Indefinite loops where the first time is different

Question

This isn't a serious problem or anything someone has asked me to do, just a seemingly simple thing that I came up with as a mental exercise but has stumped me and which I feel that I should know the answer to already. There may be a duplicate but I didn't manage to find one.

Suppose that someone asked you to write a piece of code that asks the user to enter a number and, every time the number they entered is not zero, says "Error" and asks again. When they enter zero it stops.

In other words, the code keeps asking for a number and repeats until zero is entered. In each iteration except the first one it also prints "Error".

The simplest way I can think of to do that would be something like the folloing pseudocode:

int number = 0;
do
{
    if(number != 0)
    {
        print("Error");
    }
    print("Enter number");
    number = getInput();
}while(number != 0);

While that does what it's supposed to, I personally don't like that there's repeating code (you test number != 0 twice) -something that should generally be avoided. One way to avoid this would be something like this:

int number = 0;
while(true)
{
    print("Enter number");
    number = getInput();
    if(number == 0)
    {
        break;
    }
    else
    {
        print("Error");
    }
}

But what I don't like in this one is "while(true)", another thing to avoid.

The only other way I can think of includes one more thing to avoid: labels and gotos:

    int number = 0;
    goto question;
error:
    print("Error");
question:
    print("Enter number");
    number = getInput();
    if(number != 0)
    {
        goto error;
    }

Another solution would be to have an extra variable to test whether you should say "Error" or not but this is wasted memory.

Is there a way to do this without doing something that's generally thought of as a bad practice (repeating code, a theoretically endless loop or the use of goto)?

I understand that something like this would never be complex enough that the first way would be a problem (you'd generally call a function to validate input) but I'm curious to know if there's a way I haven't thought of.

Answer 1

The following solution avoids repetition and separates the concerns of getting the input and validating it:

do
{
  print "Enter number"
  number = getInput()
} while(!numberValid(number))

bool numberValid(number) {
  if (number == 0) { 
    return true
  } else {
    print "Error!"
    return false
  }
}

Answer 2

In Ruby I would consider this to be a guard clause and I would write it as either:

do_something if condition

or

do_something unless condition

I use these sparingly though. If there is a lot of code (more than 2 statements or lines say) I prefer the traditional if then else construct.

The main time I use is is for short conditions so that I can write the single line:

procA if x=1

instead of the 3 lines:

if x=1 then
  procA
end

Answer 3

I'd separate the two behaviours The first is getting a number for the user, the second is validating it and eventually re-asking it So I'd go like this int number = 0; print "Enter number"; number = getInput(); while(number != 0) { print "Error!"; number = getInput(); } Maybe you could not like the double input line, but I think it's a nice trade off

Answer 4

You could do something like:

int number;
while((number = solicitNumber()) != 0) {
    print("Error!");
}

...

int solicitNumber() {
    print("Enter number");
    return getInput();
}