I write a short code in c

it may work well, but when I add an if ... else ... structure, a computation before the structure can not give a correct result

compilation and run can do well, but the result is wrong. Very strange

(1)
y2 = expression; //y2 = 0 always 0 error
.................

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


(2)

y2 = expression; //y2 = 0 always 0 error
.................

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


(3)
y2 = expression; // y2 = normal value; normal
.................

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


(4)

y2 = expression; // y2 = normal value; normal
.................

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

(5)
y2 = expression; // y2 = normal value; normal

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

Trouble: can not use if... else ... normally

Question: (1) if... else ... can influence a previous statement, why?
(2) How to solve the problem?