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A barcode scanner for Universal Product Codes
(UPCs) verifies the 12-digit code scanned by comparing the
code’s last digit (called a check digit ) to its own
computation of the check digit from the first 11 digits as
follows:
I. [step 1]Calculate the sum of the digits in the
odd-numbered positions (the first, third, …,
eleventh digits) and multiply this sum by 3.
II. [step 2]Calculate the sum of the digits in the
even-numbered positions (the second, fourth, …,
tenth digits) and add this to the previous
result(result of step1).
III. [step 3]If the last digit of the result from step
2 is 0, then 0 is the check digit. Otherwise,
subtract the last digit from 10 to calculate the
check digit.
IV. [step 4]If the check digit matches the final digit
of the 12-digit UPC, the UPC is assumed correct.
1. Write main function that prompts the user to enter
the 12 digits of a barcode. The program should
store the digits in an integer array.
2. Write a “check” function to calculate the check
digit, and compare it to the final barcode digit.
If the digits match, output the barcode with the
message “validated.” If not, output the barcode
with the message “error in barcode.” Also, output
with labels the results from steps 1 and 2 of the
check-digit calculations. Note that the “first”
digit of the barcode will be stored in element 0
of the array. (clue: “check” function takes two
argument and it doesnt return any value)
---------------------------------

What I have tried:

C
```#include <stdio.h>
#include <stdlib.h>

int main(void)
{   int i;
int a[12]={0,0,0,0,0,0,0,0,0,0,0,0};
int sum_odd , sum_even , last_digit ,check_digit , ans_odd , ans_even;

printf ("enter 12 digit:\n");
for (i=1;i<12;i++)
{
printf("digit %d -->\t",i);
scanf("%d",&a[i]);
if (a[i]>=10)
{a[i]=0;
printf("Error\n");

i=i-1;
}
if(i%2==0)
sum_odd=sum_odd+a[i];
else
sum_even=sum_even+a[i];

}
printf("digit 12 -->\t");
scanf("%d",&a[12]);

ans_odd=sum_odd*3;
ans_even = ans_odd + sum_even;
last_digit=ans_even%10;

return 0;
}
void check (int a[],int i)
{
int last_digit,check_digit;
if (last_digit==0)
check_digit=0;
else
check_digit=10-last_digit;

if (check_digit==a[12])
{
}
else
}```
Posted
Updated 22-Jan-21 23:48pm
v2
jeron1 22-Jan-21 17:03pm
Is there a question?

## Solution 1

Compiling does not mean your code is right! :laugh:
Think of the development process as writing an email: compiling successfully means that you wrote the email in the right language - English, rather than German for example - not that the email contained the message you wanted to send.

So now you enter the second stage of development (in reality it's the fourth or fifth, but you'll come to the earlier stages later): Testing and Debugging.

Start by looking at what it does do, and how that differs from what you wanted. This is important, because it give you information as to why it's doing it. For example, if a program is intended to let the user enter a number and it doubles it and prints the answer, then if the input / output was like this:
```Input   Expected output    Actual output
1            2                 1
2            4                 4
3            6                 9
4            8                16```
Then it's fairly obvious that the problem is with the bit which doubles it - it's not adding itself to itself, or multiplying it by 2, it's multiplying it by itself and returning the square of the input.
So with that, you can look at the code and it's obvious that it's somewhere here:
C#
```int Double(int value)
{
return value * value;
}```

Once you have an idea what might be going wrong, start using the debugger to find out why. Put a breakpoint on the first line of the method, and run your app. When it reaches the breakpoint, the debugger will stop, and hand control over to you. You can now run your code line-by-line (called "single stepping") and look at (or even change) variable contents as necessary (heck, you can even change the code and try again if you need to).
Think about what each line in the code should do before you execute it, and compare that to what it actually did when you use the "Step over" button to execute each line in turn. Did it do what you expect? If so, move on to the next line.
If not, why not? How does it differ?
Hopefully, that should help you locate which part of that code has a problem, and what the problem is.
This is a skill, and it's one which is well worth developing as it helps you in the real world as well as in development. And like all skills, it only improves by use!

## Solution 2

You need to work out more details, so start with a function for every point of your assigment. It is good style to comment the code with the parts from the assigment.

Than write the code and use some test data to verify its function. Normally it is good idea to write tests and check for the expected result.

It shouldnt be to hard because you have already the skill and only need to work it out. Take small steps and use explaining names like "sumOdd" or "sumEven" for all.