C++ :: How To Read Data Of Particular Student And Save It After Modifying

I have to read a particular student data from file using his Roll no and modify it and then save it to file again

This is the File
24 salman 98 97 96 95 94 A ;
21 faizan 88 87 86 83 85 B ;
35 Gohar 99 98 97 96 90 B ;
45 sibghat 91 92 78 85 88 C ;
009 john 89 87 91 78 95 C ;

how can i read data of roll no 35 and modify it and save it again to the file

this is my program

int r = 0 ;
while ( ar[j]!="0" ) {
//cout << "Salman majid is
" ;
if ( ar[j]==k ) {
r = j ;
getdata() ;

[Code] .....