C - use of undeclared identifier in a function. CS50 - c

Question

I am in process of rewriting my CS50 credit solution to use functions.
Stumbled upon error when defining readCardNumber().
long long readCardNumber()
{
do
{
long long result = get_long_long("Enter card number to verify: \n");
if (result < 0)
{
printf("Retry: \n");
}
}
while (result < 0);
return result;
}
I am using the CS50.h https://reference.cs50.net/cs50/get_long_long to get number. I cannot compile this solution because of :
error: use of undeclared identifier 'result'
Can someone experienced please explain whats the issue here? I did declare function at beginning of my code and declared and initialised result in function.
What would be a better way to validate that number?
https://docs.cs50.net/2018/x/psets/1/credit/credit.html - Spec for solution I am trying to rework.

Answer 1

The result variable is declared inside of the do...while block. It is not visible outside of the block, which includes the condition of the while.
You need to move the variable definition outside of the loop:
long long result;
do
{
result = get_long_long("Enter card number to verify: \n");
if (result < 0)
{
printf("Retry: \n");
}
}
while (result < 0);

Answer 2

It is a matter of scopes. In C, a scope is defined by { and }. A variable ceases to exist at end of the scope it is declared within. (Well, static variables don't but they get unaccessible. Unless you have pointers to them.)
What you need to do is to move the declaration outside the loop.
I do however want to emphasize that it is a VERY good practice to declare variables within the scope that they are used. It reduces the risk of bugs significantly. If a variable is only used inside a loop, then declare it inside the loop. It is basically the same reason as why you should not use globals unless necessary.

Answer 3

You do return result; when result is out of scope. But your code contains redundancy: you test result < 0 twice. I would recommend changing the structure to avoid this, with bonus side-effect of fixing the original problem:
long long readCardNumber(void)
{
for (;;)
{
long long result = get_long_long("Enter card number to verify: \n");
if (result >= 0)
return result;
printf("Retry: \n");
}
}

Answer 4

This is one of those inconvenient or even "unnatural" things about do/while statement: everything you declare inside the cycle body will not be visible to the condition in the while portion of the statement. The scope of those identifiers ends at the } before the while portion.
This often forces the user to declare the variable(s) before the cycle
long long result;
do
{
result = get_long_long("Enter card number to verify: \n");
if (result < 0)
printf("Retry: \n");
}
while (result < 0);
The price to pay for this solution is
unnecessary extension of result's scope beyond the cycle
it is no longer possible to meaningfully initialize the variable at the point of declaration
For many of us this is so unpleasant that we prefer to switch to the do/while(1) approach with break in the middle to terminate the cycle
do
{
long long result = get_long_long("Enter card number to verify: \n");
if (result >= 0)
break;
printf("Retry: \n");
}
while (1);
This is also not perfect, obviously. Chose for yourself, which approach you like better.
In the latter case do/while(1) serves as an "idiomatic" way to express a cycle with exit from the middle. Other people might prefer for (;;) or while(1) for that purpose, thus avoiding do/while statement altogether.