Material to Understand

• The dotimes construct. In the example below, there is a list containing 3 atoms to the right of dotimes: (1) i, the loop index variable, initialized to 0, (2) 10, the upper bound for i, when i equals 10, the loop terminates, and (3) sum, an optional return value for the loop.

    (setf sum 0)      ;; declare as a local variable above!
    (dotimes (i 10 sum)
      (setf sum (+ sum i))
    )
  

• The dolist construct. In the example below, there is a list containing 3 atoms to the right of dolist: (1) item, the loop index variable, (2) the list '(1 2 3) of which item successively takes each value, and (3) sum, an optional return value for the loop.

    (set sum 0)       ;; declare as a local variable above!
    (dolist (item '(1 2 3) sum)
      (setf sum (+ sum item))
    )
  

• The do construct.

    (do      ;; abstract version 
             ;; INITIAL-VALUE is optional
             ;; UPDATE-TECHNIQUE is optional
             ;; INTERMEDIATE-FORMS are optional
             ;; RESULT is optional
      (                      
        (local-variable-1 initial-value update-technique)
             ...
        (local-variable-n initial-value update-technique)
      )
      ( termination-test intermediate-forms result ) 
        loop-body
    )
  
    (do          ;; concrete example
      (
        (sum 0)
        (counter 0 (+ counter 1))
      ) 
      ( (>= counter 10) sum )
        (setf sum (+ sum counter))
    )
  

• The do* construct. It is identical to do, except local variables are assigned values sequentially instead of in parallel. This is very similar to the difference between let and let*.

Exercises

1. Write a function that computes the Fibonacci numbers using dotimes. Develop this function in a file.
2. Compile the function and run the compiled version.
3. Write a function that computes the Fibonacci numbers using do.
4. Write a function that computes the Fibonacci numbers using do*.
5. Use dolist to count the number of cs courses that appear in a list. For example, (count-cs '((cs 436)(cs 400)(cs 222))) should return a 3.
6. Repeat the above question using count-if.