C, Help while loop continues while not true

Question

Task: Write a char do/while loop, where the program will end if the letter is not in capital:

Solution:

Char input;

do{
    scanf("%c", &input);

} while (input <'a' || 'z'< input);

So my program says: "do this, while the input is either a or z". Why does it control all letters from a to z and how come my program ends if it's a little char instead of a capital?

I'm new to C, and I can't find an explanation anywhere, thanks in advance.

Answer 1

the problem is this statement:

while (input <'a' || 'z'< input);

as this is looking for anything not lower case letters and not taking into account the whole ascii (single char) table of possibilities.

And the criteria is for upper case which those letter are lower case.

you could use:

while ('A' <= input && input <= 'Z')

however, best to use the functionality in the header file: ctype.h because not all systems use the ASCII character set. (IBM mainframe for instance, uses EBCDIC rather than ASCII, where the alphabet is not contiguous )

Remember that the 'enter' key is not upper case, (and not allowed for in the code)

the following proposed code:

1. cleanly compiles
2. performs the desired function
3. properly checks for errors
4. uses the facilities defined in the header file ctype.h

and now the proposed code

#include <stdio.h>   // scanf(), perror()
#include <stdlib.h>  // exit(), EXIT_FAILURE
#include <ctype.h>   // isupper()


int main( void )
{
    // 'char' is all lower case: 
    // so this statement: Char input;
    // does not compile, suggest:
    char input;

    do
    {
        int scanfStatus = scanf("%c", &input);
        // always check the returned value (not the parameter value)
        if( 1 != scanfStatus )
        {
            perror( "scanf failed" );
            exit( EXIT_FAILURE );
        }
    } while ( isupper( input ) );
} // end function: main

Answer 2

Your question is:

Why does it control all letters from a to z and how come my program ends if it's a little char instead of a capital?

The answer is, because of the while test, which tests whether input <'a' or 'z'< input.

Here is some background information that will help you understand why this happens.

In your program, input is a char, and, according to the C standard, the char type is an integral type. This means that 'a' (C's way to designate a char literal) is, in fact, a number, and thus, it can be compared with comparison operators such as < or > to other (integral) numbers or other char (here, the content of input).

Now, what is the actual integral value of a char? While the integral values of the character set are implementation-defined, in general, C compilers (including Visual Studio's) will use the ASCII Character Codes.

So:

• 'a' in your code, refers to the integral value of ASCII code for the char 'a', which is 97,
• and 'z' in your code refers to the integral value of ASCII code for the char 'z', which is 122

As you can see also from the ASCII table (ASCII Character Codes Chart 1 from MSDN), the alphabet a-z has consecutive code numbers ('a' is 97, 'b' is 98, etc.), and the lowercase alphabet is effectively an ASCII code from 97 to 122.

So, in your code, the test input <'a' or 'z'< input is equivalent to input < 96 or 122 < input, and this will be true when the entered char has any ASCII value outside the range of 96 - 122, meaning, any char that is entered which is not in the range of ASCII codes for lowercase letters from 'a' to 'z' will result in the while test being true, and repeating the scanf().

Finally, as noted by other commentators or contributors, the right type is char, not Char since C is case-sensitive.