Hello, this is one examples that I always panic with C printf format specifier.
1) I did read the manpage with
Code:
man 3 printf
...... One can also specify explicitly which argument is taken, at each place where an argument is required, by writing
"%m$" instead of '%' and "*m$" instead of '*', where the decimal integer m denotes the position in the argument list of the desired
argument, indexed starting from 1. Thus,
printf("%*d", width, num);
and
printf("%2$*1$d", width, num);
are equivalent. The second style allows repeated references to the same argument.
1) Can you please explain this example on width and num for me.
2) To have further understanding I used following code from "A book on C by Al Kelley/Ira Pohl, page 503" to test,
but I met two problems. 1) A bunch of warnings as I mentioned in my last threads:
Code:
pg503.c: In function ‘main’:
pg503.c:13:2: warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘int *’ [-Wformat]
pg503.c:13:2: warning: field width specifier ‘*’ expects argument of type ‘int’, but argument 3 has type ‘char *’ [-Wformat]
pg503.c:13:2: warning: format ‘%c’ expects argument of type ‘int’, but argument 5 has type ‘char *’ [-Wformat]
pg503.c:13:2: warning: format ‘%s’ expects a matching ‘char *’ argument [-Wformat]
pg503.c:13:2: warning: format ‘%c’ expects a matching ‘int’ argument [-Wformat]
2) when I tried:
Code:
./a.out
Please input a line:
45 , ignore_this, % C read_in_this**
Note: Spaces after 45, two asterisks were put at the end on purpose, and I was expecting to see:
Code:
45
,
read_
in_this**
The program went infinite, nothing was printed. Thank you!
Last edited by yifangt; 01-23-2014 at 01:45 PM..
Reason: indentation
You give printf and scanf identical command strings, but %* means something very different to printf and scanf. It means "print this type to [format] characters wide" in printf, and "don't store this value" in scanf.
Also, you are giving printf all the same arguments too. scanf needs &i because scanf needs i's location, not its contents -- but printf needs its contents, not its location!
So, my best guess at what happened is that printf's %d prints part of a memory address (since you are on a 64-bit system, and %d expects a 32-bit integer). Next, %*s tries to get a width, and gets the other half of that memory address -- a very, very high number! So it prints several billion leading spaces before it actually gets to the string.
Last edited by Corona688; 01-23-2014 at 02:15 PM..
How stupid to use the scanf format for printf!------Thanks, Corona!
I got it now:
Code:
printf("%d\n%%\n%c\n%5s\n%c\n%s\n\n", i, c, string, string[5], string+5);
$ ./a.out
Please input a line:
45 , ignore_this, % C read_in_this**
45
%
C
read_in_this**
i
in_this**
Quote:
Also, you are giving printf all the same arguments too. scanf needs &i because scanf needs i's location, not its contents -- but printf needs its contents, not its location!
If print string, use the address of the string array(cf: string+5); if print the char of the string, use the subscription (string[5]) explicitly.
Last edited by yifangt; 01-23-2014 at 03:18 PM..
Reason: quote reply
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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01-23-2014
