literals

问题 This question already has answers here : Closed 10 years ago . Possible Duplicate: Javascript === vs == : Does it matter which “equal” operator I use? What are the differences between === vs == and !== vs != ? When should you use each one? 回答1: === is the Identity operator, and is used to test that value and type are equal. so.. "3" == 3 // true "3" === 3 // false 1 == true // true 1 === true // false "1" == true // true "1" === true // false so when you care that value and type are equal, or

问题 This question already has answers here : “is” operator behaves unexpectedly with integers (11 answers) Closed 2 years ago . Hello I am trying to understand how Python's pass by reference works. I have an example: >>>a = 1 >>>b = 1 >>>id(a);id(b) 140522779858088 140522779858088 This makes perfect sense since a and b are both referencing the same value that they would have the identity. What I dont quite understand is how this example: >>>a = 4.4 >>>b = 1.0+3.4 >>>id(a);id(b) 140522778796184

问题 I stumbled upon this function in an answer to this question: /* Note: I've formatted the code for readability. */ const char * getString() { const char *x = "abcstring"; return x; } I was surprised to discover that returning a pointer to the literal string worked, and didn't segfault like I thought it would. I always assumed that literals were pushed on to the stack or put in some other temporary memory, but where limited to the scope of the function. But here it seems that they are more

问题 My gcc compiler allows me to define an unsigned long long (i.e. 64-bit) literal as #define A_LITERAL 0x1ull --- or --- #define A_LITERAL 0x1llu Is there any difference between these two literal statements. Is this common to other C compilers? 回答1: Both are allowed by the C standard (section 6.4.4.1). The unsigned suffix u can be before or after the long l (or long long ( ll )) suffix. 回答2: Both are the same: excerpt from n3337 draft of C++11 standard: integer-suffix: unsigned-suffix long

问题 It seems that the following technique for checking the existence of an object member produces an error because the 'bar' parent object hasn't been declared before the check, which means I either have to declare it before the check or use two 'typeof' expressions, either of which would be excess code: var foo = {}, newVal = (typeof foo.bar.myVal !== 'undefined' ? foo.bar.myVal : null ); Error: foo.bar is undefined So, how do you check if a member within an undeclared object exists without

问题 r'\' in Python does not work as expected. Instead of returning a string with one character (a backslash) in it, it raises a SyntaxError. r"\" does the same. This is rather cumbersome if you have a list of Windows paths like these: paths = [ r'\bla\foo\bar', r'\bla\foo\bloh', r'\buff', r'\', # ... ] Is there a good reason why this literal is not accepted? 回答1: This is in accordance with the documentation: When an 'r' or 'R' prefix is present, a character following a backslash is included in

问题 why in the below code int *p = 22 will give compile time error and ptr will print the value successfully . int main() { /*taking a character pointer and assigning a string to it*/ char *ptr = "Stackoverflow" ; //correct /*taking a int pointer and assigning a string to it*/ int *p = 22 ; //incorrect printf("%s",ptr); // correct and print printf("%d",p); //incorrect and give compile time error. return 0; } 回答1: If you have a character array as for example char s[] = "Stackoverflow"; then the

问题 In Java 9 we have convenience factory methods to create and instantiate immutable List, Set and Map. However, it is unclear about the specific type of the returned object. For ex: List list = List.of("item1", "item2", "item3"); In this case which type of list is actually returned? Is it an ArrayList or a LinkedList or some other type of List? The API documentation just mentions this line, without explicitly mentioning that its a LinkedList: The order of elements in the list is the same as the

问题 Similar to Is hard-coding literals ever acceptable?, but I'm specifically thinking of "magic strings" here. On a large project, we have a table of configuration options like these: Name Value ---- ----- FOO_ENABLED Y BAR_ENABLED N ... (Hundreds of them). The common practice is to call a generic function to test an option like this: if (config_options.value('FOO_ENABLED') == 'Y') ... (Of course, this same option may need to be checked in many places in the system code.) When adding a new

问题 Given an array of pointers to string literals: char *textMessages[] = { "Small text message", "Slightly larger text message", "A really large text message that " "is spread over multiple lines" } How does one determine the length of a particular string literal - say the third one? I have tried using the sizeof command as follows: int size = sizeof(textMessages[2]); But the result seems to be the number of pointers in the array, rather than the length of the string literal. 回答1: If you want