How can variable field width be implemented with printf()? - c

Question

The question is :
How can variable field width be implemented using printf()? That is, instead of %8d, the width should be specified at run time.
I came across some C code on the Internet based on the question above but as I am new to C programming I haven't been able to make heads or tails of the code.
I am posting the code below:
#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

First of all, let me tell you, the code you have shown is about controlling the precision, not the field width. For a shortened form**
%A.B<format specifier>
A denotes the field width and B makes the precision.
Now, quoting the C11 standard, chapter §7.21.6.1, fprintf() (emphasis mine)
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.
and
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. [...]
So, in your case,
printf("\"%.*s\"\n", i, text);
the precision will be supplied by i which can hold different values at run-time.
The complete format (broken down in separate lines for ease of readability)
%
<Zero or more flags>
<optional minimum field width>
<optional precision>
<optional length modifier>
<A conversion specifier character>