printf("%.d", 0) 和 printf("%.1d", 0) 有什么区别？

Question

我正在重新编码 printf，现在我被精确标志封锁了一会儿。所以我读到类型转换说明符为 d 时的默认精度为 1:

所以我认为%.d和%.1d之间没有区别，但是当我测试时：

printf(".d =%.d, .1d= %.1d", 0, 0);

我确实找到了一个：

.d =, .1d= 0

Answer 1

如果在%之后使用.而不指定精度，则设置为零。

From the printf page on cppreference.com:

. followed by integer number or *, or neither that specifies precision of the conversion. In the case when * is used, the precision is specified by an additional argument of type int. If the value of this argument is negative, it is ignored. If neither a number nor * is used, the precision is taken as zero.

如果使用%d（不使用.）则默认为1：

printf("d = %d, 1d= %1d", 0, 0);
# Output:   d = 0, 1d= 0

Answer 2

C18 标准 - ISO/IEC 9899:2018 -（强调我的）状态：

"An optional precision that gives the minimum number of digits to appear for the d, i, o, u, x, and X conversions, the number of digits to appear after the decimal-point character for a, A, e, E, f, and F conversions, the maximum number of significant digits for the g and G conversions, or the maximum number of bytes to be written for s conversions. The precision takes the form of a period (.) followed either by an asterisk * (described later) or by an optional non negative decimal integer; if only the period is specified, the precision is taken as zero. If a precision appears with any other conversion specifier, the behavior is undefined."

Source: C18, §7.21.6.1/4

表示 %.d 等于 %.0d 而与 %.1d 不同。

此外：

"d,i - The int argument is converted to signed decimal in the style [-]dddd. The precision specifies the minimum number of digits to appear; if the value being converted can be represented in fewer digits, it is expanded with leading zeros. The default precision is 1. The result of converting a zero value with a precision of zero is no characters."

Source: C18, §7.21.6.1/8

这意味着如果您在 printf() 调用中使用 %.d 转换 0 值，则结果保证不会打印任何字符（这符合您的测试体验） .

Answer 3

当精度设置为零或它的值被省略时

printf( "%.d", x )'

当根据转换说明符d和i的描述时（7.21.6.1 fprintf函数）

The int argument is converted to signed decimal in the style [−]dddd. The precision specifies the minimum number of digits to appear; if the value being converted can be represented in fewer digits, it is expanded with leading zeros. The default precision is 1. The result of converting a zero value with a precision of zero is no characters.

这是一个演示程序

#include <stdio.h>

int main(void) 
{
    printf( "%.d\n", 0 );
    printf( "%.0d\n", 0 );
    printf( "%.1d\n", 0 );      
    return 0;
}

它的输出是

也就是当精度等于0或者它的值不存在时，如果指定0作为参数，则什么都不会输出。

printf("%.d", 0) 和 printf("%.1d", 0) 有什么区别？

What's the difference between printf("%.d", 0) and printf("%.1d", 0)?

c

precision

io

formatting

printf