scanf() 格式字符串中尾随空格的影响是什么? [英] What is the effect of trailing white space in a scanf() format string?
问题描述
这段代码中scanf("%d")
和scanf("%d")
有什么区别,区别在于格式中的尾随空白字符串?
What is difference between scanf("%d")
and scanf("%d ")
in this code, where the difference is the trailing blank in the format string?
#include <stdio.h>
int main(void)
{
int i, j;
printf("enter a value for j ");
scanf("%d ",&j);
printf("j is %d
", j);
printf("enter a value for i ");
scanf("%d", &i);
printf("i is %d
", i);
return 0;
}
当我在像 scanf("%d ", &j);
这样的格式说明符后添加空格时,scanf()
函数实际上是如何工作的?>
How does the scanf()
function actually work when I add spaces after the format specifier like scanf("%d ", &j);
?
推荐答案
scanf 格式中的空白字符会导致它显式读取和忽略尽可能多的空白字符.所以使用 scanf("%d ", ...
,在读取一个数字后,它会继续读取字符,丢弃所有空格,直到在输入中看到一个非空格字符.那个非-空白字符将作为输入函数读取的下一个字符.
A whitespace character in a scanf format causes it to explicitly read and ignore as many whitespace characters as it can. So with scanf("%d ", ...
, after reading a number, it will continue to read characters, discarding all whitespace until it sees a non-whitespace character on the input. That non-whitespace character will be left as the next character to be read by an input function.
使用您的代码:
printf("enter a value for j ");
scanf("%d ",&j);
printf("j is %d
", j);
它会打印第一行,然后等你输入一个数字,然后在数字之后继续等待.因此,如果您只输入 5Enter,它似乎会挂起 — 您需要在另一行中输入一些非空白字符才能继续.如果然后键入 6Enter,它将成为 i
的值,因此您的屏幕将类似于:
it will print the first line and then wait for you to enter a number, and then continue to wait for something after the number. So if you just type 5Enter, it will appear to hang — you need to type in another line with some non-whitespace character on it to continue. If you then type 6Enter, that will become the value for i
, so your screen will look something like:
enter a value for j 5
6
j is 5
enter a value for i i is 6
此外,由于大多数 scanf %-conversions 也跳过前导空格(除了 %c
、%[
和 %n
),空格 before %-conversions 是不相关的("%d"
和 "%d"
的作用相同).所以在大多数情况下,你应该避免在 scanf 转换中使用空格,除非你知道你特别需要它们来实现它们的特殊效果.
Also, since most scanf %-conversions also skip leading whitespace (all except for %c
, %[
and %n
), spaces before %-conversions are irrelevant ("%d"
and " %d"
will act identically). So for the most part, you should avoid spaces in scanf conversions unless you know you specifically need them for their peculiar effect.
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