如果在数字后留一个空格，则scanf必须确定要匹配的空格的结尾:

If you give a space after your numbers, scanf must determine the end of the whitespaces to be matched:

scanf("%d ", &a);

此处的空格表示读取并丢弃所有空格. scanf必须出现非空白字符(或EOF)，以清楚地说明 all 是什么，以便可以正确读取和丢弃它们.

Here the space means read and discard all whitespaces. A non-whitespace character (or EOF) must appear for scanf to make it clear what all is so they can be properly read and discarded.

考虑此输入流(点是字符指示符):

Consider this input stream (dots are character indicators):

   1   2
........

如果您呼叫scanf("%d")，则在呼叫后，剩余的流为

If you call scanf("%d"), then after calling, the leftover stream is

   2
....

...，在下一次读取时将丢弃空格.请注意，读取数字时，前导空格会被自动丢弃.

... where the whitespaces will be discarded at next read. Note the leading spaces are automatically discarded when reading the number.

如果您呼叫scanf("%d ")，则剩余的流为

If you call scanf("%d ") instead, the leftover stream is

2
.

您看到空格立即消失了.

You see the whitespaces are gone immediately.