comparison-operators

问题 The following program should print whether the sum of the elements of the array is positive or negative: #include <stdio.h> #define ARR_SIZE 5 int main() { int array[ARR_SIZE] = {1,-2,3,4,-5}; unsigned sum; int i; for(i=0, sum=0; i < ARR_SIZE; i++) { sum += array[i]; printf("sum %d\n ", sum); } printf("%d\n",sum); if(sum>-1) printf("non negative\n"); else printf("negative\n"); return 0; } The program doesn't do what it is supposed to; it prints 'negative' no matter what array values it

问题 I had always thought that the standard required the non-specialized template for std::equal_to<T> to call T::operator== , but I noticed the description at cppreference.com almost implies it's the other way around; certainly it doesn't mention it as a requirement. I also checked the C++11 draft standard N3337 and couldn't find any guarantees there either. If you create a class with an operator== you'd hope it would get used in all circumstances. I can't honestly think of a way to implement std

问题 I would like to expose C++ code with a std::vector<A> to python. My class A{}; does not have a comparison operator implemented. When I try BOOST_PYTHON_MODULE(libmyvec) { using namespace boost::python; class_<A>("A"); class_<std::vector<A> >("Avec") .def(boost::python::vector_indexing_suite<std::vector<A> >()); } I get an error about comparison operators. If I change the definition of A to class A { public: bool operator==(const A& other) {return false;} bool operator!=(const A& other)

问题 Under C++ or <stdbool.h> from C99, how is the less-than operator < defined for boolean values? Alternatively, explain the behaviour of this code: #ifndef __cplusplus #include <stdbool.h> #endif #include <stdio.h> int main() { bool b = -1; if(b < true) { printf("b < true\n"); } if(b < false) { printf("b < false\n"); } if(true < false) { printf("true < false\n"); } if(false < true) { printf("false < true\n"); } } Under MSVC version 10, compiled as C++ code, GCC 4.6.3-ubuntu5 compiled as C code

问题 Ruby has something called a Combined Comparison or "Spaceship" Operator, it looks like this: <=> It does the following: a <=> b := if a < b then return -1 if a = b then return 0 if a > b then return 1 Credit Is there a similar Operator in Javascript? If not, how can I end up with the same result? @madox2 suggested using Math.sign(a - b) , which works for number, but not arrays (to compare arrays you need to use array.length ). It also does not work in Internet Explorer, Safari or all Mobile

问题 Why is === faster than == in PHP? 回答1: Because the equality operator == coerces, or converts, the data type temporarily to see if it’s equal to the other operand, whereas === (the identity operator) doesn’t need to do any converting whatsoever and thus less work is done, which makes it faster. 回答2: === does not perform typecasting, so 0 == '0' evaluates to true , but 0 === '0' - to false . 回答3: First, === checks to see if the two arguments are the same type - so the number 1 and the string '1

问题 Is there a way to check if two expressions are mathematically equal? I expected tg(x)cos(x) == sin(x) to output True , but it outputs False . Is there a way to make such comparisons with sympy? Another example is (a+b)**2 == a**2 + 2*a*b + b**2 which surprisingly also outputs False . I found some similar questions, but none covered this exact problem. 回答1: From the SymPy documentation == represents exact structural equality testing. “Exact” here means that two expressions will compare equal

问题 Is there a valid class Type variable that can be used with the instanceof operator? For Example: String s = "abc"; Class<?> classType = String.class; if (s instanceof classType) { //do something } as an alternative to this: if (s.getClass() == classType) { //do something } Would there be any performance benefit? 回答1: What you're doing is not actually the same. Consider what happens with subclasses (I know you can't subclass String , so in the String case it doesn't matter). class A {} class B

问题 A few weeks ago, I have read this thread Is < faster than <=? about comparison operators in C . It was said that there is no difference in the performance between < and <= as they are interpreted as same/similar machine commands. At the same time, in our company's "best practices", it was said that we should always use "===" to compare things instead of "==". So, I started to wonder if this is always appropriate as I am used to using the "==" and "typeof ... == " and do not want to change my

问题 I have the following operator< that is supposed to sort first by a value, then by another value: inline bool operator < (const obj& a, const obj& b) { if(a.field1< b.field1) return true; else return a.field2 < b.field2; } I have the feeling this is incorrect and that you can't do that without another third comparaison test on the members variables, but I can't find any example where this doesn't work. So whould this really sort as expected? thanks edit : I would have coded it as : inline bool