How to put Break command within a Do loop over 2 variables

Question

Suppose I have the code

Do[If[f[i,j]==1,{output[i]=j,Break},Continue],{i,1,5},{j,1,1000}] 

Where $f$ is just a function.

I want the code to find the first $j$ that works for $i$, then Break and then find the first $j$ that works for the next $i$, until a $j$ has been found for each $i$.

Instead what is happening is the code seems to go through all the $j$’s for each $i$ and then it remembers the last $j$ that works as output[i].

Answer 1

First of all, the correct commands are Break[] and Continue[], you are missing the square brackets.

Secondly, you do not need Continue[].

Thirdly, use ; instead of , to join two expressions.

Lastly, Break[] exits the nearest enclosing Do, so use two separate Do statements:

Do[Do[If[f[i, j] == 1, output[i] = j; Break[]], {j, 1, 1000}], {i, 1, 5}]

Answer 2

More Mathematica-like would be on-demand calculation and memoization:

Clear[output];
output[i_] := output[i] = SelectFirst[Range[1000], f[i, #] == 1 &]

Alternatively, you can try

Clear[output];
output[i_] := output[i] =
  j /. FindInstance[f[i, j] == 1, j, PositiveIntegers, 1][[1]]

which may use better tools than simply iterating over $j$.