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Task/Parsing-RPN-calculator-algorithm/Nim/parsing-rpn-calculator-algorithm.nim

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+import math, rdstdin, strutils, tables
+
+type Stack = seq[float]
+
+proc lalign(s, x): string =
+  s & repeatChar(x - s.len, ' ')
+
+proc opPow(s: var Stack) =
+  let b = s.pop
+  let a = s.pop
+  s.add a.pow b
+
+proc opMul(s: var Stack) =
+  let b = s.pop
+  let a = s.pop
+  s.add a * b
+
+proc opDiv(s: var Stack) =
+  let b = s.pop
+  let a = s.pop
+  s.add a / b
+
+proc opAdd(s: var Stack) =
+  let b = s.pop
+  let a = s.pop
+  s.add a + b
+
+proc opSub(s: var Stack) =
+  let b = s.pop
+  let a = s.pop
+  s.add a - b
+
+proc opNum(s: var Stack, num) = s.add num
+
+let ops = toTable({"^": opPow,
+                   "*": opMul,
+                   "/": opDiv,
+                   "+": opAdd,
+                   "-": opSub})
+
+proc getInput(inp = ""): seq[string] =
+  var inp = inp
+  if inp.len == 0:
+    inp = readLineFromStdin "Expression: "
+  result = inp.strip.split
+
+proc rpnCalc(tokens): auto =
+  var s: Stack = @[]
+  result = @[@["TOKEN","ACTION","STACK"]]
+  for token in tokens:
+    var action = ""
+    if ops.hasKey token:
+      action = "Apply op to top of stack"
+      ops[token](s)
+    else:
+      action = "Push num onto top of stack"
+      s.opNum token.parseFloat
+    result.add(@[token, action, s.map(proc (x: float): string = $x).join(" ")])
+
+let rpn = "3 4 2 * 1 5 - 2 3 ^ ^ / +"
+echo "For RPN expression: ", rpn
+let rp = rpnCalc rpn.getInput
+
+var maxColWidths = newSeq[int](rp[0].len)
+for i in 0 .. rp[0].high:
+  for x in rp:
+    maxColWidths[i] = max(maxColWidths[i], x[i].len)
+
+for x in rp:
+  for i, y in x:
+    stdout.write y.lalign(maxColWidths[i]), " "
+  echo ""