Printf conversion specifiers Post: 302885114

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Top Forums Programming Printf conversion specifiers Post 302885114 by Don Cragun on Thursday 23rd of January 2014 02:50:30 PM

01-23-2014

Registered User

From our previous discussions, I assume that you understand that the call:

Code:

printf("%*d\n", 5, 1);

prints 1 as a right-justified decimal integer in a field 5 characters wide.
This can be restated as printf() "prints the 2nd argument following the format string as a decimal integer in a field whose width is specified by the 1st argument following the format string". Note that the numbers in red match the second call you had to printf:

Code:

printf("%2$*%1d\n", 5, 1);

The n$ forms to reference argument positions are most frequently used when dealing with differences in output based on localization. (For example, in the US, dates are often printed in the form MM/DD/YYYY, but in Europe, dates are frequently printed in the form DD-MM-YYYY.) If you have your format strings in a message catalog based on your current locale settings, a printf() call to print a date in the US or Europe might be:

Code:

printf(format, m, d, y);

where format would be:

Code:

%02d/%02d/%d

to print in US format, and would be:

Code:

%2$02d-%1$02d-%3$d

to print in European format. (But, using message catalogs to control which format string is used is another level of complexity that you don't need to worry about for now.)

The statement "The second style allows repeated references to the same argument." may be better explained with a different example. Suppose that you want to print more that one decimal value with a printf statement and you want a variable to control the field width, but you want the field width to be the same for all of the values printed. Try building the following sample program:

Code:

#include <stdio.h>
void
ruler(int w) {
    printf("Following output produced with width set to %d\n", w);
    printf("         1111111111222222222233333333334444444444555555555566666\n");
    printf("1234567890123456789012345678901234567890123456789012345678901234\n");
    return;
}

int
main() {
        int     width;  // field width

        for(width = 1; width <= 16; width += width) {
                ruler(width);
                printf("%*d%0*d%*d%*d\n",
                        width, 1, width, 2, width, 3, width, 4);
                printf("%2$*1$d%3$0*1$d%4$*1$d%5$*1$d\n",
                        width, 1, 2, 3, 4);
                printf("%4$*1$d%5$0*1$d%2$*1$d%3$*1$d\n\n",
                        width, 3, 4, 1, 2);
        }
        return;
}

and examine the format strings used in the printf() calls in main(). Do you understand why the width parameter has to be supplied four times in the first call, but only supplied once in the other two calls?

The output the above program produces is:

Code:

Following output produced with width set to 1
         1111111111222222222233333333334444444444555555555566666
1234567890123456789012345678901234567890123456789012345678901234
1234
1234
1234

Following output produced with width set to 2
         1111111111222222222233333333334444444444555555555566666
1234567890123456789012345678901234567890123456789012345678901234
 102 3 4
 102 3 4
 102 3 4

Following output produced with width set to 4
         1111111111222222222233333333334444444444555555555566666
1234567890123456789012345678901234567890123456789012345678901234
   10002   3   4
   10002   3   4
   10002   3   4

Following output produced with width set to 8
         1111111111222222222233333333334444444444555555555566666
1234567890123456789012345678901234567890123456789012345678901234
       100000002       3       4
       100000002       3       4
       100000002       3       4

Following output produced with width set to 16
         1111111111222222222233333333334444444444555555555566666
1234567890123456789012345678901234567890123456789012345678901234
               10000000000000002               3               4
               10000000000000002               3               4
               10000000000000002               3               4

Update:
On some systems (including MAC OS/X), flags in the %n$ can come before or after the n$; the standards say the flags should follow the n$. The above examples have been updated to use the standard form.

Last edited by Don Cragun; 01-23-2014 at 06:20 PM.. Reason: Fix incorrect placement of flags...

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