feat: update single files compile error

single_files/for.zig

@@ -2,7 +2,7 @@ const print = @import("std").debug.print;
 
 pub fn main() void {
   const string = [_]u8{ 'a', 'b', 'c' };
-  for (string) |character, index| {
+  for (string, 0..) |character, index| {
     print("{} - {}\n", .{index, character});
   }
 }

single_files/optionals.zig

@@ -1,7 +1,7 @@
 const print = @import("std").debug.print;
 
 pub fn main() void {
-  var a: ?i32 = null;
-  var b = a orelse 1;
-  print("{} {} \n", .{a, b});
+  const a: ?i32 = null;
+  const b = a orelse 1;
+  print("{?} {} \n", .{a, b});
 }

single_files/while.zig

@@ -4,7 +4,7 @@ const print = std.debug.print;
 pub fn main() void {
   var sum: i32 = 0;
   var i: i32 = 0;
-  while (i <100) : (i += 1) {
+  while (i < 100) : (i += 1) {
     sum += i;
   }
   print("Sum: {}\n", .{sum});