# Combination several If conditions in a loop

I have a Do loop with several conditions:

Do[If[i < 20, a[i] = i; If[i == 19, a[i] = i + 1], a[i] = i + 3], {i,1, 100}]


How can I rewrite the If condition in the Do loop? Is there any better way to show that?

With Piecewise you can write it in classical mathematical form:

a[i_] = Piecewise[{{i, i <= 18}, {i + 1, i == 19}}, i + 3]


$$\begin{cases} i & i\le18 \\ i+1 & i=19 \\ i+3 & \text{True} \end{cases}$$

Array[a, 100]
(*    {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17,
18, 20, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37,
38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54,
55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71,
72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88,
89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103}    *)


Going further, PiecewiseExpand can be very helpful in combining and simplifying such expressions.

Here's an alternative approach. Instead of a loop

Do[If[i < 20, a[i] = i; If[i == 19, a[i] = i + 1], a[i] = i + 3], {i, 1, 100}]


use pattern matching

aa[i_ /; i == 19] = i + 1;
aa[i_ /; i < 20] = i;
aa[i_ /; i <= 100] = i + 3;

And @@ Table[a[j] == aa[j], {j, 1, 100}]
(* True *)


Sometimes using Which is easier to understand compared to nested If.

Do[
Which[
i == 19, aa[i] = i + 1,
i < 20, aa[i] = i,
True, aa[i] = i + 3],
{i, 1, 100}]

And @@ Table[a[j] == aa[j], {j, 1, 100}]
(* True *)

a=Join[Range,{20},Range[23,103]]

Range@100 /. x_ /; x >= 20 -> x + 3 /. 19 -> 20