c - 如何使用 printf() 实现可变字段宽度？

Question

问题是:

How can variable field width be implemented using printf()? That is, instead of %8d, the width should be specified at run time.

基于上述问题，我在互联网上发现了一些 C 代码，但由于我是 C 编程新手，所以我无法弄清楚这些代码。我发布以下代码:

#include <stdio.h>

int main()
{
   const char text[] = "Hello world";
   int i;
   for ( i = 1; i < 12; ++i )
   {
      printf("\"%.*s\"\n", i, text);
   }


    return 0;
}

Answer 1

首先，让我告诉你，你所显示的代码是关于控制精度的，而不是字段宽度。对于缩写形式^**

 %A.B<format specifier>

A 表示字段宽度，B 表示精度。

现在，引用 C11 标准，第 §7.21.6.1 章，fprintf()(强调我的)

Each conversion specification is introduced by the character %. After the %, the following appear in sequence:

[..]

• 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 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.

和

As noted above, a field width, or precision, or both, may be indicated by an asterisk. In this case, an int argument supplies the field width or precision. [...]

所以，就你而言，

printf("\"%.*s\"\n", i, text);

精度将由i提供，它可以在运行时保存不同的值。

<小时/>

完整格式^{(为了便于阅读，分成单独的行)}

%
<Zero or more flags>
<optional minimum field width>
<optional precision>
<optional length modifier>
<A conversion specifier character>