanf("%s\n", a);
    return a;
}

int main(void)
{
    char *h = method1();
    printf("%s\n", h);
    return 0;
}

当我运行上面的代码时，提示要求我两次输入(我在代码中只使用一次 scanf ).为什么会这样?

When I run the code above, the prompt is asking me twice for input (I only use scanf once in the code). Why is that?

(我输入了"jo"；它要求更多输入，所以我再次输入了"jo".然后它只打印了一次"jo".)

(I entered 'jo'; it asked for more input, so I entered 'jo' again. Then it only printed out 'jo' once.)

推荐答案

在我的scanf手册页中

From my scanf manual page

格式字符串中的空格(例如，空白，制表符或换行符)与输入中任意数量的空格匹配，包括无空格.其他所有东西都只能匹配自己.

White space (such as blanks, tabs, or newlines) in the format string match any amount of white space, including none, in the input. Everything else matches only itself.

因此使用 scanf(％s \ n"，a)，它将扫描字符串，后跟可选的空格.由于在第一条换行符之后可能会有更多的空格，因此在第一条换行符之后scanf不会执行，而是查找下一步.您会注意到，您可以输入任意数量的换行符(或制表符或空格)，scanf仍将等待更多换行符.

Thus with scanf ("%s\n", a) it will scan for a string followed by optional white space. Since after the first newline more whitespace may follow, scanf is not done after the first newline and looks what's next. You will notice that you can enter any number of newlines (or tabs or spaces) and scanf will still wait for more.

但是，当您输入第二个字符串时，空格序列被定界并且扫描停止.

However, when you enter the second string, the sequence of whitespace is delimited and scanning stops.

使用 scanf(％s"，a)不扫描尾随空白.

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