Basic question on "if" routing

[AdrianH]

I am starting to learn C for use with a small AVR single board computer. The board came with C code examples and I am learning how C works by modifying the code and adding sections etc.

I am stuck with use of the "If" decision making. I have a book and have also tried to follow the example on the tutorial here.

Rather then put all my code down I have abbreviated a lot, sorry for this but I am more interested in the programme flow. I have attached an jpg image of the wanted flow control with the continues all going out of the if block.

Below is my attempt at replicating it in C with what I believe is the correct formatting.

My question is, am I correct? There may well be a better way and I would love to see it but for now yes or no and if no where have I gone wrong. Sorry for the long pre-amble.

if (a)	{
		if (b){
				do 1;
				continue;
			}
		else
			{
			if (c){
					if (d)
						{
						do 2;
						continue;
						}
					else
						{
						continue;
						}
					}
				else
				{
				if (e) 
					do 3;
				else 
					continue;
				}
			}
		}
else
	continue;

Cheers

Adrian

[master5001]

C has something called a switch case statement.

Example:

switch(condition)
{
  case 1: // This just signifies that the condition is equal to 1
     break;
  case 2: // ditto, but 2
     break;
  case 3:
     break;
  default: // Last but never least, this is what happens if none of your special cases are matched.
}

[Elysia]

if (a)
{
	if (b)
	{
		do 1;
		continue;
	}
	else
	{
		if (c)
		{
			if (d)
			{
				do 2;
				continue;
			}
			else
			{
				continue;
			}
		}
		else
		{
			if (e) 
				do 3;
			else 
				continue;
		}
	}
}
else
	continue;

You need to learn to indent properly or you end up with unreadable code.
And you should also choose a style and stick with it.
I notice but one error. A missing instruction in the block highlighted by red.

[AdrianH]

Elysia;

Thanks for the response, took me a while to notice even after your response to see I had missed a continue; from that section.

Sorry for the formatting my editor notepad++ and the forum may well have different tab settings it did not look as bad when I entered it first off. And a style or recognised style is something I guess I will pick up as I go along.

master5001;

Thanks for your pointer to an alternative method I will try that as I go.

Cheers

Adrian

[master5001]

One thing to be aware of about each case statement is that must refer to a constant.

For example:

switch(key_pressed)
{
  case user_key_1:
     open_menu();
     break;

   ...

Won't cut it since you can't have a variable as a case. Just constants. I know some dirty tricks around that, however since they are dirty they are also potentially undefined and are definitely undefined according to the standard.

[AdrianH]

Hi again master!

OK some terms are still quite vague to me, so please persevere.

When you say a constant is that like... value = 6 or similar?

My book shows an example with a character variable c: and goes on with an example on determining what keyboard character has been used, so I thought it was mean for strings.

In my present use (a) (b) (c) are all function calls that will return 1 or 0.

As I say a long way to go, but I will get there, slowly.

Adrian

[Elysia]

(In C) Yes, constant means a value (like a number) or a #define.
In C++, it has a different meaning - meaning that any expression that can be determined at compile time, whether it be a #define, a number or a constant variable.
Of course, since you're using C, the first line applies to you.

[matsp]

(In C) Yes, constant means a value (like a number) or a #define.
In C++, it has a different meaning - meaning that any expression that can be determined at compile time, whether it be a #define, a number or a constant variable.
Of course, since you're using C, the first line applies to you.

Although modern C also has adopted the "constant variable" concept from C++, so it's the same in C if we assume that the C99 standard is being used.

Certainly, function calls are NOT constants in either of the two languages. Even if the function always returns a constant value, the call to the function is still a function call [even if the compiler can under some circumstances replace the entire function call with a constant value, it is not a constant from the stricter sense that it needs to be to be a valid case statement].

It is valid (in both C and C++) to have a statement like this, however:

   case 1:
   case 1+2:

and it will be the same as case 1 and case 3 respectively. Anything that the compiler can figure out what it is during the compile process.


--
Mats

[Elysia]

Certainly, function calls are NOT constants in either of the two languages. Even if the function always returns a constant value, the call to the function is still a function call [even if the compiler can under some circumstances replace the entire function call with a constant value, it is not a constant from the stricter sense that it needs to be to be a valid case statement].

But that will change, at least to an extent, in the coming C++ standard! Yay.
I don't know how long C will be left out of that feature, if not always.