how can i get the following figure using the for loop

Question

how can i get the following figure using the for loop.

n=5
For[i=1,i<5,i++,For[j=1,j<n,j++,A=(i+j)]]
A//MatrixForm

The code I found is above but not working.where is the mistake?

Answer 1

Welcome to MMA SE! What the code above does is simply repeatedly overwrite the value of A. In each iteration of the loop, it evaluates A = i + j, so the A at the end of the loop is simply i + j for the last i, j in the loop.

You could do a For loop where you initialize A to a table, and then set different parts of A, e.g. A[[i,j]] = "*". That's not advisable in Mathematica, but it would look like this:

n=6;
A = ConstantArray["", {n,n}];
For[i=1,i<=n,i++,For[j=1,j<=i,j++,A[[i,j]] = "*"]];
A // Grid

(Note an important change from the given code: we use j <= i, not j <= n.)

But it's far easier to simply use Table (or Array, for an alternative approach) to generate matrices, with a conditional statement in each entry that test if i < j:

Table[If[i < j, "", "*"], {i, 6}, {j, 6}] // Grid

Another way: you could also use LowerTriangularize on a 6 by 6 ConstantArray of "*", and replace all the resulting 0s in the upper triangle with "":

(LowerTriangularize[ConstantArray["*", {6, 6}]] /. (0 -> "")) // Grid