some easy questions

# include <stdio.h>
void main ()
{
int a,y;
a=10;y=0;
do {
a+=2;
y+=a;
if (y>50)break; //多余的判断 10+2能大于50么？
printf ("a=%d y=%d\n",a,y);
}while (a=14); //12能等于14么？ 就算等于也应该是＝＝不是＝，满足了条件后循环是空的，让人郁闷～
printf ("a=%d y=%d\n",a,y);
}

a=12 y=12
a=16 y=28
a=16 y=44
a=16 y=60
why the result is not 18,60

# include <stdio.h>
void main ()
{
int a,i;
for (a=1,i=-1;-1<=i<1;i++) //这里不对
{
a++;printf ("%2d",a);
}
printf ("%2d",i); //这里无法理解，因为for不对
}


#include<stdio.h>
void main()
{
int i=5;
do {
switch (i%2)
{
case 4:i--;break; 4/2的余数是5，口算都算得出来，2个case在这里没用处
case 6:i--;continue; //case只能用break退出循环
}
i--;i--; //2个i-- ？
printf ("%d",i);
}while (i>0); //whiel条件又是空循环
}

I don't understand at all,I hope you can explain it to me >_<