Use of undeclared identifier 'cardNum'

Question

I'm using a function of the library CS50 on cs50's environment, and this is how you store a value on a variable gathering it with an input:

long i = get_long("Enter a long: ");

Here comes my code:

#include <stdio.h>
#include <cs50.h>
#include <math.h>

int main(void)
{

do
{
    long cardNum = get_long("Card number:\n");
}
while (0 <= cardNum <= 9999999999999999);


if (340000000000000 <= cardNum <= 379999999999999)
{

...and so on.

The issue WAS

use of undeclared identifier 'cardNum'

I've added:

long cardNum;

BEFORE the do-while loop; PROBLEM SOLVED.

Well you *shouldn't* use `long cardNum`, it should be `char cardNum[32]` and input a string. That lets you get directly to any digit, and does not lose leading zeros. You are going the wrong way. — Weather Vane, Jun 01 '20 at 19:28
Because if the user enters `001122334455` then `getlong()` will return the `long` value `1122334455`. Now please tell me the first 2 digits of the card number, and whether *enough* digits were entered. — Weather Vane, Jun 01 '20 at 19:32
@mkrieger1 - it's a terrible idea to use integer types for account numbers like this: they look like numbers, but the only sane way to handle this is with strings. — Steve Friedl, Jun 01 '20 at 19:33
@mkrieger1 various digits or sets of digits of the card number have particular meanings. In my example, what was the 2nd digit of the card number? You make life very difficult by entering a string, converting that to an integer, and reversing that back to individual digits with `/` and `%`, when you have that information to begin with. — Weather Vane, Jun 01 '20 at 19:37
@WeatherVane While what you are saying is true for more advanced development, this is clearly a school task for beginners. — klutt, Jun 01 '20 at 21:56
@klutt I suppose so, but packing an input string into an integer and then unwinding it is just hard work! — Weather Vane, Jun 01 '20 at 21:59
@SusannaDafna Take the [tour] and you'll see how things work here — klutt, Jun 01 '20 at 22:28
bro, I'm taking a tour on C and Python and for real, nightmares. How do I post as solved? — Susanna Dafna, Jun 01 '20 at 22:29

Abhishek Bhagate · Answer 1 · 2020-06-01T20:02:16.807

0

You are using cardNum in the do block. Declare it outside the block of do and your error will be gone.

Correct way -

long cardNum = 0;
do
{
    cardNum = get_long("Card number:\n");
}
while (0 <= cardNum && cardNum <= 9999999999999999);

In your case, you were getting error because cardNum is only scoped for do block and isn't visible inside the while because its scope ends as soon as it leaves the do block. This is why you are getting use of undeclared identifier 'cardNum' as error. The compiler sees it as an undeclared variable inside the while() function.

NOTE : you are better off using long long int because maximum range of long is implementation based and might sometimes just be limited to 2,147,483,647 (most probably on 32 bit systems). You can also use int cardNum[20] or whatever your maximum limit is instead of 20.

edited Jun 01 '20 at 20:02

answered Jun 01 '20 at 19:29

Abhishek Bhagate

5,583
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Well, `LONG_MAX` is `9223372036854775807`. So, I guess it can hold that. Corrected the second point – Abhishek Bhagate Jun 01 '20 at 19:45
`LONG_MAX` is implementation-defined. – Eric Postpischil Jun 01 '20 at 19:55
I have no idea what you are suggesting me now. Only Abhishek seems to be replying to the error I'm referring; No, I can't use a string as this variable, it's for the CS50's problem set 2, and it says it shall be a float. The input is a float, and I should not use arrays. – Susanna Dafna Jun 01 '20 at 20:46
Plus, I tried to declare the variable outside of the do while loop, it generated basically the same error, saying that the variable was already declared before the loop, (0 in the case), so... – Susanna Dafna Jun 01 '20 at 20:47

Use of undeclared identifier 'cardNum'

1 Answers1