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POUnreproducible string comparison, forces elsif failure
 

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  1. POUnreproducible string comparison, forces elsif failure
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    <p>In answering <a href="https://stackoverflow.com/questions/928563/code-golf-evaluating-mathematical-expressions">this code golf question</a>, I ran across a problem in my answer.</p> <p>I've been testing this and I cannot even get these two comparisons to work in the code, despite the fact that IRB has the right behavior. I <em>really</em> need some help here.</p> <p>Here's the code, below that will be an explanation of the problem.</p> <pre><code>def solve_expression(expr) chars = expr.split '' # characters of the expression parts = [] # resulting parts s,n = '','' # current characters while(n = chars.shift) if (s + n).match(/^(-?)[.\d]+$/) || (!chars[0].nil? &amp;&amp; chars[0] != ' ' &amp;&amp; n == '-') # only concatenate when it is part of a valid number s += n elsif (chars[0] == '(' &amp;&amp; n[0] == '-') || n == '(' # begin a sub-expression p n # to see what it breaks on, ( or - negate = n[0] == '-' open = 1 subExpr = '' while(n = chars.shift) open += 1 if n == '(' open -= 1 if n == ')' # if the number of open parenthesis equals 0, we've run to the end of the # expression. Make a new expression with the new string, and add it to the # stack. subExpr += n unless n == ')' &amp;&amp; open == 0 break if open == 0 end parts.push(negate ? -solve_expression(subExpr) : solve_expression(subExpr)) s = '' elsif n.match(/[+\-\/*]/) parts.push(n) and s = '' else parts.push(s) if !s.empty? s = '' end end parts.push(s) unless s.empty? # expression exits 1 character too soon. # now for some solutions! i = 1 a = parts[0].to_f # left-most value is will become the result while i &lt; parts.count b,c = parts[i..i+1] c = c.to_f case b when '+': a = a + c when '-': a = a - c when '*': a = a * c when '/': a = a / c end i += 2 end a end </code></pre> <p>The problem occurs in the assignment of <code>negate</code>.</p> <p>I need negate to be true when the character just before a expression is a dash, but the condition isn't even working. Both <code>n == '-'</code> and <code>n[0] == '-'</code>, the form of quotation doesn't matter, wind up FALSE every time. Yet, I've been using this exact comparison and <code>n == '('</code> works correctly every time!</p> <p>What is going on? Why doesn't <code>n == '-'</code> work, when <code>n == '('</code> does? This is encoded in UTF-8 w/o BOM, UNIX linebreaks.</p> <p>What's wrong with my code?</p>
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