{"record":{"id":"c04480264ca77a37","repo":"TheAlgorithms/C-Sharp","slug":"invalid-postfix-expression-leftover-operands","errorCode":null,"errorMessage":"Invalid postfix expression: Leftover operands.","messagePattern":"Invalid postfix expression: Leftover operands\\.","errorType":"exception","errorClass":"InvalidOperationException","httpStatus":null,"severity":"error","filePath":"Algorithms/Stack/InfixToPostfix.cs","lineNumber":85,"sourceCode":"\n                if(char.IsDigit(ch))\n                {\n                    stack.Push(ch - '0');\n                    continue;\n                }\n\n                if (IsOperator(ch))\n                {\n                    EvaluateOperator(stack, ch);\n                    continue;\n                }\n\n                throw new InvalidOperationException($\"Invalid character in expression: {ch}\");\n            }\n\n            if (stack.Count != 1)\n            {\n                throw new InvalidOperationException(\"Invalid postfix expression: Leftover operands.\");\n            }\n\n            return stack.Pop();\n        }\n\n        private static void ProcessInfixCharacter(char c, Stack<char> stack, StringBuilder postfixExpression)\n        {\n            if (IsOperand(c))\n            {\n                postfixExpression.Append(c);\n                return;\n            }\n\n            if (c == '(')\n            {\n                stack.Push(c);\n                return;\n            }","sourceCodeStart":67,"sourceCodeEnd":103,"githubUrl":"https://github.com/TheAlgorithms/C-Sharp/blob/96e2905cab7bc6b33ac0a34ee5bb82ddccbcbb6c/Algorithms/Stack/InfixToPostfix.cs#L67-L103","documentation":"PostfixExpressionEvaluation throws InvalidOperationException when, after consuming all tokens, the stack does not hold exactly one result. This means the string was not a valid postfix expression — too many operands or missing operators.","triggerScenarios":"Evaluating strings like \"2 3 4\" (operands without enough operators) or \"2 3 + 5\" where one operand remains unconsumed.","commonSituations":"Hand-written postfix with a typo; concatenating outputs of multiple conversions; truncating a valid expression.","solutions":["Check the token count: a valid postfix expression of n binary operators needs n+1 operands.","Generate postfix programmatically via InfixToPostfixConversion instead of writing it by hand.","Catch InvalidOperationException and validate the expression shape before retrying."],"exampleFix":"// before\nint r = evaluator.PostfixExpressionEvaluation(\"2 3 4\");\n// after\nint r = evaluator.PostfixExpressionEvaluation(\"2 3 4 +\"); // becomes (3+4) with 2? use \"2 3 + 4 -\" etc.","handlingStrategy":"validation","validationCode":"int operands = postfix.Count(char.IsLetterOrDigit); int operators = postfix.Count(c => \"+-*/^\".Contains(c)); bool valid = operands == operators + 1;","typeGuard":null,"tryCatchPattern":"try { int r = evaluator.PostfixExpressionEvaluation(postfix); } catch (InvalidOperationException) { /* expression is not valid postfix */ }","preventionTips":["Check operand/operator count relation before evaluation","Generate postfix programmatically rather than by hand","Add round-trip tests: infix -> postfix -> evaluate"],"tags":["postfix","evaluation","malformed-input"],"backgroundTag":"invalid-argument-format","analyzedSha":"96e2905cab7bc6b33ac0a34ee5bb82ddccbcbb6c","analyzedAt":"2026-09-13T17:04:01.438Z","contentChangedAt":"2026-09-13T17:04:01.438Z","schemaVersion":2},"datasetVersion":"2026-09-16T04:17:20.429Z"}