Set variable text column width in printf

Question

In order to determine the size of the column in C language we use %<number>d. For instance, I can type %3d and it will give me a column of width=3. My problem is that my number after the % is a variable that I receive, so I need something like %xd (where x is the integer variable I received sometime before in my program). But it's not working.

Is there any other way to do this?

In general, instead of "not working" you should copy-paste the exact error message you received. — luser droog, Aug 19 '11 at 04:29
@luserdroog There is no error message when `printf` fails. It just silently gives incorrecy output. — S.S. Anne, Sep 01 '19 at 23:18
Is this related to the eight years old question? And what output do you get and what are you expecting? What does the call look like? — luser droog, Sep 02 '19 at 08:31

score 118 · Accepted Answer · edited Mar 15 '21 at 09:42

118

You can do this as follows:

printf("%*d", width, value);

From Lee's comment:
You can also use a * for the precision:

printf("%*.*f", width, precision, value);

Note that both width and precision must have type int as expected by printf for the * arguments, type size_t is inappropriate as it may have a different size and representation on the target platform.

edited Mar 15 '21 at 09:42

chqrlie

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answered Aug 18 '11 at 10:23

Oliver Charlesworth

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@SouravKannanthaB: no this does not work for `scanf`: the `*` means *suppress assignment* and no pointer argument is provided for this conversion specification. There is no way to specify the number as an argument, but you can compose a format string with `snprintf` as a work-around. `scanf_s` from optional and controversial Annex K attempted to address related issues, but the specification is inadequate for simple use. – chqrlie Mar 15 '21 at 09:39

score 8 · Answer 2 · edited Aug 28 '13 at 00:23

Just for completeness, wanted to mention that with POSIX-compliant versions of printf() you can also put the actual field width (or precision) value somewhere else in the parameter list and refer to it using the 1-based parameter number followed by a dollar sign:

A field width or precision, or both, may be indicated by an asterisk ‘∗’ or an asterisk followed by one or more decimal digits and a ‘$’ instead of a digit string. In this case, an int argument supplies the field width or precision. A negative field width is treated as a left adjustment flag followed by a positive field width; a negative precision is treated as though it were missing. If a single format directive mixes positional (nn$) and non-positional arguments, the results are undefined.

E.g., printf ( "%1$*d", width, value );

Your example doesn't fit the language. It should be `"%*1$d"` — Jim Balter, Mar 02 '14 at 20:18
Furthermore, you should specify both argument by means of `n$`, like so: `printf("%2$*1$d", width, value);`. — Stefan van den Akker, Aug 07 '14 at 11:26

Set variable text column width in printf

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