FAQ: Functions: Scope & Flexibility - Review

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This community-built FAQ covers the “Review” exercise from the lesson “Functions: Scope & Flexibility”.

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2

I’m trying to build a function that does the same thing as in the example ( 7/ 9 ).

add_together()

which takes two arguments and, if they’re numbers, adds them, and if they’re strings/characters, concatenates them.

Since i’m using a template, i don’t have any relevant fns.cpp, but my fns.hpp is as follows:

template
T add_together(T input1, T input2){
return input1 + input2;
}

I know that strings can be concatenated using the “+” operator, and yet when i try to run my test main(), as follows:

#include <iostream>
#include “fns.hpp”
#include <string>
int main() {
std::cout << add_together(1,2) << “\n”;
std::cout << add_together(“A”, “B”) << “\n”;
}

I get the following:
error: invalid operands of types 'constant char*' and 'constant char*' to binary 'operator+'
After some googling i see that constant characters cannot be added together, but a constant character can be added to a string, but if, within my main(), i specify as follows:

std::cout << add_together(std::string “A”, “B”) << “\n”;

then the error changes to:
error: expected primary-expression before string constant
std::cout << combine(std::string "A", "B" << "\n";

and i have no clue what that means. Any ideas?

2 Likes
4

I decided to challenge myself by using one function inside of another function. It took me a lot of debugging to get it straight, but it was worth it! My idea was to create a program which calculates kinetic energy from an object’s mass, displacement (that’s distance in a particular direction), and the time taken to make the movement. Here is the main.cpp file of my code:

#include <iostream>
#include "fns.hpp"

int main() {
  std::cout << kinetic_energy(80, 25, 16) << "\n";
}

//KE = 0.5 * 80 * ((25 / 16) * (25 / 16))
//KE = 97.6562 Joules

Here is the fns.hpp file of my code:

double kinetic_energy(double mass, double displacement, double time);
double average_velocity(double displacement, double time);

Here is the fns.cpp file of my code:

double average_velocity(double displacement, double time) {
  return displacement / time;
}
double kinetic_energy(double mass, double displacement, double time) {
  return 0.5 * mass * (average_velocity(displacement, time) * average_velocity(displacement, time));
}

Thank you, Codecademy for the fact that I was able to code this! Keep up the good work!

3 Likes
5

did you fix it ?!!
i am interesting

6

I know this is an old comment, but I was scrolling through and trying to troubleshoot some projects as a learning exercise, and figured I’d post what I worked out.

I think the main problem is the arguments “A” and “B” aren’t passing as a string, they are passing as constant char*, meaning that they can’t be modified in any way. It’s something to do with the argument passing as a pointer, rather than an object. If you declare the string as an object outside of the statement, you can then pass the string as the argument to be concatenated.

// fns.hpp

template <typename T>

T add_together(T input1, T input2){

return input1 + input2;
}

// main.cpp

#include <iostream>
#include "fns.hpp"


int main() {

std::string str_A = "A";
std::string str_B = "B";

  std::cout << add_together(1, 2) << "\n";
  std::cout << add_together(str_A, str_B) << "\n";

}

The output of this program is:
3
AB

1 Like
7

I’d agree with your here. So far as I’m aware string literals always evaluate that way. So "A" is basically equivalent to const char name[] = {'A', '\0'}, a constant character array terminated by null (without the assigned name though, it’s just an lvalue). I think the literal would always be stored away as data at compile time anyway.

The major issue being that "A" is not a C++ string object at that point and when named inside the function it’s treated like an array name would be, a pointer to this first value in an array. So you cannot use + with another array pointer (it doesn’t even know how long the character array is at that point).

The literal evaluation is also why the following is perfectly valid-

const char *s1 = "RED";
// equivalent to... const char name[] = {'R', 'E', 'D', '\0'};

where s1 is the first pointer in an array of values (like name is in the equivalent example). Like a pointer passed to a function though we don’t know the length from the pointer alone and rely on the null termination to prevent overflows (string literals are always null terminated so far as I’m aware).

Since the underlying object created by the string literal is immutable the following at least throws a warning and may fail to compile depending on current iso standard, compiler and flags-

char *s2 = "RED";  // <-- Bad practice even if the compiler allows it

Assuming it even complied, attempting to modify any values pointed to by s2 and the neighbouring const chars, e.g. s2[0] = 'B' would be either undefined or broken behaviour that would probably just lead to a crash.

1 Like
8

Been stuck on this for a day now defiantly restarting the lesson to understand it better but what am i do doing wrong?

main.cpp:#include
#include
#include “fns.hpp”
int main() {
int length, width;
rectangle(length, width);
int area = rectangle(length, width);
double perimeter = rectangle(length, width);

std::cout << "The area of your rectangle is " << area<< "\n";

std::cout << "The perimeter of your rectangle is " << perimeter << "\n";

return 0;

}

fns.hpp:
//Delcarations

void rectangle(int& length, int& width);

int rectangle(int length, int width);

double rectangle(double length, double width);

fns.cpp:
//Definition
void rectangle(int& length, int& width){
std::cout << “Enter the length of your rectangle.\n”;
std::cin >> length;
std::cout << “Enter the width of your rectangle.\n”;
std::cin >> width;
}

int rectangle(int length, int width){
return length * width;
}

double rectangle(double length, double width){
return 2 * (length + width);
}