Why am I getting this NoSuchMethodError? - dart

Question

I have this code
String encrypt(String x) {
String out;
var _x = x.codeUnits;
List dict;
/* <dict_assignment> */
dict[0] = 'a';
dict[1] = 'b';
dict[2] = 'c';
dict[3] = 'd';
dict[4] = 'e';
dict[5] = 'f';
dict[6] = 'g';
dict[7] = 'h';
dict[8] = 'i';
dict[9] = 'j';
/* </dict_assignment> */
_x.toList().forEach((i) {
var _i = i.toString();
_i.split("").forEach((k) {
var _k = int.parse(k);
print(_k);
print(dict[_k]);
out += dict[_k];
});
});
return out;
}
(Yes I'm writing HTML tags as comments in Dart...sue me)
(Idk why my indentations are messed up)
For some reason when I use this same function with a random string like this
var x = encrypt("hmm interesting");
I keep getting this
Unhandled exception:
NoSuchMethodError: The method '[]=' was called on null.
Receiver: null
Tried calling: []=(0, "a")
#0 Object.noSuchMethod (dart:core-patch/object_patch.dart:54:5)
Please help me I'm actually confused why this is happening

Answer 1

You have not initialized your dict variable, so it contains null.
If you change List dict; to List dict = []; then that would start working.
You also haven't initialized out.
The remainder of the code is leaning towards being overly complicated, and can be optimized as well. Here is a suggestion:
String encrypt(String x) {
var out = StringBuffer();
const dict = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j'];
for (var i in x.codeUnits) { // x.codeUnits is a list. Use for-in to iterate it.
for (var k in i.toString().codeUnits) {
var _k = k ^ 0x30; // Best way to convert code unit for 0-9 into integer 0-9.
// print(_k);
// print(dict[_k]);
out.write(dict[_k]); // Use a StringBuffer instead of repeated concatenation.
}
}
return out.toString();
}
It does not appear to be a decryptable encryption. The string "77" and the string "ᖳ" (aka "\u15b3") both encrypt to "ffff".
Or, if you want to "code-golf" rather than be readable or close to the original, it can also be a one-liner:
String encrypt(String x) => [
for (var i in x.codeUnits)
for (var k in "$i".codeUnits) "abcdefghij"[k ^ 0x30]
].join("");