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exam 2019-08-26, moved some old news
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| Original file line number | Diff line number | Diff line change |
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| @@ -0,0 +1,7 @@ | ||
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| def inc(x): | ||
| #jupman-raise | ||
| return x + 1 | ||
| #/jupman-raise |
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| @@ -0,0 +1 @@ | ||
| from B1_solution import * |
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| @@ -0,0 +1,163 @@ | ||
| DEBUG = True | ||
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| def debug(msg): | ||
| if DEBUG: | ||
| print("DEBUG: ", msg.replace('\n', '\n' + (' '*8))) | ||
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| class Backpack: | ||
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| def __init__(self, max_weight): | ||
| """ Creates a Backpack with given max_weight. | ||
| - if max_weight is negative, raises ValueError | ||
| """ | ||
| if max_weight < 0: | ||
| raise ValueError("Expected a non-zero weight, got instead: %s " % max_weight) | ||
| self._elements = [] | ||
| self._max_weight = max_weight | ||
| #jupman-raise | ||
| self._weight = 0 | ||
| #/jupman-raise | ||
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| def size(self): | ||
| """ RETURN the number of items in the backpack | ||
| - MUST run in O(1) | ||
| """ | ||
| #jupman-raise | ||
| return len(self._elements) | ||
| #/jupman-raise | ||
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| def max_weight(self): | ||
| """ Return the maximum allowed weight | ||
| """ | ||
| return self._max_weight | ||
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| def weight(self): | ||
| """ Return the backpack total current weight | ||
| ************ MUST RUN IN O(1) *************** | ||
| """ | ||
| #jupman-raise | ||
| return self._weight | ||
| #/jupman-raise | ||
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| def is_empty(self): | ||
| """ RETURN True if the backpack empty, False otherwise | ||
| - MUST run in O(1) | ||
| """ | ||
| #jupman-raise | ||
| return len(self._elements) == 0 | ||
| #/jupman-raise | ||
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| def __str__(self): | ||
| """ Return a string like | ||
| Backpack: weight=8 max_weight=10 elements=[('a',5), ('b',3)] | ||
| """ | ||
| #jupman-raise | ||
| return "Backpack: weight=%s max_weight=%s\n elements=%s" % (self._weight, self._max_weight,self._elements) | ||
| #/jupman-raise | ||
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| def peek(self): | ||
| """ RETURN the top element in the stack (without removing it!) | ||
| - if stack is empty, raise IndexError | ||
| - Must run in O(1) | ||
| """ | ||
| #jupman-raise | ||
| if len(self._elements) == 0: | ||
| raise IndexError("Empty backpack !") | ||
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| return self._elements[-1] | ||
| #/jupman-raise | ||
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| def push(self, item, w): | ||
| """ Adds item of weight w on the top of the backpack. | ||
| - if w is negative, raises ValueError | ||
| - if w is heavier than topmost item, raises ValueError | ||
| - if max_weight is exceeded, raises ValueError | ||
| - MUST run in O(1) | ||
| """ | ||
| #jupman-raise | ||
| debug("Pushing (%s,%s)" % (item, w)) | ||
| if w < 0: | ||
| raise ValueError("Expected a non-negative number, got instead %s" % w) | ||
| candidate_weight = self._weight + w | ||
| if not self.is_empty(): | ||
| if w > self.peek()[1]: | ||
| raise ValueError("Pushing weight greater than top element ! %s > %s", (w, self.peek()[1])) | ||
| if candidate_weight > self._max_weight: | ||
| raise ValueError("Can't exceed max_weight ! (%s > %s)" % (candidate_weight, self._max_weight)) | ||
| self._weight = candidate_weight | ||
| self._elements.append((item, w)) | ||
| #/jupman-raise | ||
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| def pop(self): | ||
| """ Removes the top element in the backpack and RETURN it | ||
| as a tuple (element_id, weight) like ('a', 3) | ||
| - if backpack is empty, raise IndexError | ||
| - MUST run in O(1) | ||
| """ | ||
| #jupman-raise | ||
| if len(self._elements) == 0: | ||
| raise IndexError("Empty stack !") | ||
| else: | ||
| debug("Popping %s " % str(self._elements[-1])) | ||
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| el = self._elements.pop() | ||
| self._weight -= el[1] | ||
| return el | ||
| #/jupman-raise | ||
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| # NOTE: this function is implemented *outside* the class ! | ||
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| def remove(backpack, el): | ||
| """ | ||
| Remove topmost occurrence of el found in the backpack, | ||
| and RETURN it (as a tuple name, weight) | ||
| - if el is not found, raises ValueError | ||
| - DO *NOT* ACCESS DIRECTLY FIELDS OF BACKPACK !!! | ||
| Instead, just call methods of the class! | ||
| - MUST perform in O(n), where n is the backpack size | ||
| - HINT: To remove el, you need to call Backpack.pop() until | ||
| the top element is what you are looking for. You need | ||
| to save somewhere the popped items except the one to | ||
| remove, and then push them back again. | ||
| """ | ||
| #jupman-raise | ||
| rem = [] | ||
| ret = None | ||
| while not backpack.is_empty(): | ||
| tup = backpack.peek() | ||
| if tup[0] == el: | ||
| ret = backpack.pop() | ||
| break | ||
| else: | ||
| rem.append(backpack.pop()) | ||
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| # restores backpack | ||
| while len(rem) != 0: | ||
| restored = rem.pop() | ||
| backpack.push(restored[0], restored[1]) | ||
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| if ret: | ||
| return ret | ||
| else: | ||
| raise ValueError("Couldn't find element %s" % el) | ||
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| #/jupman-raise | ||
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