# FLUTTER – Problem with List constructor and null safety mode

## Issue

I would like to use this "Levenshtein" function to assess similarities between two strings (to check if user has committed a spelling mistake).

Since I work on null safe mode, it points out an error with the LIST constructor :

``````List<List<int>> d =  List.generate(sa + 1, (int i) =>  List(sb + 1));
``````

What can I write to replace `List(sb+1));` ?

``````int levenshtein(String a, String b) {
a = a.toUpperCase();
b = b.toUpperCase();
int sa = a.length;
int sb = b.length;
int i, j, cost, min1, min2, min3;
int levenshtein;
// ignore: deprecated_member_use
List<List<int>> d =  List.generate(sa + 1, (int i) =>  List(sb + 1));
if (a.length == 0) {
levenshtein = b.length;
return (levenshtein);
}
if (b.length == 0) {
levenshtein = a.length;
return (levenshtein);
}
for (i = 0; i <= sa; i++) d[i][0] = i;
for (j = 0; j <= sb; j++) d[0][j] = j;
for (i = 1; i <= a.length; i++)
for (j = 1; j <= b.length; j++) {
if (a[i - 1] == b[j - 1])
cost = 0;
else
cost = 1;
min1 = (d[i - 1][j] + 1);
min2 = (d[i][j - 1] + 1);
min3 = (d[i - 1][j - 1] + cost);
d[i][j] = min(min1, min(min2, min3));
}
levenshtein = d[a.length][b.length];
return (levenshtein);
}
``````

## Solution

You can use `List.generate` for the inner list as well.

``````List<List<int>> d =  List.generate(sa + 1, (int i) =>  List.generate(sb + 1, (int j) => 0));
``````

Also, if they’re all going to be initialized to 0 you can just do this too:

``````List<List<int>> d = List.filled(sa + 1, List.filled(sb + 1, 0));
``````