Error in floating point comparision

#include<stdio.h>
#include<conio.h>
void main()
{
clrscr();
   float a=0.7;
   if(a<0.7)
   {printf("c");
   }
   else
   {  printf("c++");
   }
   getch();
}

Consider this problem as mentioned above this program runs correctly for the 0.7 but when i change value to 0.8 it behaves in a weird manner and does not produce right result.