Data validation

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Once bad data gets into a system, such as a database, it is very difficult to find and remove it. When copies of that bad data are made, it gets even harder.

It is a good idea to keep bad data out of a system.

Letting users enter any data they want into a system can cause problems.

Many of our programs have assumed that another program is providing the data and that that program kept bad data out of this program.

At the user interface, bad data needs to be kept out.

In many web systems, users are not allowed to proceed until the data meets certain requirements.

email addresses
phone numbers
credit card numbers
... and so on ...

In programming, the term validation refers to checking that what the user typed was expected. If not, the user is asked to provide additional input.

The related term verification refers to ensuring that the program is correct according to specifications.

In validation, the data needs to meet a certain format.

It is not, in general, possible to actually check that email addresses, phone numbers, etc., are, in reality, the correct ones when entered.

For example, one way to verify an email address is to send a message with a code in a link to the email address. The user then needs to click on the link to verify that the email account actually exists. (But, the user may be a machine/bot and not human - another issue).

Here is the pseudo-code that uses a loop for getting data and validating it.

set valid is false while not valid do ask for data d1 get data d1 if data is valid then set valid to true else tell user the input was not valid end if end while

Note: C does not directly support true and false so 1 is usually used for true and 0 is used for false.

Here is a C program that shows validation.

Here is the C code.

#include <stdio.h>

int main() {
	int min1 = 1;
	int max1 = 10;
	int num1;
	int valid1 = 0;
	while (valid1 == 0) {
		printf("Enter a number from %d to %d: \n",min1,max1);
		scanf("%d",&num1);
		printf("You entered number %d.\n",num1);
		if ((num1 >= min1) && (num1 <= max1)) {
			valid1 = 1;
			}
		else {
			printf("The number %d is not valid. Please try again.\n",num1);
			printf("\n");
			}
		}
	printf("The valid number is %d\n",num1);
	return 0;
	}

Here are some examples of input and output for the above program code.

Here is an example input.

0
11
5

For the above example input, here is the expected output.

Enter a number from 1 to 10: 
You entered number 0.
The number 0 is not valid. Please try again.

Enter a number from 1 to 10: 
You entered number 11.
The number 11 is not valid. Please try again.

Enter a number from 1 to 10: 
You entered number 5.
The valid number is 5

Here is an example input.

6

For the above example input, here is the expected output.

Enter a number from 1 to 10: 
You entered number 6.
The valid number is 6

Here is an example input.

-2
4

For the above example input, here is the expected output.

Enter a number from 1 to 10: 
You entered number -2.
The number -2 is not valid. Please try again.

Enter a number from 1 to 10: 
You entered number 4.
The valid number is 4

Note that the example above uses a loop.

In actual programs, the above would never be written.

Instead, the program would validate the data and, if there is a problem, raise an error or just halt the program with an appropriate error message.

On a web page, for example, an error message would be displayed and no action would take place until the input data is validated.