Thread: strange error while adding a long to char

Hello,

I am writing a program where I have to take a string and add an input number(key) to each element of the string thus creating a sort of cipher. I have the line that does the addition to each element as

Code:

for(int i = 0; i < strlen(input); i++)
    {
        printf("%c", input[i] + (int)((key + 26) % 26));
    }

This works as long as I have (int) type cast as shown above. But the function that implements the above code accepts the key as a long to accommodate for bigger integers. so if I just do not typecast to int and just do

Code:

for(int i = 0; i < strlen(input); i++)
    {
        printf("%c", input[i] + ((key + 26) % 26));
    }

the compiler throws an error saying

error: format specifies type 'int' but the argument has type 'long' [-Werror,-Wformat]
[I] printf("%c", input + ((key + 26) % 26));

I do not follow:

1) Why I have to force the int typecast while a long is just an int with higher range.

2) Why does the error say that my format specifies int while I am clearly specifying %c and not %d?

1) Because the compiler warns you when you use the incorrect type in the printf format, and you have told the compiler to treat all warnings as errors. Also, a variable argument list has no set type for its arguments, so the compiler doesn't really know to cast that long to an int. So it ends up passing a long to printf, but printf tries to read an int. In systems where long is larger than int, that could be a problem.

2) Character literals in C are of type int, as to why... probably to make the code easier to manage on the processor level.

Imagine %c is a door big enough to fit a wheel barrow 80cm wide.

When you use "key", your compiler says, "well... that is 160cm wide [because it is of type "long"], so we'll put the result of this little calculation into one that big".

You then go to wheel it into an 80cm door and you get the warning... As you should...


When you use the cast, what you are saying is - When you get the output of that little calculation, just use a 80cm wheel barrow for the result, I know what I am doing.