Scheme Lisp compatible atoi imlpementation. Handles negation and trailing zeros.

atoi.rkt

#lang sicp

;; power impl
;; x - number
;; y - power
;; z - preserved base
(define (^-impl x y z) (if (<= y 0)
                    x
                    (^-impl (* z x) (- y 1) z)))

;; power
;; x - number
;; y - power
;; returns:
;; x <= 0 => 1
;; x > 0 => (x^y)
(define (^ x y) (if (<= y 0)
                    1
                    (^-impl x (- y 1) x)))

;; atoi
;; x - carry string as list (expected numeric)
;; y - carry integer
;; z - carry power
(define (atoi-impl x y z) (if (<= (length x) 0)
                              y
                              (atoi-impl (cdr x) (+ y (* (string->number (string (car x))) (^ 10 z))) (- z 1))))

;; skip heading char
;; x - string to list
;; y - string to skip
(define (skip-heading-char x y)
 (if (equal? (string (car x)) y)
     (skip-heading-char (cdr x) y)
     x))

;; atoi
;; x - string
(define (atoi x)
  (let ((negative? (equal? (string (car (string->list x))) "-")))
    (let ((x-list (skip-heading-char (if negative? (cdr (string->list x)) (string->list x)) "0")))
      (* (atoi-impl x-list 0 (- (length x-list) 1)) (if negative? -1 1)))))

(display (atoi "00133742069"))