1

I have a pretty simple C++ console program. It's working fine, but I have some issues. I have 2 functions. First function called "input" asks from user to input numbers from 6 to 10. In that function I declared:

if ((a[i][j] < 6) || (a[i][j] > 10))
{
    cout<<"Invalid input!";
    return 0; 
}

Second function called "output", prints out those numbers from first function.

In the main it is like:

int main ()
{
    ...
    input (matrix, number);
    output (matrix, nubmer);
}

My question is this. When I input number that isn't 6-10, my program still do "output" function, so it prints some random numbers. How can I break whole program in exact time when input rules are broken? Without any random output and stuff, just to print "Invalid output", then to start program from the start?

CRT_flow
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  • Post complete code. Does it print 'Invalid input' in first place? – Ajay Jun 19 '11 at 18:59
  • Give me a few seconds to post the whole code, its in my native language, so u wont understand. – CRT_flow Jun 19 '11 at 19:00
  • If your input function return 0 on invalid input, does it return another value on success? You could check the return value of the input function in a loop. – Fox32 Jun 19 '11 at 19:00

6 Answers6

7

You may check the return value of input() before calling output(). You can use the exit() function to terminate the program

Maxim Krizhanovsky
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3
 if(input(matrix,number) != 0) output(matrix,number); 
 // You don't {} for a single instruction

This should fix your problem.

2

The usual way to indicate errors is to throw an exception.

int main() {
    ...
    try {
        input(matrix, number);
        output(matrix, number);
    } catch(std::exception& e) {
        std::cout << "Error: " << e.what();
    }
}

In input:

if ((a[i][j] < 6) || (a[i][j] > 10))
{
    throw std::runtime_error("Invalid input!");
}
Puppy
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    You obviously know more about C++ than I do. But I never use exceptions to check if the user input is valid. I use exceptions just for exceptional situations, which should never occur. It is expected that users make nonsensical inputs. – Lucas Jun 19 '11 at 19:09
  • @Lucas: Program termination is an exceptional condition. – Puppy Jun 19 '11 at 19:35
  • Is it? Most programs have to terminate at some point. I would make program termination in this case just part of my normal control flow. But I guess it's debatable. – Lucas Jun 19 '11 at 19:44
2

If your input function return 0 on invalid input and another value on success you can run a loop until your input is valid

...

while(input(matrix,number) == 0);

...
Fox32
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2

An idiomatic code would be this:

while(input (matrix, number)) //idiomatic loop
{
    output (matrix, nubmer);
}

The input() function should return false or 0 when the input is invalid, and true or non-zero, when the input is valid.

The loop above has the same form as we write std::istream idioms:

 std::string word;
 while( std::cin >> word ) 
 {
     //..
 }

 //Or

 ifstream file;
 //...
 while( file >> word ) 
 {
     //..
 }

 //Or
 while( std::getline(file, word)) 
 {
     //..
 }

I just explained this idiom here, as to how it works:

Community
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Nawaz
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1

There are plenty of solutions. One is to declare the input function to return an integer. On success input can return 0. On failure (the input is outside the range 6-10) input returns a negative number (-1 for simplicity). Inside your main function, use a while loop to continue calling input until it returns 0 for success. Each time input fails you can print a message telling the client to enter try again. If you want to exit immediately after the first input failure, you can call exit(1) inside your main.

Chris
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