I want to have a function making a new integer like this:
void makeInt(string str, int x);
void makeInt(string str, int x) {
int str = x;
}
int main() {
makeInt(name,4);
}
But now the question is, how can I make the string, name the integer? So that int name = 4. Because now it would only give out an error. Is that actually possible?
This is by the way in C++
You can, use C preprocessor macro.
#define makeInt(str, x) int str = x
int main() {
makeInt(name, 4);
}
Other than that, there is no way to do that in C++.
But I can't see any reason why would you want to do that.
If you want to store a variable with name inputted from user, and then retrieve the variable with name inputted from user, consider using std::map<std::string, int>.
#include <map>
#include <iostream>
#include <string>
int main(){
std::map<std::string, int> values;
values["name"] = 4;
std::cout << values["name"];
}
will prints 4.
Related
I am writing a code to insert an integer at an index of the string, but after providing the integer to add as string, insert function is not giving the correct output.
It is giving the error that :
no matching member function to call for insert string
This is my code:
#include <iostream>
using namespace std;
int main()
{
string s = "45564528";
int x = 8;
s.insert(s.begin()+5,to_string(x));
cout<<s<<endl;
return 0;
}
The expected output is 455648528.
Looking at the documentation for std::string::insert() shows that it takes a char or an iterator range, not a std::string, which std::to_string() naturally returns. At least, this is the case for the overloads that take an iterator for the first argument.
#include <iostream>
#include <string> // CHANGED: Include what you use
// using namespace std; // CHANGED: Bad practice
int main()
{
std::string s = "45564528";
int x = 8;
// CHANGED: Create string from the int, and use the iterator range overload
// to account for multi-digit numbers
auto tmp = std::to_string(x);
s.insert(s.begin()+5, tmp.begin(), tmp.end());
std::cout << s << '\n'; // CHANGED: std::endl is rarely actually needed
return 0;
}
There is an overload that lets you insert another std::string, but the first argument must be an index and not an iterator. So this would work as well:
#include <iostream>
#include <string>
int main()
{
std::string s = "45564528";
int x = 8;
s.insert(5, std::to_string(x));
std::cout << s << '\n';
return 0;
}
I have a pointer to an int object.
int x = 10;
int *ptr;
ptr = &x;
I want to write a function GetString(int *ptr) which would return me the string ptr. How do I do that in C++?
There isn't a direct way to extract a variable's string name in C++. The language has only rudimentary reflection features, see the type_traits standard library and SO post Why does C++ not have reflection?.
But you can use preprocessor stringification # to hack something similar: define
#define STRINGIFY(x) #x
then the preprocessor substitutes STRINGIFY(ptr) with the string "ptr".
Note: if you want to stringify the result of a macro expansion use two levels of macros.
#include <iostream>
#include <map>
const std::string& getString(int *ptr, const std::map<int*, std::string>& mp){
return mp.find(ptr)->second;
}
int main()
{
std::map<int* , std::string> mp;
int x = 10;
mp.insert({&x, "ptr1"});
int y = 9;
mp.insert({&y, "ptr2"});
std::cout << getString(&y, mp);
}
Demo
I have a class BankAccount with two string members - name and num. What I want is to assign values to these objects when I create them (when the constructor is called). However the compiler says No instance of constructor matches the argument list when I try to create an object.
I would like to ask why is that?
// hwk-2.cpp : This file contains the 'main' function. Program execution begins and ends there.
//
#include "pch.h"
#include <iostream>
#include <string>
#include <stdio.h>
using namespace std;
class BankAccout {
char name[23];
char num[15];
double sum;
public:
BankAccout(char *nm, char *nr, double s) {
strcpy(name,nm);
strcpy(num, nr);
sum = s;
}
};
int main()
{
BankAccout k("Peter", "0403940940", 34.21);
}
as a coffee break exercise here is more idiomatic version
#include "pch.h"
#include <iostream>
#include <string>
class BankAccount {
std::string name_;
std::string num_;
double sum_;
public:
BankAccount(std::string name, std::string num, double sum) {
name_ = name;
num_ = num;
sum_ = sum;
}
};
int main()
{
BankAccount k("Peter", "0403940940", 34.21);
}
The signature of the constructor does not match.
This one will match:
BankAccount(const char *nm, const char *nr, double s);
EDIT:
The reason is the way you are calling the constructor in the main function. You are giving literal strings as parameters. These literals are const, you cannot change them at runtime. Thus you will pass pointers to const char*.
This is very obvious if you look at this opposing example. This is a way that would be compatible with the old signature BankAccout(char *nm, char *nr, double s);.
int main(int argc, char* argv[])
{
char name[] = "hello";
char number[] = "1234";
std::cout << "name before: " << name << std::endl;
BankAccount k(name, number, 8.5);
// name and number are not const,
// you can change them :
name[2] = 'x';
name[3] = 'x';
std::cout << "name after: " << name << std::endl;
return 0;
}
An even simpler version, if you don’t need to have additional functionality in the class: just use a struct.
#include <string>
struct BankAccount {
std::string name;
std::string number;
double balance;
};
int main() {
BankAccount account{"Joy", "44", 43.};
}
im new in c++ (and not to old in programming...) and i have problem with handling vectors and strucs in class.
basically i have a vector and a array of pointers to struct members in the class and i want work on the in my methos but im doing something worng/
here is my movement.h
#pragma once
using namespace std;
class movement
{
private:
static const int MAX_ROW_PER_TRACKER = 100;
static const int MIN_TO_START_CALC = 30;
static const int MAX_TRACKERS = 20;
struct tracker
{
int id;
double a[MAX_ROW_PER_TRACKER];
double b[MAX_ROW_PER_TRACKER];
double c;
};
vector<int> trackersOrder[MAX_TRACKERS] = {};
tracker* trackersArr[MAX_TRACKERS];
public:
movement();
void addRow(int a, int b, int c);
~movement();
};
and my movement.cpp
#include "stdafx.h"
#include "movement.h"
#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;
movement::movement()
{
}
void movement::addRow(int id, int a, int b)
{
int index;
vector<int>::iterator searchID = find(trackersOrder.begin(), trackersOrder.end(), ID);
if (searchID == trackersOrder.end())
{
vector<int>::iterator freeLocation = find(trackersOrder.begin(), trackersOrder.end(), 0);
index = freeLocation - trackersOrder.begin();
trackersOrder.insert(trackersOrder.begin + index, id);
structArr[index] = new tracker;
structArr[index]->id = id;
structArr[index]->a[0] = a;
structArr[index]->b[0] = b;
structArr[index]->c = 0;
}
}
movement::~movement()
{
}
so when i send to method "addRow" id, and b i want to first check if i allready have this id in my vector (the vector just give me the index for the structs array) and if not then if put the id in the first empty place in the vector and on the structs array/
but from some reasin its look to me that the methid dont reconized the vector and the structs. can you help me understand why?
p.s - i can bet that i have more mistakes in my code, its my firs try with pointers and ect. (im comming from the good life in Matlab) so i will be happy to learn on them also
thank you very much!
The main problem
The problem is that in your code, trackersOrder is not a vector but an array of vectors:
vector<int> trackersOrder[MAX_TRACKERS] = {}; // array of MAXTRACKERS vectors !!
The solution
If you define it as simple vector, it should work better:
vector<int> trackersOrder;
If you want to set its size do it in the movement constructor:
movement::movement() : trackersOrder(MAX_TRACKERS)
{
}
Other issues
There is a case typo with an ID that should be id.
auto searchID = find(trackersOrder.begin(), trackersOrder.end(), id); // by the way auto is easier + ID corrected
There are a missing () after a begin whicn transforms unfortunately your iterator arithmetic into function pointer arithmetic (sic!!):
trackersOrder.insert(trackersOrder.begin() + index, id); // corrected
Finally, there are a couple of structArr that should be replaced by trackersArr.
The result does finally compile (online demo)
I need to do the following:
const char* my_var = "Something";
REGISTER(my_var);
const char* my_var2 = "Selse";
REGISTER(my_var2);
...
concst char* all[] = { OUTPUT_REGISTERED }; // inserts: "my_var1, my_var2, ..."
REGISTER and OUTPUT_REGISTERED are preprocesor macros. This would be great for large number of strings, like ~100. Is it possible to accomplish this?
PS. The code belongs to level-0 "block" – i.e. it is not inside any function. AFAIK, I cannot call regular functions there.
#include <iostream>
#include <vector>
using namespace std;
vector<const char*>& all()
{
static vector<const char*> v;
return v;
}
struct string_register
{
string_register(const char* s)
{
all().push_back(s);
}
};
#define REGISTER3(x,y,sr) string_register sr ## y(x)
#define REGISTER2(x,y) REGISTER3(x,y,sr)
#define REGISTER(x) REGISTER2(x, __COUNTER__)
REGISTER("foo");
REGISTER("bar");
int main()
{
}