on net: using printf to add two numbers(without using any operator) like following:
main()
{
printf("Summ = %d",add(10,20))
return 0;
}
int add(int x,int y)
{
return printf("%*d%*d",x,' ',y,' ');
}
Could anyone please explain, how this works:
return printf("%*d%*d",x,' ',y,' ');
Note: This fails when i call "sum" like following:
sum(1,1) or sum(3,-1)
There are two central concepts here:
printf() returns the number of characters printed.%*d format specifier causes printf() to read two integers from its arguments, and use the first to set the field width used to format the second (as a decimal number).So in effect the values being added are used as field widths, and printf() then returns the sum.
I'm not sure about the actual d formatting of the space character, at the moment. That looks weird, I would have gone with an empty string instead:
static int sum(unsigned int a, unsigned int b)
{
return printf("%*s%*s", a, "", b, "");
}
int main(void)
{
unsigned int a = 13, b = 6;
int apb = sum(a, b);
printf("%d + %d = %d?\n", a, b, apb);
return 0;
}
The above works and properly computes the sum as 19.
s. See the code. I just think this is clearer.
' ' is an int, so the %d formatting will print 32. And since the field width is a minimum (meaning that the output will not be truncated if it is larger than the specified width), add(1,1) will return 4. Your solution with %*s and "" is thus much better (although the trick won't work with negative numbers of course).sum() take unsigned int args instead to make it clear it won't work with negative numbers, well spotted.
printf returns the number of printed characters.
Here, it is used in the add function to generate a string only composed of 10 + 20 spaces, by using the format string.
So the printf in the add function will return 30.
Then, this result is simply printed with printf (his main purpose).
Note: it might be evident, but this printf usage has to be avoided. It's very dirty as it generates useless outputs. Imagine: add(10000,10000)...
First, printf returns the number of characters it prints.
Second, in the format specifier %*d, * means the minimum number of characters to be printed, but the width is not from the format string itself, but from the additional argument.
With all together, the job is done, but it won't work well on small numbers like 1 because of %d in the format specifier, a better solution could be :
("%*c%*c", a, ' ', b,' ');
char dummy; return snprintf(&dummy, 1, "%*s%*s", x, "", y, "");instead to avoid useless output.add. Then your question is about the functionsum. Those are different functions.