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copied!<p>Unfortunately you <code>operator_stack</code> list is empty therefore it returns a <code>IndexError</code> BTW if you want to find the last element of a list use and make sure to make it to <code>None</code> if its empty:</p> <p>So use:</p> <p><code>peek = operator_stack[-1] if operator_stack else None</code></p> <p>instead of: </p> <p><code>peek = operator_stack[len(operator_stack)-1]</code></p> <p>Also when debugging your code it is clearly visible from the comments that these lines in:</p> <p><em>line 49</em> :<code>while not operator_stack and prec[peek] >= prec[element]:</code></p> <p><em>line 59</em> : <code>while not operator_stack:</code></p> <p>should actually look like:</p> <p><em>line 49</em> :<code>while operator_stack and prec[peek] >= prec[element]:</code></p> <p><em>line 59</em> : <code>while operator_stack:</code></p> <p>Finally add a if statement to check if <code>peek</code> is <code>None</code></p> <p>A short version would be</p> <pre><code>#line 18 peek = operator_stack[-1] if operator_stack else None #line 49 if peek is not None: while operator_stack and prec[peek] >= prec[element]: #Append the pop'd element of the operator stack to the #output_queue list. output_queue.append(operator_stack.pop()) #Append whatever is left (+,-,*,/) to the operator stack operator_stack.append(element) #line 59 while operator_stack: #Append the last element in the stack until the stack is empty. output_queue.append(operator_stack.pop()) </code></pre> <p>If you <a href="https://stackoverflow.com/questions/20129563/why-wont-my-list-populate-indexerror-list-out-of-range/20129672#comment30000816_20129672">doubt</a> what <code>while operator_stack:</code> means, see this simple example:</p> <pre><code>>>> a = [2,5,6] >>> while a: # a is not empty right? ... print 2 # so print 2 ... break # and break 2 </code></pre>

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