PTA counts student performance by grade
int set_grade( struct student *p, int n ){
int i,count=0;
for(i=0;i<n;i++){
if((*p).score<60){
(*p).grade='D';
count++;
}else if((*p).score<70){
(*p).grade='C';
}else if((*p).score<85){
(*p).grade='B';
}else if((*p).score<=100){
(*p).grade='A';
}
p++;
}
return count;
}