Suppose I have a variable, double x, as a result of some calculations, which can have any value, including zero, and I need it passed to std::cout. How can I avoid printing x if its value is zero?
As an example, this will print 1+<value_of_x> if x, else just 1:
std::cout << (x ? "1+" : "1") << x << '\n';
Is there a way to make the same but for x? Something like the following nonsense:
std::cout << (x ? ("1+" << x) : "1") << '\n';
I should probably add that I am not advanced in C++.
You could say
std::cout << (x ? "1+" + std::to_string(x) : "1") << '\n';
but
if (x)
std::cout << "1+" << x << '\n';
else
std::cout << "1" << '\n';
is perhaps more readable.
(I consider this largely a matter of personal preference.)
If x is 0, don't print it:
if (x != 0)
std::cout << x << '\n';
Any further variations should be self-evident.
Using an if statement would be a simple and readable approach:
if (x)
std::cout << "1+" << x;
else
std::cout << "1";
std::cout << '\n';
Or even:
std::cout << "1";
if (x) std::cout << "+" << x;
std::cout << '\n';
But, if you really want to print out the value inline, you can define a custom operator<< to format the value however you want:
struct to_coefficient_str
{
double m_value;
to_coefficient_str(double value) : m_value(value) {}
void print(std::ostream &out) const
{
out << "1";
if (m_value)
out << "+" << m_value;
}
};
std::ostream& operator<<(std::ostream &out, const to_coefficient_str &ce)
{
ce.print(out);
return out;
}
Then you can use it like this:
std::cout << to_coefficient_str(x) << '\n';
Related
I wrote a generic class for handling and executing a function pointer. This is a simplified equivalent of std::function and std::bind. To handle member functions I use cast to internal EventHandler::Class type. Question: is it ok to cast it that way? Will it work in all cases when invoking handled function?
template <typename ReturnType, typename... Arguments>
class EventHandler
{
class Class {};
ReturnType (Class::*memberFunction)(Arguments...) = nullptr;
union {
Class *owner;
ReturnType(*function)(Arguments...) = nullptr;
};
public:
EventHandler() = default;
EventHandler(EventHandler &&) = default;
EventHandler(const EventHandler &) = default;
EventHandler &operator=(EventHandler &&) = default;
EventHandler &operator=(const EventHandler &) = default;
EventHandler(ReturnType (*function)(Arguments...)) :
function(function)
{
}
template <typename Owner>
EventHandler(Owner *owner, ReturnType (Owner::*memberFunction)(Arguments...)) :
memberFunction((ReturnType (Class::*)(Arguments...)) memberFunction),
owner((Class *) owner)
{
}
template <typename Owner>
EventHandler(const Owner *owner, ReturnType (Owner::*memberFunction)(Arguments...) const) :
memberFunction((ReturnType (Class::*)(Arguments...)) memberFunction),
owner((Class *) owner)
{
}
ReturnType operator()(Arguments... arguments)
{
return memberFunction ?
(owner ? (owner->*memberFunction)(arguments...) : ReturnType()) :
(function ? function(arguments...) : ReturnType());
}
};
The implementation provides handle for a global function, a member function and a const member function. Obviously there is volatile and const volatile that is not show here for clarity.
EDIT
All the code below is just a representation of all of kinds of supported functions.
class Object
{
public:
double y = 1000;
Object() = default;
Object(double y) : y(y) {}
static void s1(void) { std::cout << "s1()" << std::endl; }
static void s2(int a) { std::cout << "s2(a:" << 10 + a << ")" << std::endl; }
static void s3(int a, float b) { std::cout << "s3(a:" << 10 + a << ", b:" << 10 + b << ")" << std::endl; }
static int s4(void) { std::cout << "s4(): "; return 10 + 4; }
static Object s5(int a) { std::cout << "s5(a:" << 10 + a << "): "; return Object(10 + 5.1); }
static float s6(int a, Object b) { std::cout << "s6(a:" << 10 + a << ", b:" << 10 + b.y << "); "; return 10 + 6.2f; }
void m1(void) { std::cout << "m1()" << std::endl; }
void m2(int a) { std::cout << "m2(a:" << y + a << ")" << std::endl; }
void m3(int a, float b) { std::cout << "m3(a:" << y + a << ", b:" << y + b << ")" << std::endl; }
int m4(void) { std::cout << "m4(): "; return ((int) y) + 4; }
Object m5(int a) { std::cout << "m5(a:" << y + a << "): "; return Object(y + 5.1); }
float m6(int a, Object b) { std::cout << "m6(a:" << y + a << ", b:" << y + b.y << "); "; return ((int) y) + 6.2f; }
void c1(void) const { std::cout << "c1()" << std::endl; }
void c2(int a) const { std::cout << "c2(a:" << y + a << ")" << std::endl; }
void c3(int a, float b) const { std::cout << "c3(a:" << y + a << ", b:" << y + b << ")" << std::endl; }
int c4(void) const { std::cout << "c4(): "; return ((int) y) + 4; }
Object c5(int a) const { std::cout << "c5(a:" << y + a << "): "; return Object(y + 5.1); }
float c6(int a, Object b) const { std::cout << "c6(a:" << y + a << ", b:" << y + b.y << "); "; return ((int) y) + 6.2f; }
};
void f1(void) { std::cout << "f1()" << std::endl; }
void f2(int a) { std::cout << "f2(a:" << a << ")" << std::endl; }
void f3(int a, float b) { std::cout << "f3(a:" << a << ", b:" << b << ")" << std::endl; }
int f4(void) { std::cout << "f4(): "; return 4; }
Object f5(int a) { std::cout << "f5(a:" << a << "): "; return Object(5.1); }
float f6(int a, Object b) { std::cout << "f6(a:" << a << ", b:" << b.y << "); "; return 6.2f; }
Here is the usage example for all of the above functions
int main()
{
std::cout << "=== Global functions" << std::endl;
EventHandler ef1(f1); ef1();
EventHandler ef2(f2); ef2(2);
EventHandler ef3(f3); ef3(3, 3.1f);
EventHandler ef4(f4); std::cout << ef4() << std::endl;
EventHandler ef5(f5); std::cout << ef5(5).y << std::endl;
EventHandler ef6(f6); std::cout << ef6(6, Object(6.1)) << std::endl;
std::cout << std::endl;
std::cout << "=== Member static functions" << std::endl;
EventHandler es1(Object::s1); es1();
EventHandler es2(Object::s2); es2(2);
EventHandler es3(Object::s3); es3(3, 3.1f);
EventHandler es4(Object::s4); std::cout << es4() << std::endl;
EventHandler es5(Object::s5); std::cout << es5(5).y << std::endl;
EventHandler es6(Object::s6); std::cout << es6(6, Object(6.1)) << std::endl;
std::cout << std::endl;
std::cout << "=== Member functions" << std::endl;
Object object(20);
EventHandler em1(&object, &Object::m1); em1();
EventHandler em2(&object, &Object::m2); em2(2);
EventHandler em3(&object, &Object::m3); em3(3, 3.1f);
EventHandler em4(&object, &Object::m4); std::cout << em4() << std::endl;
EventHandler em5(&object, &Object::m5); std::cout << em5(5).y << std::endl;
EventHandler em6(&object, &Object::m6); std::cout << em6(6, Object(6.1)) << std::endl;
std::cout << std::endl;
std::cout << "=== Member const functions" << std::endl;
const Object constObject(30);
EventHandler ec1(&constObject, &Object::c1); ec1();
EventHandler ec2(&constObject, &Object::c2); ec2(2);
EventHandler ec3(&constObject, &Object::c3); ec3(3, 3.1f);
EventHandler ec4(&constObject, &Object::c4); std::cout << ec4() << std::endl;
EventHandler ec5(&constObject, &Object::c5); std::cout << ec5(5).y << std::endl;
EventHandler ec6(&constObject, &Object::c6); std::cout << ec6(6, Object(6.1)) << std::endl;
system("pause");
return 0;
}
Finally - to the point - here an example that shows how much easier in use is the EventHandler I prepared when compared to std::function interface. And actually the reason of such approach.
EventHandler<float, int, Object> example;
example = f6;
example(7, Object(7.1));
example = EventHandler(&object, &Object::m6);;
example(8, Object(8.1));
It’s undefined behavior to call a function through a function pointer(-to-member) of a different type. (Some practical reasons for this rule are that the object’s address might need to be adjusted to call a member function of a base class or that a vtable might be involved.) You can use type erasure to allow calling member functions on objects of different types (which is what std::bind does), or you can (restrict to member functions and) add the class type as a template parameter.
Of course, the usual answer is to just use std::function with a lambda that captures the object in question and calls whatever member function. You can also take the C approach and define various functions with a void* parameter that cast that parameter to a known class type and call the desired member function.
Is there a way to create a function which you can use between two << operators in an ostream?
Let's assume the function's name is usd, and might look something like:
std::ostream& usd(std::ostream& os, int value) {
os << "$" << value << " USD";
return os;
}
Then I would like to use it like:
int a = 5;
std::cout << "You have: " << usd(a) << std::endl;
Which would print:
You have: $5 USD
I would prefer a solution without the need for a class.
If you must use a class I would prefer not to mention the class at all when using the usd function. (For example how the std::setw function works)
EDIT:
In my implementation I intend to use the std::hex function, the one described above was just a simplified example but probably shouldn't have.
std::ostream& hex(std::ostream& os, int value) {
os << "Hex: " << std::hex << value;
return os;
}
So I am not sure if a function returning a simple string is sufficient.
To obtain the usage you described:
int a = 5;
std::cout << "You have: " << usd(a) << std::endl;
You'd simply need usd(a) to return something that you have an ostream<< operator for, like a std::string, and no custom ostream<< operator is needed.
For example:
std::string usd(int amount)
{
return "$" + std::to_string(amount) + " USD";
}
You can write other functions to print in other currencies, or to convert between them, etc but if all you want to handle is USD, this would be sufficient.
If you used a class representing money, you could write an ostream<< for that class and you wouldn't need to call a function at all (given that your default ostream<< prints USD)
class Money
{
int amount;
};
std::ostream& usd(std::ostream& os, Money value) {
os << "$" << value.amount << " USD";
return os;
}
int main(int argc, char** argv)
{
Money a{5};
std::cout << "You have: " << a << std::endl; // Prints "You have: $5 USD"
return 0;
}
I don't know how to do this without a class. However, it is easy to do with a class.
struct usd {
int value;
constexpr usd(int val) noexcept : value(val) {}
};
std::ostream& operator<<(std::ostream& os, usd value) {
os << "$" << value.value << " USD";
return os;
}
for hex
struct hex {
int value;
constexpr hex(int val) noexcept : value(val) {}
};
std::ostream& operator<<(std::ostream& os, hex value) {
os << "Hex: " << std::hex << value.value;
return os;
}
usage
int a = 5;
std::cout << "You have: " << usd(a) << std::endl;
std::cout << "You have: " << hex(a) << std::endl;
Normally it has no sense and is very unsafe, but only theoretically if there is a way,
Here is example:
#include<iostream>
struct A {
uint32_t &get() {
return *reinterpret_cast<uint32_t *>(this);
}
void set(const uint32_t val) {
*this = *reinterpret_cast<const A *>(&val);
}
};
struct B : A {
uint16_t a;
uint16_t b;
void set_b(const uint32_t val) {
*this = *reinterpret_cast<const B *>(&val);
}
};
main() {
B k;
k.a = 0x1234;
k.b = 0x5678;
std::cout << std::hex << k.get() << " : " << k.a << " " << k.b << std::endl;
k.set_b(0x87654321);
std::cout << std::hex << k.get() << " : " << k.a << " " << k.b << std::endl;
k.set(0xaabbccdd);
std::cout << std::hex << k.get() << " : " << k.a << " " << k.b << std::endl;
}
I get this result:
56781234 : 1234 5678
87654321 : 4321 8765
87654321 : 4321 8765
But I except that last should be:
aabbccdd : ccdd aabb
So, why overwriting data in structure from parent not working?
Experiment:
I make one experiment, that I add one variable into struct A, then set function was working as expected (but final structure was bigger)
Of course there exists different ways how to deal with this (for example with unions) but I only playing with this and I interested why this is not working.
In the class A the set function is really
void set(const uint32_t val) {
(*this).operator=(*reinterpret_cast<const A *>(&val));
}
That will invoke the automatically generated A::operator= function. But since A doesn't have any member variables to be copied, it does nothing.
And now that you've done your experiment, please don't do anything like that ever again.
This question already has answers here:
Pattern to avoid nested try catch blocks?
(16 answers)
Closed 6 years ago.
I have a yaml-cpp which always converts into a std::string, and sometimes also into something else. For example, if the string actually is "3.14", it would also convert into double. I'd first like to try int, then double, then bool, and if that doesn't work, convert to a std::string. Alright, so let's nest those try-catches:
try {
const int a = node.as<int>();
std::cout << "int!" << a << std::endl;
} catch (YAML::BadConversion) {
try {
const double a = node.as<double>();
std::cout << "double!" << a << std::endl;
} catch (YAML::BadConversion) {
try {
const bool a = node.as<bool>();
std::cout << "bool!" << a << std::endl;
} catch (YAML::BadConversion) {
const std::string a = node.as<std::string>();
std::cout << "string!" << a << std::endl;
}
}
}
Hm, the deeper and deeper nesting tells me that this isn't the best way to write that code.
Any suggestions on how to improve the design here? Flat nesting would certainly be advised.
You may put it in a function like:
template<typename N, typename T>
bool tryParseNode(N& node, T& val) {
try {
val = node.as<T>();
return true;
} catch (YAML::BadConversion) {
return false;
}
}
then:
int a;
double d;
bool b;
std::string s;
if (tryParseNode(node, a) {
std::cout << "int!" << a << std::endl;
}
else if (tryParseNode(node, d) {
std::cout << "double!" << d << std::endl;
}
else if (tryParseNode(node, b) {
std::cout << "bool!" << b << std::endl;
}
else if (tryParseNode(node, s) {
std::cout << "string!" << s << std::endl;
}
Try the other way round:
Convert into to a string, then try bool, etc.
Everything within a single try-catch and ignore the exception.
Using exceptions for normal control flow is considered bad practice. In this case, the as method uses the `YAML::convert::decode' method to attempt to convert the node into the requested type returning a false if it fails instead of throwing an exception.
int anInt;
double aDouble;
bool aBool;
if (YAML::convert <int>::decode (node, anInt))
std::cout << "int!" << anInt << std::endl;
else
if (YAML::convert <double>::decode (node, aDouble))
std::cout << "double!" << aDouble << std::endl;
else
if (YAML::convert <bool>::decode (node, aBool))
std::cout << "double!" << aBool << std::endl;
else
std::cout << "string!" << node.as <std::string> () << std::endl;
which could be further simplified to
template <typename value_type>
std::optional <value_type> decode (YAML::Node const & Node)
{
value_type Value;
if (YAML::convert <value_type>::decode (node, Value))
return { Value };
else
return {};
}
if (auto anInt = decode <int> (node))
std::cout << "int!" << *anInt << std::endl;
else
if (auto aDouble = decode <double> (node))
std::cout << "double!" << *aDouble << std::endl;
else
if (auto aBool = decode <bool> (node))
std::cout << "double!" << *aBool << std::endl;
else
std::cout << "string!" << node.as <std::string> () << std::endl;
I have been searching on Google an in this forum for a while, but I could not find any answer or tip for my problem. Tutorials couldn't help me either...
I want to redistribute some points, stored in a vector p_org. (x-value is stored as double).
Therefore I have the function distribute, which is defined in maths.h
distribute_tanh(&p_org_temp,&p_new_temp,iz,spacing[0],spacing[1],l_rot[(kk+1)*iz-2],status);
The function distribute_tanh does look like this:
inline void distribute_tanh (std::vector<double> *p_org, std::vector<double> *p_new, const int n_points, double spacing_begin, double spacing_end, const double total_length, double status){
//if status == 0: FLAP, if status == 1: SLAT
std::cout << "spacing_begin: " << spacing_begin << " spacing_end: " << spacing_end << std::endl;
double s_begin = spacing_begin / total_length;
double s_end = spacing_end / total_length;
double A = sqrt(s_end/s_begin);
double B = 1 / (sqrt(s_end*s_begin)*n_points);
std::cout << "A: " << A << " B: " << B << std::endl;
std::vector<double> u (n_points);
std::vector<double> sn (n_points);
double dx;
double dy;
std::cout << "Control at the beginning: p_org: " << (p_org) << " p_new: " << (p_new) << " n_points: " << n_points << " s_begin: " << s_begin << " s_end: " << s_end << " total_length: " << total_length << std::endl;
//problem no. 1
for (int i=0;i<n_points;i++){
if (B > 1.001) {
if (B < 2.7829681) {
double Bq=B-1;
dy=sqrt(6*Bq)*(1-0.15*Bq+0.057321429*pow(Bq,2)-0.024907295*pow(Bq,3)+0.0077424461*pow(Bq,4)-0.0010794123*pow(Bq,5));
} else if (B > 2.7829681) {
double Bv=log(B);
double Bw=1/B-0.028527431;
dy=Bv+(1+1/Bv)*log(2*Bv)-0.02041793+0.24902722*Bw+1.9496443*pow(Bw,2)-2.6294547*pow(Bw,3)+8.56795911*pow(Bw,4);
}
u[i]=0.5+(tanh(dy*(i*(1.0/n_points)-0.5))/(2*tanh(dy/2)));
}
else if (B < 0.999) {
if (B < 0.26938972) {
dx=M_PI*(1-B+pow(B,2)-(1+(pow(M_PI,2))/6)*pow(B,3)+6.794732*pow(B,4)-13.205501*pow(B,5)+11.726095*pow(B,6));
} else if (B > 0.26938972) {
double Bq=1-B;
dx=sqrt(6*Bq)*(1+0.15*Bq+0.057321429*pow(Bq,2)+0.048774238*pow(Bq,3)-0.053337753*pow(Bq,4)+0.075845134*pow(Bq,5));
}
u[i]=0.5+(tan(dx*(i*(1.0/n_points)-0.5))/(2*tan(dx/2)));
}
else {
u[i]=i*(1.0/n_points)*(1+2*(B-1)*(i*(1.0/n_points)-0.5)*(1-i*(1.0/n_points)));
}
sn[i]=u[i]/(A+(1.0-A)*u[i]);
std::cout << "sn(i): " << sn[i] << std::endl;
std::cout << "p_org[n_points]: " << &p_org[n_points-1] << std::endl;
if(status==0){
//p_new[i]=p_org[0]+(total_length*sn[i]);
std::cout << "FLAP maths.h" << std::endl;
}
//Here is the problem no. 2
else if(status==1){
//p_new[i]=p_org[0]-(total_length*sn[i]);
std::cout << "SLAT maths.h" << std::endl;
}
//std::cout << "p_new in math: " << p_new << std::endl;
}
}
My problem is, that I am unable to access the value of p_org or p_new. At the beginning I would like to give out the value of p_org and p_new. If I try it with a *, the compiler is complaining: error: no operator "<<" matches these operands
operand types are: std::basic_ostream> << std::vector>
std::cout << "Control at the beginning: p_org: " << (*p_org) << " p_new: " << (*p_new) << " n_points: " << n_points << " s_begin: " << s_begin << " s_end: " << s_end << " total_length: " << total_length << std::endl;
If I leave the * off, I get the addresses of p_org and p_new.
At the end of the code I would like to write the new value to p_new. If I use * to access the value, the compiler is complaining, if I leave it off, its complaining too with the following message:
error: no operator "-" matches these operands
operand types are: std::vector<double, std::allocator<double>> - double
p_new[i]=p_org[0]-(total_length*sn[i]);
^
I tried to understand both problems, but until now I had no success.
Thanks for your advice.
Your issue with the compiler error can be cut down to a very simple program.
#include <vector>
void foo(std::vector<int>* pV)
{
pV[0] = 10; // error.
}
int main()
{
std::vector<int> v(10);
foo(&v);
}
The issue is that operator[] as done above works for objects and references, not pointers. Since pv is a pointer, you must dereference it first to obtain the object, and then apply [] to the dereferenced pointer.
void foo(std::vector<int>* pV)
{
(*pV)[0] = 10; // No error
}
The other form of calling operator[] can be also used, but is a bit more verbose:
void foo(std::vector<int>* pV)
{
pv->operator[](0) = 10; // No error
}
However, to alleviate having to do this, pass the vector by reference. Then the "normal" way of using operator[] can be used.
#include <vector>
void foo(std::vector<int>& pV)
{
pV[0] = 10; // No error.
}
int main()
{
std::vector<int> v(10);
foo(v);
}