Expressions: Operators and OperandsExpressions as Program InputsExploring an Example Prefix Addition ProgramHow the Prefix Addition Program Can FailExample Program Header FileExample Program Source File / Usage FunctionExample Program Main FunctionExample Program Parsing FunctionMore Generally When Debugging Code May HelpAnother Motivation Debugging “Core” DumpsDebugging on Various PlatformsSome Essential Debugging CommandsCSE 332: C++ expressionsExpressions: Operators and Operands•Operators obey arity, associativity, and precedenceint result = 2 * 3 + 5; // assigns 11•Operators are often overloaded for different typesstring name = first + last; // concatenation•An lvalue gives a location; an rvalue gives a value–Left hand side of an assignment must be an lvalue–Prefix increment and decrement take and produce lvalues–Posfix versions (and &) take lvalues, produce rvalues•Beware accidentally using the “future equivalence” operator, e.g., if (i = j) instead of if (i == j)•Avoid type conversions if you can, and only use named casts (if you must explicitly convert types)CSE 332: C++ expressionsExpressions as Program Inputs•E.g., what if we wanted prefix arithmetic, not infix./prefix_adder + 8 + 9 10•Can write a program to parse prefix expressions+ 8 + 9 10 + + 8 9 10•These are equivalent to their infix versions(8 + (9 + 10)) ((8 + 9) + 10)•Key idea: walk through expression, calculate value++891012 34 5++8910123 45same resultdifferent orderCSE 332: C++ expressions Exploring an Example Prefix Addition Program•A debugger helps us observe a program’s behavior–Step through (and into) functions–Watching the call stack and variable values•But, before we start using the fancy tools…–What do we expect the program to do?–How might the program fail?–Can we make predictions and test them?•Thinking: the most powerful way to explore or debug–Scientific method should guide what you do•Hypothesis  prediction  experiment  analysis (and repeat)–Tools can help you follow this disciplined approach fasterCSE 332: C++ expressionsHow the Prefix Addition Program Can Fail•Too few arguments in expression./prefix_adder + 8 + 9•Cannot calculate result+ 8 + 9 (needs another value to finish 2nd + operation)•Try this on your own in the studio, for practice++8912 34 ???CSE 332: C++ expressionsExample Program Header File // prefix_adder.h//// author: Chris Gill [email protected]//// purpose: Declarations for a simple prefix adder program, which// takes the command line arguments as a prefix addition // expression and computes an integer result.#ifndef PREFIX_ADDER_H#define PREFIX_ADDER_H// Function prototypes.void usage (char * program_name);int parse_and_compute (int & current_index, int last_index, char *argv[]);#endif /* PREFIX_ADDER_H */CSE 332: C++ expressionsExample Program Source File / Usage Function// prefix_adder.cc//// author: Chris Gill [email protected]//// purpose: definitions for a simple prefix adder program, which// takes the command line arguments as a prefix addition // expression and computes an integer result.#include "prefix_adder.h"#include <iostream> // For std output stream and manipulators.#include <string> // For standard C++ strings.#include <sstream> // For standard string streams.#include <cstring> // For C-style string functions// Helper function to print out the program's usage message.void usage (char * program_name) { cout << "Usage: " << program_name << " <argument> [<argument>]..." << endl << "Purpose: computes program arguments as prefix addition expression" << endl;}CSE 332: C++ expressionsExample Program Main Functionint main (int argc, char *argv[]){ // A few useful constants for argument positions const int minimum_arguments = 2; const int starting_index = 1; const int program_name_index = 0; if (argc < minimum_arguments || strcmp (argv[starting_index], "--help") == 0) { usage (argv[program_name_index]); return 1; } try { // Pass the current and last index to use, and the array, to the // expression parsing function, and store the result. int current_position = starting_index; int value = parse_and_compute (current_position, argc - 1, argv); // Print out the result, and return success value. cout << "The value calculated is " << value << endl; return 0; } catch (...) { cout << "caught exception" << endl; return -1; }}CSE 332: C++ expressionsExample Program Parsing Function// Helper function to parse the input symbols and compute a value.int parse_and_compute (int & current_index, int last_index, char *argv[]) { // make sure we're still in the argument range if (current_index > last_index) { throw 1; } // look for a single-symbol addition operator if (strlen (argv[current_index]) == 1 && *(argv[current_index]) == '+') { int first_operand = parse_and_compute (++current_index, last_index, argv); int second_operand = parse_and_compute (current_index, last_index, argv); return first_operand + second_operand; } // treat anything else as an integer else { int result; istringstream i (argv[current_index++]); i >> result; return result; }}CSE 332: C++ expressionsMore Generally When Debugging Code May Help•When your program crashes–Finding out where it crashed–Examining program memory at that point•When a bug occurs in your program–Watching program variables –Identifying where a problem occurs (and why)•Tracing through a program’s behaviors–Learning what a program does–Good basic testing approachCSE 332: C++ expressionsAnother Motivation Debugging “Core” Dumps•A program dumps a memory image when it crashes–creates a file (called “core” on Linux) and then exits–this happens when a program receives certain signals:•Segmentation fault–accessed memory it does not own–or dereferenced a 0 pointer•Bus error–divided by zero–corrupted stack•Debuggers are very useful to examine “core” files–When your program crashes within Visual Studio–On Linux: gdb myprogram coreCSE 332: C++ expressionsDebugging on Various Platforms•We’ll use Visual Studio’s graphical debugger–Much more on this as we go through the semester–Set command line arguments in the project properties –It’s also good to know about common text-based