comparision strings from struct in if statement - c++

Question

I wrote a c++ code and used struct, I want to compare two string in struct type.but error occures:
#include <iostream>
using namespace std;
#define NumOfStudents 2
#define NumOfCourses 3
struct Student{
int stdnum, FieldCode, age;
double average, marks, res[NumOfCourses];
char Fname[20], Lname[20], cmp[20];
};
int main(){
struct Student students[NumOfStudents];
int i, j;
// char cmp[20];
cout << "\t **********************************************************************\n";
cout << "\t *++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++*\n";
cout << "\t *+ FIRST-NAME || LAST-NAME || STUDENT-NUMBER || FIELD-CODE || AGE +*\n";
cout << "\t *++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++*\n";
cout << "\t **********************************************************************\n";
for(i=0; i<NumOfStudents; i++){
cout << "STUDENT #" << i+1 << ": ";
cin >> students[i].Fname >> students[i].Lname >> students[i].stdnum >> students[i].FieldCode >> students[i].age;
}
cout << "what student do you want?[Enter family]\n";
cin >> students[j].cmp;
for(i=0; i<NumOfStudents; i++){
if(students[j].cmp == students[j].Lname){ //The error occurs here
for(j=0; j<NumOfCourses; j++){
cout << "\nCOURCE #" << j+1 << ": ";
cin >> students[j].marks;
}
}
else
cout << "The Student with name " << students[j].cmp << " doesn't exist!!!";
}
return 0;
}
In the code I commented place the error occurred...
thank you

Answer 1

Of course this comparison
if(students[j].cmp == students[j].stdnum){
is invalid because data member cmp has type char[20] and stdnum has type int. It is not clear what you are going to compare.
EDIT: I see you updated your post and wrote
if(students[j].cmp == students[j].Lname){
However this statement is also invalid because arrays have no the comparison operator. In this condition the both arrays are converted to pointers to their first elements and the pointers are compared. So the condition will be equal always to false. Instead you have to use strandard C function strcmp
if( std::strcmp( students[j].cmp, students[j].Lname ) == 0 ){
It would be simpler if you would use std::string instead of character arrays. You need to include header <string>
In this case the structure will look the following way
struct Student{
int stdnum, FieldCode, age;
double average, marks, res[NumOfCourses];
string Fname, Lname, cmp;
};
and you could use the comparison operator
if( students[j].cmp == students[j].Lname ){

Answer 2

This should do the trick for you.
if(strcmp(students[i].cmp, students[i].Lname) == 0)

Answer 3

If you are sure about using char string, you should use strcmp function to compare them.
(http://www.cplusplus.com/reference/cstring/strcmp/)

Answer 4

You are using a variable j which has no value assigned to it. Because of that, you are accessing an unknown member in the array, probably something off the array's boundaries (an index larger than NumOfStudents).
I think you want if(students[i].cmp == students[i].stdnum){ instead of if(students[j].cmp == students[j].stdnum){, still, it should be noted that you are comparing two char arrays, which does not have your intended effect of comparing the two strings. It will probably compare the position of the arrays in the memory(will always evaluate to false). Instead use strcmp, and use commands from the <cstring> library when dealing with cstrings (char arrays).
Or just save yourself the trouble and use the string class, which overloads the = operator and lets you compare two strings using =.