hello everyone,                                i have created a programme which works fine and not in the way intended by problem statement. here is the problem statement copied as it is from HtDP. -- Exercise 11.3.5.   Develop a program that visualizes a student riot. In preparation of a student riot, a small group of students meets to make paint-filled balloons. The typical riot uses <code class="scheme">'red</code> only. Then, on the evening of the riot, the students enter a university's progressive theater with the balloons and throw them all over the seats. The program's only input should be a natural number, which represents the number of balloons thrown. The visualization should use a canvas that contains a black grid and the positions of the balls.Assume a random distribution of the balls over the theater's seats. Each box in the grid represents a seat. Configure the program so the change of one variable definition changes the number of columns in the grid and a change to another changes the number of rows. Hint: Develop auxiliary functions that draw some given number of lines in the vertical and the horizontal direction.  ---- my solution works not with function call :                   (riot n) but with                                                                   (draw-baloons (riot n)) also < riot > consumes only one agument but i had to use radius of disk (red baloons) as an extra in its body as shown down here. may you help me on modifying <riot > according to the problem statement? "arnuld" ---------------------- start of programme ------------------------------------------- (define ROWS 5) (define COLS 5) ;; i created variable definitions of COLS & ROWS because problem wants so ;; hence you can change the values of COLS & ROWS here in variable definitions and effect will be ;; transfered into the variables & functions which use these 2 variables ;; same is for canvas (define canvas-x 300) (define canvas-y 300) (define dist-ROWS (quotient canvas-y ROWS)) (define dist-COLS (quotient canvas-y COLS)) (define radius 5) (define (riot n)   (cond     [(zero? n) empty]     [else (cons radius (riot (sub1 n)))])) (define (draw-balloons a-lon)   (cond     [(empty? a-lon) true]     [else (draw-all a-lon)])) (define (draw-all a-lon)   (and (draw-solid-disk  (make-posn (random canvas-x)                                     (random canvas-y))                          (first a-lon)                          'red)        (sleep-for-a-while 1)        (draw-balloons (rest a-lon)))) ;; ------------ draw columns and rows here ------------------------------- (define (draw-COLS COLS)   (cond     [(zero? COLS) true]     [else (and (draw-solid-line (make-posn (* COLS dist-COLS) 0)                                 (make-posn (* COLS dist-COLS) canvas-y) 'black)                (draw-COLS (sub1 COLS)))])) ;; change the values of COLS & ROWS in variable definitions and effect will be here into these  ;; functions (define (draw-ROWS ROWS)   (cond     [(zero? ROWS) true]     [else (and (draw-solid-line (make-posn 0 (* ROWS dist-ROWS))                                 (make-posn canvas-x (* ROWS dist-ROWS)) 'black)                (draw-ROWS (sub1 ROWS)))])) ;; tests (start canvas-x canvas-y) (draw-COLS COLS) (draw-ROWS ROWS) (draw-balloons (riot 20)) (sleep-for-a-while 1) (stop) ------------------------ end of programme ------------------------------------------ -- "the great intellectuals"