[SOLVED] C question about updating a variable for a placeholder

lleb · 07-18-2013, 12:22 PM

in the printf (" foo %d", d);

is there a way to use a for loop to update a space for the place holder. so if i wanted %4d as the start, and then add 2 every time until i get to 10...

%4d first time
%6d 2nd
%8d etc... until this has been done 10 times.

I am trying to create a multiplication table with the following layout.

for my first row i just used printf to manually spread out the top row of numbers:

Code:

printf("    1  2  3  4  5  6  7  8  9  10");	// print out of top row

so nothing fancy there. for the multiplication im just using a simple for loop.

Code:

for (i = 1; i <= 10; i+=1);

ill use that to update the end of the printf statement with a*i. this is where my question comes in about the while loop for updating the # for the place holder.

Thanks. If it can be done please point me in the right direction to accomplish this task. examples are good, exact code is NOT what i am after as this is homework. Please do not do my homework for me, but help and guidance is greatly appreciated.

jmwebb2112 · 07-18-2013, 12:50 PM

You should be able to use the syntax printf( "%*d", number, int ), where you supply the value of the number of spaces in the parameter that corresponds with the placement of the asterisk. For example:

int spaces = 5
int value = 10;
printf( "%*d", spaces, value );

thirdm · 07-18-2013, 12:55 PM

One of us is slightly confused I think. When you say this...

Quote:

%4d first time
%6d 2nd
%8d etc... until this has been done 10 times.

... what do you mean by first time, 2nd time, etc.? Are you thinking you're calling printf 10 times in the loop each time with a larger width in the format string? If you think about it, or do it on paper, that's not right. If you did it that way, then the first output's right side would be at 4, 2nd six away from there, third one eight away from that new end -- won't line up at all. Remember that once you've called printf, the output stream has advanced to the end of what you output. It almost sounds like, if I'm understanding you, that you're expecting some kind of overlay output where it goes back to the beginning of the line with each call, like you're on a typewriter and doing a carriage return with no linefeed.

Now, you could build the format string in a loop (a format string doesn't have to be a string literal) and then perhaps call printf or another function in that family once with all the arguments accumulated into somekind of stdarg variable arg thingy, but that would be crazy. Rather there isn't a need for different format spec widths here at all other than perhaps the differences in widths between 9 and 10. Each call to printf with each number in your loop should have the same width.

As an aside, don't start loop indexes from one. It's the C idiom to count from zero (as all sane languages do). Also, it's more idiomatic to increase the loop variable with the ++ operator rather than +=. So an idiomatic loop is written like this...

for (i = 0; i < 10; i++)
...

or maybe (particularly for C++ programmers)

for (i = 0; i != 10; ++i)
...

lleb · 07-18-2013, 01:51 PM

thank you both. im sorry i forgot to give an example of the desired display, this might clear up some of the confusion

Code:

    1  2  3  4
1   1  2  3  4
2   2  4  6  8
3   3  6  9  12
4   4  8  12 16

I need to do that from 1 - 10 in both row and column.

the first row is displayed via printf, i was thinking for the following 10 row to start with the variable, then use that variable in a counting type loop to multiply my for loop with the i above.

am i at least on the right track?

the only reason i didnt start at 0, was 0 * foo is always 0, so i was going to skip that row entirely. no need to waste CPU time, or display for a 0 row.

lleb · 07-18-2013, 02:30 PM

never mind, i was trying to make it much more complicated then i needed. i was able to embed a loop inside of a loop.

Code:

#include<stdlib.h>
#include<stdio.h>

// create the top to skip a number holder slot, then print 1 - 10 in a line with spaces
// create the side to skip a line, then print 1 - 10 down no spaces between lines
// align row/colum with sum of value of the top/side spaced properly

int main(void)
{

	int rows, columns;	// input variables as integers to make table easier to read and calculate

	// for loop with initial condition = 1, test will end at 10, and update is increase by 1 this should go across the top
	for ( rows = 1; rows <=10; rows+=1)
	{
		// for loop with initial condition = 1, test will end at 10, and update is increase by 1 this should go down.  This loop is embeded into the first loop.
		for ( columns = 1; columns <= 10; columns+=1)
		{
		// this should take the sum of the column and the row and spread them out by tabs \t
		printf("%d\t", columns * rows);
		}
	// to make it easier to read, add a space between lines
	printf("\n");
	}
}

this way each time the inner loop runs then it updates the outer loop to include the \n. this works as desired.

Code:

imac:ENG-3211 user$ gcc -o hw_4_2 hw_4_2.c
imac:ENG-3211 user$ ./hw_4_2 
1	2	3	4	5	6	7	8	9	10	
2	4	6	8	10	12	14	16	18	20	
3	6	9	12	15	18	21	24	27	30	
4	8	12	16	20	24	28	32	36	40	
5	10	15	20	25	30	35	40	45	50	
6	12	18	24	30	36	42	48	54	60	
7	14	21	28	35	42	49	56	63	70	
8	16	24	32	40	48	56	64	72	80	
9	18	27	36	45	54	63	72	81	90	
10	20	30	40	50	60	70	80	90	100

thanks for the help earlier. they got me looking for a much better solution and my head onto the right track.