format - 漂亮地打印固定宽度字段中的值

Question

我想在固定宽度字段中漂亮地打印一个由列名和值组成的矩阵。这是我写的代码:

(defstruct
  (matrix
    (:print-function (lambda (my-matrix stream depth)
                     (declare (ignore depth))
                     (let* ((dimensions (matrix-dimensions my-matrix))
                            (column-names (matrix-column-names my-matrix))
                            (values (matrix-values my-matrix))
                            (number-of-rows (nth 0 dimensions))
                            (number-of-columns (nth 1 dimensions)))
                       (progn (loop :for i :upto number-of-columns
                                 :do (if (< i number-of-columns)
                                         (format stream (nth i column-names))
                                         (format stream "~%")))
                              (loop :for j :upto (1- number-of-rows)
                                 :do (loop :for k :upto number-of-columns
                                        :do (if (< k number-of-columns)
                                                (format stream (write-to-string (aref values j k)))
                                                (format stream "~%")))))))))
  dimensions
  column-names
  values)

但是，如果我运行此代码，我会得到以下结果:

CL-USER> (make-matrix :dimensions '(3 2)
                  :column-names '("Michael" "Alexandra")
                  :values (make-array '(3 2) :initial-contents '((54 34) (65 78) (49 38))))
MichaelAlexandra
5434
6578
4938

如何编辑我的 :print-function (更具体地说是它的格式部分)，以便列名和值都整齐地打印在固定宽度字段中？

我读到有一个 ~w 指令的格式用于此目的，但并没有真正理解它是如何工作的。

Answer 1

在 format 指令中使用 "~NN":

(defstruct
  (matrix
    (:print-function (lambda (my-matrix stream depth)
                       (declare (ignore depth))
                       (let* ((dimensions (matrix-dimensions my-matrix))
                              (column-names (matrix-column-names my-matrix))
                              (values (matrix-values my-matrix))
                              (number-of-rows (nth 0 dimensions))
                              (number-of-columns (nth 1 dimensions)))
                         (loop :for i :upto number-of-columns
                           :do (if (< i number-of-columns)
                                   (format stream "~10s" (nth i column-names))
                                   (terpri stream)))
                         (loop :for j :upto (1- number-of-rows)
                           :do (loop :for k :upto number-of-columns
                                 :do (if (< k number-of-columns)
                                         (format stream "~10s" (aref values j k))
                                         (terpri stream))))))))
  dimensions
  column-names
  values)

PS。 let 中不需要 progn