Passing in values to functions

I remember that there was a way to pass a value into a function sometimes, but not always. If you didn't pass the value into the function, then either that variable doesn't exist for the lifetime of the function or the variable is automatically set to a predefined value.

For example, I have a function prototype

Code:

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printCommand(Line *head, int lineNumber)

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And sometimes I want to call this function without the int being called. Also, is it possible to instead send a string to the function as well?

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is it possible to instead send a string to the function as well

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The C language does not support sending/returning arrays by value, but you may pass them in by reference - i.e. A pointer to the first char in the array

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And sometimes I want to call this function without the int being called

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If you are referring to overloading the function, the answer is no. What you can do is use a pointer to pass the int into the function and when you want to not use the int pass a NULL into the function -> Your function could test for this and behave differently if the int was a NULL pointer

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Originally Posted by Click_here

If you are referring to overloading the function, the answer is no. What you can do is use a pointer to pass the int into the function and when you want to not use the int pass a NULL into the function -> Your function could test for this and behave differently if the int was a NULL pointer

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Oh I see. Is that how some common library functions such as printf and scanf work?

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Originally Posted by Mentallic

Oh I see. Is that how some common library functions such as printf and scanf work?

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They use a "variable argument list" using the macros defined in stdarg.h. That might be what you want, but you'd have to explain exactly what you're trying to accomplish to get the best advice.

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Originally Posted by oogabooga

They use a "variable argument list" using the macros defined in stdarg.h. That might be what you want, but you'd have to explain exactly what you're trying to accomplish to get the best advice.

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Well, each node in my linked list is a line of text, and I'm required to - depending on user input - print out either a specific line number (hence the int lineNumber) or also if they input '+' or '-' to print out the next/previous line respectively.
I've settled with changing the function prototype to

Code:

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printCommand(Line *head, int lineNumber)

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because I think it'll keep things more clear in the end if I just converted the strings to numbers if they are numbers, because I'm going to also be taking in a lot more different types of character input down the track.

Entering a NULL pointer to printf is a big no.

If you are interested in the way they work, look up "va_list"/ "va_start"/ "va_end".

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Originally Posted by Mentallic

Well, each node in my linked list is a line of text, and I'm required to - depending on user input - print out either a specific line number (hence the int lineNumber) or also if they input '+' or '-' to print out the next/previous line respectively.

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Here are some possibilities.


Use negative line numbers as special codes:

Code:

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#define CMD_NEXT -1
#define CMD_PREV -2

void print(Line *head, int lineNumber) {
    switch (lineNumber) {
    case CMD_NEXT:
        // print next line
        break;
    case CMD_PREV:
        // print prev line
        break;
    default:
        if (lineNumber > 0)  // or >= 0
            // print line lineNumber
        else
            // error
    }
}

// calls
print(line, 20);        // print line 20
print(line, CMD_NEXT);  // print next line
print(line, CMD_PREV);  // print prev line

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Use an extra parameter:

Code:

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void print(Line *head, char command, int lineNumber) {
    switch (command) {
    case 'N':
        // print next line
        break;
    case 'P':
        // print prev line
        break;
    case 'L':
        // print line lineNumber
        break;
    default:
        // error
    }
}

// calls
print(line, 'L', 20); // print line 20
print(line, 'N', 0);  // print next line
print(line, 'P', 0);  // print prev line

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Use a string:

Code:

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void print(Line *head, char *command) {
    int lineNumber;
    switch (command[0]) {
    case 'N':
        // print next line
        break;
    case 'P':
        // print prev line
        break;
    case 'L':
        // print line lineNumber
        lineNumber = atoi(command + 1);
        break;
    default:
        // error
    }
}

// calls
print(line, "L20"); // print line 20
print(line, "N");  // print next line
print(line, "P");  // print prev line

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Thanks for showing me a few options. I had already began using negative values to represent traversing the list but decided to go with strings instead since it seemed to make things a bit clearer.

Oh and it works now, so again thanks.