Encrypt a number

Hello everyone. I am self learning C from the Dietel and Dietel book C How to program. I am doing an assignment at the end of the chapter that requires a user to enter a four digit integer that will be encrypted as follows. Replace each digit with the result of adding 7 to the digit and getting the remainder after dividing the new value by 10. Then swap first digit with third, and second with fourth. Then display the digit. I think I can solve the first part of the problem. I am not sure how I will switch the digits places. Any suggestions is appreciated. So far the material have only covered while loop, if, else if, and I can only use what's available to me up to this point. I look forward to your suggestions. Thank you.

Comments

[Matt Ellen]

To calculate the result of the swap you'll do:

third*1000 + fourth*100 + first*10 + second.

To get the individual digits you can use division and modulo arithmetic.

For example given the starting number 1234, to get the second digit:

(1234 mod 1000) ÷ 100. (This will give 2.)

You might need to cast to int to get rid of the fractional part.

[Val]

So what I did so far was form a new number based on the requirement.
`
int main(void)
{
int number, newNumber,remainder,diviser = 1000;

printf("%s","Enter a four digit number: ");
scanf("%d",&remainder);

while(remainder != 0) {

    number = remainder / diviser;
    remainder %= diviser;
    newNumber += ((number + 7) % 10) * diviser;
    diviser /= 10;
}
    printf("%s%d","New formed number: ",newNumber);
return 0;

}
`
I'll work now on implementing the swap.

[Val]

Thanks I'll give it a try.

[Val]

I've just had someone show me that printf function can format number with specifications. So it totally eliminates the rest of the code. Pretty cool.

`int main(void)
{
int number = 0,newNumber = 0,remainder,diviser = 1000;

printf("%s","Enter a four digit number: ");
scanf("%d",&remainder);

//breaking up the numbers into left most digit
while(remainder != 0) {
    number = remainder / diviser;
    remainder %= diviser;
    /*
    forming a new number by adding 7 to each digit and
    getting its remainder, then forming a new
    number by multiplying to appropriate
    power with each pass
    */
    newNumber += ((number + 7) % 10) * diviser;
    diviser /= 10;
}
printf("%s%d\n","New formed number: ",newNumber);
//swap of the digits
remainder = newNumber % 100;
number = newNumber / 100;
remainder = remainder * 100 + number;
printf("\n%s%04d","Encrypted number: ",remainder);

return 0;

}

`

[Val]

Hi everyone. So this is how I solve the above problem. I am not sure If I took the shortcuts to come up with this. If I did please let me know. Program works as is, the only formatting I didn't do is the new encrypted digit is evaluated to 0000, there is no order to swap anything. I left it as is.

`
int main(void)
{
int number,newNumber = 0,remainder,diviser = 1000;

printf("%s","Enter a four digit number: ");
scanf("%d",&remainder);

while(remainder != 0) {
    number = remainder / diviser;
    remainder %= diviser;
    newNumber += ((number + 7) % 10) * diviser;
    diviser /= 10;
}
printf("%s%d\n","New formed number: ",newNumber);
//swap of the digits
remainder = newNumber % 100;
number = newNumber / 100;
remainder = remainder * 100 + number;
if (remainder < 1000) {
    if(remainder < 100){
        printf("%s%d","Encrypted number is : 00",remainder);
    }else {
        printf("\n%s%d","Encrypted number: 0",remainder);
    }
}else {
    printf("\n%s%d","Encrypted number: ",remainder);
}

return 0;

}

`