Error occurred when I use variables in the shift operation of C

Question

Last night, my friend tell me a situation as following shows:

in c/c++ ,we write two lines code like this:

int a = 0;
printf("%d\n", 1 << -1);
printf("%d\n", 1 << (a-1));

It is amazing there are two different results, previous is 0, and the other is -2147483648.

We do not know why, and anyone can give me a help.

any word is welcome.

Answer 1

Having a negative operand on the right hand side of a left-shift operator is undefined behavior in C. You shouldn't be amazed at any results from doing it.