evaluation | 易学教程

How is if statement evaluated in c++?

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问题 Is if ( c ) the same as if ( c == 0 ) in C++? 回答1: No, if (c) is the same as if (c != 0) . And if (!c) is the same as if (c == 0) . 回答2: I'll break from the pack on this one... " if (c) " is closest to " if (((bool)c) == true) ". For integer types, this means " if (c != 0) ". As others have pointed out, overloading operator != can cause some strangeness but so can overloading " operator bool() " unless I am mistaken. 回答3: If c is a pointer or a numeric value, if( c ) is equivalent to if( c !=

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

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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 language to language? The Java Tutorials has a list illustrating operator precedence . The equality operators

C : is there “lazy evaluation” when using && operator, as in C++?

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问题 I would like to know if this looks correct : while((next !=NULL) && (strcmp(next->name, some_string) < 0) { //some process } I mean, if next is NULL , then the second part of the expression won't be ever tested by the compiler? I have heard that in C++ it's the case (but I'm not even sure of it). Can someone confirm me that I won't get strange errors on some compilers with that? 回答1: Yes && is short circuited and you are using it correctly. If next is NULL string compare will never happen.

Dynamic source code in C++ [closed]

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问题 Closed . This question needs to be more focused. It is not currently accepting answers. Want to improve this question? Update the question so it focuses on one problem only by editing this post. Closed 4 years ago . How to process dynamic source code in C++? Is it possible to use something like eval("foo")? I have some functions that need to be called depending on user's choice: void function1 (); void function2 (); ... void functionN (); int main (int argv, char * argv []) { char * myString

Bash: evaluate a mathematical term?

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问题 echo 3+3 How can I evaluate such expressions in Bash, in this case to 6? 回答1: in shells such as zsh/ksh, you can use floats for maths. If you need more maths power, use tools like bc/awk/dc eg var=$(echo "scale=2;3.4+43.1" | bc) var=$(awk 'BEGIN{print 3.4*43.1}') looking at what you are trying to do awk '{printf "%.2f\n",$0/59.5}' ball_dropping_times >bull_velocities 回答2: echo $(( 3+3 )) 回答3: expr is the standard way, but it only handles integers. bash has a couple of extensions, which only

Math Expression Evaluation

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问题 What is the best way to implement a python program that will take a string and will output its result according to operator precedence (for example: "4+3*5" will output 19). I've googled for ways to solve this problem but they were all too complex, and I'm looking for a (relatively) simple one. clarification: I need something slightly more advanced than eval() - I want to be able to add other operators (for example a maximum operator - 4$2 = 4) or, also I am more interested in this

Evaluate string with math operators [duplicate]

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This question already has an answer here: Evaluating string “3*(4+2)” yield int 18 [duplicate] 14 answers Is there an easy way to evaluate strings like "(4+8)*2" So that you'd get the int value of 24? Or is there a lot of work needed to get this done...? manojlds Use Ncalc: Expression e = new Expression("(4+8)*2"); Debug.Assert(24 == e.Evaluate()); http://ncalc.codeplex.com/ Also, this question had been previously asked and has some interesting answers including Ncalc : Evaluating string "3*(4+2)" yield int 18 Someone else added this and then it got deleted. I thought it was pretty cool

Best and shortest way to evaluate mathematical expressions

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There are many algorithms to evaluate expressions, for example: By Recursive Descent Shunting-yard algorithm Reverse Polish notation Is there any way to evaluate any mathematical expression using C# .net reflection or other modern .net technology? Further to Thomas's answer, it's actually possible to access the (deprecated) JScript libraries directly from C#, which means you can use the equivalent of JScript's eval function. using Microsoft.JScript; // needs a reference to Microsoft.JScript.dll using Microsoft.JScript.Vsa; // needs a reference to Microsoft.Vsa.dll // ... string expr = "7 + (5

Why does 1+++2 = 3?

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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

SQL UPDATE order of evaluation

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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