EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, sub code=0x0). error while making data structure - swift

Question

I am practicing my array form of data structure with swift.
I made a class "student"
and there are functions like display() and delete()
However, the application is not working.
There is an error message that
EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, sub code=0x0).
I think this error is about "optional" problem.
Here is my code.
class student
{
var studentArray = [[String]?]()
var numberOfStudents : Int = 10;
func display()
{
for (var i = 0; i < numberOfStudents ; i++)
{
print("{");
for (var j = 0; j < 2; j++)
{
print(studentArray[i]![j] + " ");
}
print("}");
}
}
func delete( value : String)
{
var i = 0
for ( i = 0; i < numberOfStudents ; i++)
{
if (value == studentArray[i]![1])
{
break;
}
}
if (i == numberOfStudents - 1 )
{
print("not found");
}
else
{
for (var k = i; k < numberOfStudents - 1 ; k++)
{
studentArray[k]![1] = studentArray[k+1]![1];
studentArray[k]![0] = studentArray[k+1]![0];
}
numberOfStudents--;
}
}
}
var hello = student()
hello.studentArray = [["0","0ee"],["9","9ee", ]]
hello.display() // I have a error at this point
hello.studentArray
Could anyone explain what is about it for me?

Answer 1

There are several mistakes in your code. The actual error is caused by your numberOfStudents variable, which is hard coded to 10, even though the array only contains 2 elements. Use studentArray.count in your for loop, not 10. Then read the Swift manual. You should not be using optionals nor C-style for loops in this example.
Here's how I would do it...
class Student { // Capitalise your classes
// Unnecessary whitespace removed
var studentArray: [[String]] = [] // No need for optionals here
/*
var numberOfStudents : Int = 10; // var is useless & wrong, also no need for semi-colon
*/
func display() {
/* A Swift-ier way to do this is
for student in studentArray {
print("{")
for field in student {
print(field + " ")
}
print("}")
}
However, using indexing:
*/
for i in 0 ..< studentArray.count {
print("{")
for j in 0 ..< studentArray[i].count { // Don't *know* this will be 2
print(studentArray[i][j] + " ") // Don't need semi-colons unless you want to put multiple statements on the same line
}
print("}")
}
}
/* func delete() not used in question, so removed from answer */
}
var hello = Student()
hello.studentArray = [["0","0ee"], ["9","9ee", ]] // Note spurious (but not wrong) comma
hello.display()
hello.studentArray