counting number of outputs in For loop

Question

I am using For loop and If within it. For example:

For[a=0, a < 21, a = a + 1,
 If[EvenQ[a], Print[a,"\n"]]
]

Certainly, the program will print all even numbers below 21 starting from 0. I would like to Print the output along with counting number, like:

1. 0 
2. 2
3. 4

etc.

I will be glad if you suggest me some command for the same.

Answer 1

i = 1;
For[a = 0, a < 21, a = a + 1, If[EvenQ[a], Print[i++, ". ", a]]]

1. 0
2. 2
3. 4

(and so on)

Another style of coding for your consideration:

Cases[Range[0, 21], _?EvenQ]

TableForm[%, TableHeadings -> Automatic]

See also Sow and Reap, e.g. Creating a table/Matrix during a For loop

Answer 2

I suggest reading this Q and A: Alternatives to procedural loops and iterating over lists in Mathematica as it is instructive and useful.

Just some variants to illustrate Sow and Reap and MapIndexed:

Reap[(i = 1; 
    For[a = 0, a < 21, If[EvenQ@a, Sow[{i++, a}]]; a = a + 1])][[-1, 
   1]] // TableForm

or

(i = 1); Range[0, 21] /. {x_?EvenQ :> {i++, x}, 
   x_?OddQ :> Sequence[]} // TableForm

or

MapIndexed[{#2[[1]], #1} &, Range[0, 21, 2]] // TableForm

There are many ways to do things. That is part of the fun.