operator-precedence | 易学教程

Is Python's order of evaluation of function arguments and operands deterministic (+ where is it documented)?

阅读更多关于 Is Python's order of evaluation of function arguments and operands deterministic (+ where is it documented)?

问题 C doesn\'t guarantee any evaluation order so a statement like f(g1()+g2(), g3(), g4()) might execute g1() , g2() , g3() , and g4() in any order (although f() would be executed after all of them) What about Python? My experimentation for Python 2.7 shows that it appears to be left-to-right order of evaluation but I wonder if this is specified to be the case. Test program: def somefunc(prolog, epilog): print prolog def f(a, b, *args): print epilog return f def f(text): print text return 1

Why does 1+++2 = 3?

阅读更多关于 Why does 1+++2 = 3?

问题 How does Python evaluate the expression 1+++2 ? How many ever + I put in between, it is printing 3 as the answer. Please can anyone explain this behavior And for 1--2 it is printing 3 and for 1---2 it is printing -1 回答1: Your expression is the same as: 1+(+(+2)) Any numeric expression can be preceded by - to make it negative, or + to do nothing (the option is present for symmetry). With negative signs: 1-(-(2)) = 1-(-2) = 1+2 = 3 and 1-(-(-2)) = 1-(2) = -1 I see you clarified your question to

Why is a Dictionary “not ordered”?

阅读更多关于 Why is a Dictionary “not ordered”?

I have read this in answer to many questions on here. But what exactly does it mean? var test = new Dictionary<int, string>(); test.Add(0, "zero"); test.Add(1, "one"); test.Add(2, "two"); test.Add(3, "three"); Assert(test.ElementAt(2).Value == "two"); The above code seems to work as expected. So in what manner is a dictionary considered unordered? Under what circumstances could the above code fail? Jon Skeet Well, for one thing it's not clear whether you expect this to be insertion-order or key-order . For example, what would you expect the result to be if you wrote: var test = new Dictionary

Operator precedence in Scala

阅读更多关于 Operator precedence in Scala

I like Scala's propose of operator precedence but in some rare cases, unmodified rules may be inconvenient, because you have restrictions in naming your methods. Are there ways to define another rules for a class/file, etc. in Scala? If not, would it be resolved in the future? Thomas Jung Operator precedence is fixed in the Scala Reference - 6.12.3 Infix Operations by the first character in the operator. Listed in increasing order of precedence: (all letters) | ^ & = ! < > : + - * / % (all other special characters) And it's not very probable that it will change. It will probably create more

Reordering factor gives different results, depending on which packages are loaded

阅读更多关于 Reordering factor gives different results, depending on which packages are loaded

问题 I wanted to create a barplot in which the bars were ordered by height rather than alphabetically by category. This worked fine when the only package I loaded was ggplot2. However, when I loaded a few more packages and ran the same code that created, sorted, and plotted my data frame, the bars had reverted to being sorted alphabetically again. I checked the data frame each time using str() and it turned out that the attributes of the data frame were now different, even though I\'d run the same

Why does the expression a = a + b - ( b = a ) give a sequence point warning in c++?

阅读更多关于 Why does the expression a = a + b - ( b = a ) give a sequence point warning in c++?

问题 Following is the test code: int main() { int a = 3; int b = 4; a = a + b - (b = a); cout << \"a :\" << a << \" \" << \"b :\" << b << \"\\n\"; return 0; } Compiling this gives the following warning: > $ g++ -Wall -o test test.cpp test.cpp: In function ‘int main()’: > test.cpp:11:21: warning: operation on ‘b’ may be undefined > [-Wsequence-point] Why can the operation be undefined? According to my understanding, first the subexpression (b = a) should be evaluated because of higher precedence of

SQL UPDATE order of evaluation

阅读更多关于 SQL UPDATE order of evaluation

问题 What is the order of evaluation in the following query: UPDATE tbl SET q = q + 1, p = q; That is, will \"tbl\".\"p\" be set to q or q + 1 ? Is order of evaluation here governed by SQL standard? Thanks. UPDATE After considering Migs\' answer, I ran some tests on all DBs I could find. While I don\'t know what the standard says, implementations vary. Given CREATE TABLE tbl (p INT NOT NULL, q INT NOT NULL); INSERT INTO tbl VALUES (1, 5); -- p := 1, q := 5 UPDATE tbl SET q = q + 1, p = q; I found

Logical AND, OR: Is left-to-right evaluation guaranteed?

阅读更多关于 Logical AND, OR: Is left-to-right evaluation guaranteed?

问题 Is left-to-right evaluation of logical operators ( && || ) guaranteed? Let\'s say I have this: SDL_Event event; if (SDL_PollEvent(&event)) { if (event.type == SDL_QUIT) { // do stuff } } Is this guaranteed to be the same as this? SDL_Event event; if (SDL_PollEvent(&event) && event.type == SDL_QUIT) { // do stuff } This can also be very important, let\'s say we have two requirements, a and b . Requirement a is much more likely to fail then b . Then it\'s more efficient to say if (a && b) than

C# conditional AND (&&) OR (||) precedence

阅读更多关于 C# conditional AND (&&) OR (||) precedence

问题 We get into unnecessary coding arguments at my work all-the-time. Today I asked if conditional AND (&&) or OR (||) had higher precedence. One of my coworkers insisted that they had the same precedence, I had doubts, so I looked it up. According to MSDN AND (&&) has higher precedence than OR (||). But, can you prove it to a skeptical coworker? http://msdn.microsoft.com/en-us/library/aa691323(VS.71).aspx bool result = false || true && false; // --> false // is the same result as bool result =

In Java, what are the boolean “order of operations”?

阅读更多关于 In Java, what are the boolean “order of operations”?

问题 Let\'s take a simple example of an object Cat . I want to be sure the \"not null\" cat is either orange or grey. if(cat != null && cat.getColor() == \"orange\" || cat.getColor() == \"grey\") { //do stuff } I believe AND comes first, then the OR. I\'m kinda fuzzy though, so here are my questions: Can someone walk me through this statement so I\'m sure I get what happens? Also, what happens if I add parentheses; does that change the order of operations? Will my order of operations change from