I am really new to the C language and recently asked for code help on stack overflow and a really great person gave me this code.

for(int m = 1; m < 262144; m <<= 1) putc("\n12345"[!!(96221 & m)   !!(26520 & m)   !!(18192 & m)   !!(1536 & m)   !!(1024 & m)], stdout);

Which prints out

1
123
12345
123
1

This was exactly what I was looking for.

However I do not know how this code works and also I have google’d these things above but I can not find/ understand it.

Especially with ‘ <<=, All those weird ginormous numbers. ‘ these two.

So, can someone explain how this code works? And explain what each part of the codes are?

CodePudding user response：

I'm not sure the person who gave you this code was brilliant so much as devious... this looks like deliberately obfuscated code :)

m <<= 1

The above means "shift the bits in m left by one position", which has the effect of multiplying m by 2.

putc(..., stdout);

putc( is a function to print a single character to a file (in this case, to stdout)

"\n12345"[!!(96221 & m)   !!(26520 & m)   !!(18192 & m)   !!(1536 & m)   !!(1024 & m)]

This line computes the character to print. "\n12345" is an array of characters to choose from, and then the text inside the [brackets] is computing an index (between 0 and 5) into that array; the character at that index will be the one that is printed.

The !! prefixes are double-applications of the boolean not-operator, used to convert the value in parentheses to 1 if it was non-zero and to 0 if it was zero.

The (blah & m) pieces are checking the given integer-constant to see if a particular bit is set in that constant, or not. If the particular bit is set, 1 is added to the sum that will be used to calculate the index into the array. I don't think I'm smart enough to give more details on why those particular numeric values were chosen :)