evaluation

How would I get something similar to the following?: (evaluate-text "(+ 1 2)") ; resolves to 3 (load-string "(+ 1 2)") user> (eval (read-string "(+ 1 2)")) 3 You probably shouldn't ever need to do this. Macros and fns make this kind of thing unnecessary 99% of the time. This is quite brittle, and can be unsafe if these strings are coming from user input, and so on. How similar does it have to be? Clojure's eval works on lists, so: (eval (list + 1 2)) #=> 3 来源： https://stackoverflow.com/questions/1885448/how-do-you-evaluate-a-string-as-a-clojure-expression

I'm wondering if it's guaranteed that in a Java program, the boolean expression on the right of a conjunction (exp2 above) will NOT be evaluated as long as the expression on the left (exp1) evaluated to false. I'm wondering because I have an expression like the following: if (var != null && var.somePredicate()) // do something If Java is not guaranteed to stop evaluating (var != null && var.somePredicate()) after it sees that var is null, then it may try to evaluate var.somePredicate() which would throw a NullPointerException. So my question is, does Java guarantee a certain behavior when it

A few days ago I asked by which criteria the compiler decides whether or not, to compute a constexpr function during compile time. When does a constexpr function get evaluated at compile time? As it turns out, a constexpr is only evaluated during compile-time, if all parameters are constant expressions and the variable you are assigning it to is are constant expression as well. template<typename base_t, typename expo_t> constexpr base_t POW(base_t base, expo_t expo) { return (expo != 0 )? base * POW(base, expo -1) : 1; } template<typename T> void foobar(T val) { std::cout << val << std::endl;

Puzzle for the R cognoscenti: Say we have a data-frame: df <- data.frame( a = 1:5, b = 1:5 ) I know we can do things like with(df, a) to get a vector of results. But how do I write a function that takes an expression (such as a or a > 3 ) and does the same thing inside. I.e. I want to write a function fn that takes a data-frame and an expression as arguments and returns the result of evaluating the expression "within" the data-frame as an environment. Never mind that this sounds contrived (I could just use with as above), but this is just a simplified version of a more complex function I am

I would like to find a javascript parser that can handle and evaluate simple expressions. The parser should be able to evaluate the regular mathematical expressions, and support custom functions with parameters. It also has to support strings handling. String concatenation with || operator support is preferred, but it is okay if + will do the trick. Examples of an expression that should be handled by the parser: 3 * (2 + 1) - 1 2 * func(2, 2) func('hello world', 0, 5) || ' you' Has anyone implemented such a thing or where can I find something similar? I have a modified version of an

This question already has an answer here: Is the shortcircuit behaviour of Python's any/all explicit? 4 answers I wish to use the Python all() function to help me compute something, but this something could take substantially longer if the all() does not evaluate as soon as it hits a False . I'm thinking it probably is short-circuit evaluated, but I just wanted to make sure. Also, is there a way to tell in Python how the function gets evaluated? TerryA Yes, it short-circuits: >>> def test(): ... yield True ... print('one') ... yield False ... print('two') ... yield True ... print('three') ...

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? Yes && is short circuited and you are using it correctly. If next is NULL string compare will never happen. Yes, in C++ short circuit and and or operators are available. Here 's a question answered in the C-faq on the

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 = new char [100]; ... myString = "1" //user input cout << eval("function"+myString)(); } How is it usually done? UPD : Based on slacy's and clinisbut's answers I think I need to make a function registry. I suppose it should be made as an array of pointers to functions. Here's the question, how do I declare an array of

echo 3+3 How can I evaluate such expressions in Bash, in this case to 6? 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 echo $(( 3+3 )) expr is the standard way, but it only handles integers. bash has a couple of extensions, which only handle integers as well: $((3+3)) returns 6 ((3+3)) used in conditionals, returns 0 for true (non-zero) and 1 for false