i and j are the indexes of characters to be displayed in the ith iteration i = 0 initially and go upto length of string j = length of string initially in each iteration of i , we increment i and decrement j , we print character only of k == i or k == j
for ( int i = 0 ; i < len ; i ++ ) { int j = len - 1 - i ; for ( int k = 0 ; k < len ; k ++ ) { if ( k == i k == j ) cout << str [ k ] ; else cout << " ▁ " ; } cout << endl ; } }