The mistaken value is assigned to y when execute 'y = expression;'

in gdb, 'p y' will give the mistaken value after runing the above statement.

but ' p expression' gives the right value.

block2 and block3 are like this:

if(a->is_root) c = m2 - y2;
else f = m2 - y2;

As said before:
only one statement: if(a->is_root) c = m2 - y2; // y2 normal
only one statement: if(!a->is_root) f = m2 - y2; // y2 normal

but:

if(a->is_root) c = m2 - y2;
else f = m2 - y2;

or

if(a->is_root) c = m2 - y2;
if(!a->is_root) f = m2 - y2;

both make y2 abnormal. 'y2 = expression' is far away from if__ else __ structure

Thanks for your teaching