pascal

(define (pascal row col)
    (cond ((< row 2) 1)
          ((= col 0) 1)
          ((= row col) 1)
          (else (+ (pascal (- row 1) col)
               (pascal (- row 1) (- col 1))))))

(define (pascal-row row)
    (define (pascal->string row col)
        (number->string (pascal row col)))
    (define (col-iter base-str start-col)
        (if (= start-col row)
            (string-append base-str
                           (pascal->string row start-col))
            (col-iter (string-append base-str
                                     (pascal->string row start-col)
                                     " ")
                      (+ start-col 1))))
    (col-iter "" 0))

(define (center-string string size)
    (define num-spaces (floor (/ (- size (string-length string)) 2)))
    (string-append (make-string num-spaces)
                   string))

(define (pascal-triangle centered rows)
    (define cols (string-length (pascal-row (- rows 1))))
    (define (pascal-row-centered row)
        (center-string (pascal-row row) cols))
    (define pascal-row-func (if centered pascal-row-centered pascal-row))
    (define (pt-iter base-str btm-row)
        (if (= btm-row 0)
            (string-append (pascal-row-func btm-row)
                           base-str)
            (pt-iter (string-append "\n"
                                    (pascal-row-func btm-row)
                                    base-str)
                     (- btm-row 1))))
    (pt-iter "" (- rows 1)))