Use function in nested if statement to trigger action - c++

Question

Below shows the main program with an if statement. If the if statement is true, then it moves onto triggering the function.
If what in the function is true, then x = true, which triggers the final action. The, x from the aux function still comes out as undeclared.
What am I doing wrong?
void aux(bool x) {
std::string text;
std::getline(std::cin, text);
if (text.find("be ") != std::string::npos ||
text.find("do ") != std::string::npos ||
text.find("can ") != std::string::npos ||
text.find("but ") != std::string::npos) {
x = true;
}
}
int main() {
std::string text;
std::getline(std::cin, text);
if (text.find("7 ") != std::string::npos) {
aux();
{
if (x);
{
std::cout<<"A function to equal a group of words"<<std::endl;
}
}
}
getchar();
return 0;
}

Answer 1

You may want to change your aux function to return a boolean instead of taking it as a parameter:
bool aux() {
if (/* your checks here */) {
return true;
} else {
return false;
}
}
The bool to the left of the function name, in place of void indicates that the result of calling your function will be a boolean.
Then in main you would use aux like this:
if (aux()) {
std::cout<<"A function to equal a group of words" <<std::endl;
}

Answer 2

Seems like you don't know about local and global variable.
You can use pass by reference to solve the problem as follows:
void aux(bool &x)
{
std::string text;
std::getline(std::cin, text);
if(text.find("be ") != std::string::npos || text.find("do ") != std::string::npos ||
text.find("can ") != std::string::npos || text.find("but ") != std::string::npos)
{
x = true;
}
else{
x = false;
}
}
int main()
{
bool x = false;
if (text.find("7 ") != std::string::npos)
{
aux(x);
//{ un-necessary brackets.
if (x);
{
std::cout<<"A function to equal a group of words" <<std::endl;
}un-necessary brackets.
//}
}
getchar();
return 0;
}
Problem you have:
You have following mistakes in your code.
1.You have defined a function like this:
void aux(bool x){
//your code here
}
But you haven't called it by passing any variable as parameter/argument. Do as following:
aux(var1);//var1 is boolean variable
2 . The change in x doesn't appear there because the x will be treated as a new variable inside the aux(var1) function. So if you use pass by reference, then the changes will be persistence to the variable (here x).

Answer 3

You seem to be operating on the assumption that 'x' should be available to your main() function after calling aux(). C/C++ does not work that way - there is no 'x' in the scope of your main function because you never declared anything named x.
Furthermore, it will not have the value you want unless you return x from your aux() function and assign it to something in your main function.
You should familiarize yourself with the topic of 'scope' in C/C++ as that will prevent further misunderstanding, but the basic problem you need to solve with your sample is:
You arent passing a bool to your aux() function, so it wont work (it is a parameter for aux()
You arent returning the value of x from your aux function
You arent declaring a boolean in your main function to take the value returned from aux() function

Answer 4

It is completely vague what is your expectations about bool x? If you want to change the x and x is outter variable then you need either pass it by the reference or return the value. Return of the value is much more logical.
bool
aux()
{
std::string text;
std::getline(std::cin, text);
if(text.find("be ") != std::string::npos ||
text.find("do ") != std::string::npos ||
text.find("can ") != std::string::npos ||
text.find("but ") != std::string::npos)
return true;
return false
}
Now it is also logical to use the result of aux() without dangling x with random value:
int main()
{
std::string text;
std::getline(std::cin, text);
if (text.find("7 ") != std::string::npos)
{
if (aux()) // We dont need the variable here since we use return value of the aux
std::cout<< "A function to equal a group of words" << std::endl;
else
std::cerr << "Bad string detected" << std::endl;
}
getchar();
return 0;
}
Now it starts to work