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 ) Console . Write ( str [ k ] ) ; else Console . Write ( " ▁ " ) ; } Console . WriteLine ( " " ) ; } }