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macOS My F Statement is not working...

cybrscot

cybrscot

macrumors 6502
Original poster
Dec 7, 2010
282
0
Somewhere in Southeast Asia
I'm not having any luck with my IF statement. I've tried changing around the variables, etc, but I'm not sure why it won't work. It does compile, but the output is always 0. I don't know why. I'm using a %d placeholder for the wind = variable. I think that C treats characters as int, so I should be able to use the %d placeholder for text, right? Any ideas? I have already written a similar program, that worked, but I don't see the difference. So I'm at a loss on this one.

Code: 
#include <stdio.h>

main ()
{

	int velocity, wind ;
	int calm, light, breezy, gale, storm, hurricane;
	
	printf ("Enter wind velocity (knots): ") ;
	scanf ("%d", &velocity ) ;
	
	if (velocity < 1 )
		wind = calm ;
	else if (velocity < 4 )
		wind = light ;
	else if (wind < 28 )
		wind = breezy ;
	else if (velocity < 48 )
		wind = gale ;
	else if (velocity < 64 )
		wind = storm ;
	else if (velocity > 63 )
		wind = hurricane ;
		
	printf ("Conditions are... %d\n", wind) ;
	
	return ; 
	
}


Code: 
Light airScott-Deans-MacBook-Pro:~ scottdean$ gcc ~/documents/wind.c
Scott-Deans-MacBook-Pro:~ scottdean$ ./a.out
Enter wind velocity (knots): 2
Conditions are... 0
 
What is calm set to? What is light set to? What are breezy, gale, storm, and hurricane set to? I don't see any assignment statement setting their value. They could be anything at all. What are you expecting them to be set to? If you DID print a character value (you have no char variables or constants) as an integer value, what would you expect to see?

-Lee
 
make an enum

It looks to me like you either intended to create an enum (but never did), or you intended to initialize your variables (but never did).
 
hmmm, I guess I thought that each "wind = statement" was connected to it's above "if expresson", so that when it tested the velocity against the expression, it would print the wind = (calm, gale, hurricane, whatever is associated with that if). So if it tested velocity at 0, it's true that it's less than 1, so I thought it might print the wind = calm, which is why in my final printf I use (%d, wind) and was thinking that it would print the word that = wind for the tested true statement.

Damn, what an terrible piece of code. Okay, back to the drawing board
 
cybrscot said:
Damn, what an terrible piece of code. Okay, back to the drawing board
Click to expand...

That's a big step in becoming a programmer! (EDIT: http://xkcd.com/844/)

You made calm, light, breezy, gale, storm, hurricane separate int variables that are just like velocity, wind and you didn't initialize them to a value.

As mydogisbox said, look at enums, and if your goal is to print

Code: 
Conditions are... hurricane
then wind needs to be a string (char *) and use %s in your printf format string. As it stands you could only get

Code: 
Conditions are... 1

EDIT:Also this is a good place to learn about switch/case/break.

B
 
balamw said:
That's a big step in becoming a programmer! (EDIT: http://xkcd.com/844/)

You made calm, light, breezy, gale, storm, hurricane separate int variables that are just like velocity, wind and you didn't initialize them to a value.

As mydogisbox said, look at enums, and if your goal is to print

Code: 
Conditions are... hurricane
then wind needs to be a string (char *) and use %s in your printf format string. As it stands you could only get

Code: 
Conditions are... 1

EDIT:Also this is a good place to learn about switch/case/break.

B
Click to expand...

Actually, I have another thread with that question. I DID try to do it in a switch/case/break, but I'm running into the problem that the case is supposed to be constant, and I need the cases to be a range of numbers that the user may input. How to do? I'll go check that other thread, haven't yet.

Thanks
Scott
 
Also, as far as the char* and %s, the book hasn't covered those yet, so I think I'm supposed to write this without those. The question in the book did ask that after the user input the wind velocity in knots, the program return the value, calm, gale, etc. So the author isn't looking for 0, or 1. I need to figure out how to get the corresponding word to print.

Question from Book:
Write a program that asks the user to enter a wind velocity (in knots), then displays the corresponding description.

Velocity Description
Less than 1 Calm
1-3 Light Air
4-27 Breeze
etc
etc
etc.
 
With the tools you have at hand you can put the printf in the if statements.

Code: 
#include <stdio.h>

main ()
{

	int velocity;
	
	printf ("Enter wind velocity (knots): ") ;
	scanf ("%d", &velocity ) ;
	
	if (velocity < 1 )
		printf ("Conditions are calm\n") ;
	else if (velocity < 4 )
		printf ("Conditions are light\n") ;
	else if (wind < 28 )
		printf ("Conditions are breezy\n") ;
	else if (velocity < 48 )
		printf ("Conditions are gale\n") ;
	else if (velocity < 64 )
		printf ("Conditions are storm\n") ;
	else if (velocity > 63 )
		printf ("Conditions are hurricane\n") ;
			
	return ; 
	
}

You could use an expression to take your ranges to an integer that could be used in a switch/case with an enum, but it sounds like this is where the author wants you.

B
 
balamw said:
With the tools you have at hand you can put the printf in the if statements.

Code: 
#include <stdio.h>

main ()
{

	int velocity;
	
	printf ("Enter wind velocity (knots): ") ;
	scanf ("%d", &velocity ) ;
	
	if (velocity < 1 )
		printf ("Conditions are calm\n") ;
	else if (velocity < 4 )
		printf ("Conditions are light\n") ;
	else if (wind < 28 )
		printf ("Conditions are breezy\n") ;
	else if (velocity < 48 )
		printf ("Conditions are gale\n") ;
	else if (velocity < 64 )
		printf ("Conditions are storm\n") ;
	else if (velocity > 63 )
		printf ("Conditions are hurricane\n") ;
			
	return ; 
	
}

You could use an expression to take your ranges to an integer that could be used in a switch/case with an enum, but it sounds like this is where the author wants you.

B
Click to expand...

Wow, you are right on the money! I'm very disappointed though that I was overcomplicating the code. What you did was really pretty simple. I was going about it entirely the wrong way. Damn, up till now I've been pretty good about writing the correct "kind" of code, albeit some mistakes in the code. But on this one what I thought was appropriate was way off the mark. Thanks for the code and the time you took to help me. I appreciate it very much. Below was my most recent "attempt" at fixing my previous code, it still didn't work of course. LOL! :D Look at it, it's ridiculously complex compared to yours!!!

Code: 
#include <stdio.h>

main ()
{

	int velocity ;
	char calm, light, breezy, gale, storm, hurricane ;
	
	calm       < 1 ;
	light      < 4 ;
	breezy     < 28 ;
	gale       < 48 ;
	storm      < 64 ;
	hurricane  > 63 ;
	
	printf ("Enter wind velocity (knots): ") ;
	scanf ("%d", &velocity ) ;
	
	if (velocity < 1 )
		velocity = calm ;
	else if (velocity < 4 )
		velocity = light ;
	else if (velocity < 28 )
		velocity = breezy ;
	else if (velocity < 48 )
		velocity = gale ;
	else if (velocity < 64 )
		velocity = storm ;
	else if (velocity > 63 )
		velocity = hurricane ;
		
	printf ("Conditions are... %s\n", velocity) ;    /* I see mistake here I have %s, velocity is an int, I changed it, but still get output 0 */
	
	return ; 
	
}
 
Last edited:
The other folks have given you some good explanations already. I think it's important to step back and look carefully at what you're trying to do -- sometime it's useful to just go over the basics again.

These statements:

Code: 
int velocity, wind ;
int calm, light, breezy, gale, storm, hurricane;

These define a bunch of variables, each of type "int". The names of the variables are velocity, wind, calm, light, etc. However, variable names are only used by YOU as a programmer. They could just as easily have been named x or VARIABLE_Y or intVar02312 or aTypeOfWindThatFeelsCalm or whatever. It will make no difference to how the code works, except that using a name like "velocity" is more readable to you than a name like "x".

As others have pointed out, when you created a new variable called, say, "calm", the compiler presumably automatically assigned it a value of 0. By the way, you can't rely on a compiler to do this -- some won't -- so it's best practice to always explicitly set a default value or else make sure the variable gets set to some value. Otherwise, you might get unexpected results.

Strings, like the actual word "calm", are a whole other kettle of fish in C, as a string in C is actually an array of characters, and you'll need to understand some basics about arrays and "pointers" in order to use them.

cybrscot said:
Look at it, it's ridiculously complex compared to yours!!!
Click to expand...

Hmm... uh, without meaning to offend, Scott, that code suggests to me that you're still not quite understanding how variables work. I think it would be good for you to step back and practice playing with variables. Maybe write a program that just declares a few variables, does something with them (maybe x = y + z or something), prints out some values, stuff like that...
 
Thanks notjustjay. You're right. When I initialize a variable, must it be with an = symbol?

like
x = y + z

or can it be with relational operators too?

x < y + z
or
x =<y + z (read, x is less than y + z)

In my terrible example code that you looked at, I did
calm < 1 ;
light < 4 ;
etc
etc
 
cybrscot said:
In my terrible example code that you looked at, I did
calm < 1 ;
light < 4 ;
etc
etc
Click to expand...

I think you are greatly confusing things... At some point, you must do something like:

calm = 1;

When you do:

if (calm < 4) { do stuff }

You can do:

calm = x < y;

Which would put a 0 or 1 in there depending if it is true or false.

Calm has to be set to something so you can compare it. I think you greatly confuse things when you start mixing your assignments and comparisons. You can do this (and there are some cases to actually do this), but I think you won't get the results you are expecting.
 
Last edited:
ulbador said:
I think you are greatly confusing things... At some point, you must do something like:

calm = 1;

When you do:

if (calm < 4) { do stuff }

Calm has to be set to something so you can compare it. I think you greatly confuse things when you start mixing your assignments and comparisons. You can do this (and there are some cases to actually do this), but I think you won't get the results you are expecting.
Click to expand...

So the compiler can't figure out that 0, 1, 2, 3 are < 4?
If I set calm = 1: then the user must input 1 to return the value "calm"
So I would have to do this????
calm = 1
calm = 2
calm = 3 {would this have worked? Although way too many numbers and variables to initialize, I'm just curious if it would have worked.
 
cybrscot said:
Thanks notjustjay. You're right. When I initialize a variable, must it be with an = symbol?

like
x = y + z

or can it be with relational operators too?

x < y + z
or
x =<y + z (read, x is less than y + z)
Click to expand...

You have to use an = sign, yes. "Set x to be equal to something".

Relational operators are only used for comparing things, for example inside an if statement.

You can't say

x < y + z

But you can say

if (x < y + z)
{ // then do something
}

Or you can even say

q = x < y + z;

Which says "determine whether 'x < y + z' is TRUE or FALSE, and assign that answer to q". q will end up with a value of 0 if that expression "x < y + z" is FALSE, and non-zero if it is TRUE.

Basically an expression is "MAKE this happen" and a condition is "DETERMINE whether this is true or false".

Edit: One thing to keep in mind with an expression like "x < y + z" is that it is confusing. Which part is the computer going to calculate first? Will it figure out the sum of y + z, and then see if x is less than the total? Or will it look at "x < y", determine if that part is true (and give it a value of 0 if it is false), then add z to the result? I'm not sure. To be explicitly clear, you should write expressions like that using brackets, e.g.

q = x < (y + z);

or

q = (x < y) + z;
 
Last edited:
cybrscot said:
If I set calm = 1: then the user must input 1 to return the value "calm"
Click to expand...

Careful there. You just wrote the value "calm" in quotes, as if you expect the actual word "calm" (letters C, A, L, M) to appear on the screen. As I said, it won't. calm is just a variable name. The word "calm" has nothing to do with any of this. If you want to see the word "calm", that's a couple of steps ahead of where we are right now.
 
notjustjay said:
You have to use an = sign, yes. "Set x to be equal to something".

Relational operators are only used for comparing things, for example inside an if statement.

You can't say

x < y + z

But you can say

if (x < y + z)
{ // then do something
}

Or you can even say

q = x < y + z

Which says "determine whether 'x < y + z' is TRUE or FALSE, and assign that answer to q". q will end up with a value of 0 if that expression "x < y + z" is FALSE, and non-zero if it is TRUE.

Basically an expression is "MAKE this happen" and a condition is "DETERMINE whether this is true or false".
Click to expand...


Thank you very much!! Trust me, I am getting better at this stuff! These are all questions I would ask a teacher (if I had one) for clarification, but I don't have one, only the friendly great folks here on the forum!

Time for me to go to bed, so I've got to stop thinking about this code stuff, once my mind gets going on it, it can keep me up until 4am, (actually does every night, but I've got to be at work early tomorrow!) Thanks everybody!
 
cybrscot said:
So the compiler can't figure out that 0, 1, 2, 3 are < 4?
If I set calm = 1: then the user must input 1 to return the value "calm"
So I would have to do this????
calm = 1
calm = 2
calm = 3 {would this have worked? Although way too many numbers and variables to initialize, I'm just curious if it would have worked.
Click to expand...

I think where you are having your disconnect is with the actual variable name:

int calm = 0;

With that variable, the name of it is "calm". But that is just the name you use to reference it in your code.

That is not the name that gets printed to the screen when you try to print it out. When you try to print it out, you are printing out whatever the VALUE of the variable is. When you do:

printf("The variable is: %s", calm);

This tells the program to print the VALUE of calm as a "string". This doesn't mean that it is going to print "calm", this means it is going to try to convert whatever is stored IN the variable to a string and print it out.
 
You're welcome. Good luck :)

Here's a quick little program for you to study and play with, that will hopefully help you understand what's going on.
Code: 
#include <stdio.h>

main ()
{
	int myNumber = 71;  // Ssh, don't tell anyone!
	int yourGuess = 0;
	
	printf ("Guess what number I am thinking of: ") ;
	scanf ("%d", &yourGuess ) ;
	
	if (yourGuess < myNumber )
		printf("Sorry, %d is too low!\n", yourGuess);
	else if (yourGuess > myNumber )
		printf("Sorry, %d is too high!\n", yourGuess);
	else if (yourGuess == myNumber )
		printf("Yes, %d is correct!\n", yourGuess);
	return; 
}

You only get one shot at the guess (there is no "go back and guess again" loop). Also, that last "else if" isn't really necessary because if your guess is NOT too low AND your guess is NOT too high then it MUST be equal, so I could have just used an "else" instead of "else if".
 
Last edited:
cybrscot said:
I was going about it entirely the wrong way. Damn, up till now I've been pretty good about writing the correct "kind" of code, albeit some mistakes in the code.
Click to expand...

I think your instinct is right, you just don't have the tools to do what your instinct tells you yet.

Here's some sample code that might help you get your switch/case working.

Code: 
#include <stdio.h>

int main(int argc, char *argv) {

	int velocity;
	
	printf ("Enter wind velocity (knots): ") ;
	scanf ("%d", &velocity ) ;

	int condition;

        condition = (velocity > 1) + (velocity > 4) + (velocity > 28) + (velocity > 48) + (velocity > 64);
        printf ("%d\n", condition);

}

condition will have a value between 0 and 5 corresponding to {calm, light, breezy, gale, storm, hurricane}.

B
 
balamw said:
condition = (velocity > 1) + (velocity > 4) + (velocity > 28) + (velocity > 48) + (velocity > 64);

condition will have a value between 0 and 5 corresponding to {calm, light, breezy, gale, storm, hurricane}.
Click to expand...

I love utterly simplistic code like this. So perfect and elegant.
 
ulbador said:
I love utterly simplistic code like this. So perfect and elegant.
Click to expand...

I believe this is only true for C++, where "true" evaluates to 1 when converted to an int. I've read that with straight-up C, the only guarantee is that false is 0 and true is some non-zero value, possibly 1, possibly even -1.
 
notjustjay said:
I believe this is only true for C++, where "true" evaluates to 1 when converted to an int. I've read that with straight-up C, the only guarantee is that false is 0 and true is some non-zero value, possibly 1, possibly even -1.
Click to expand...

http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf

Each of the operators < (less than), > (greater than), <= (less than or equal to), and >= (greater than or equal to) shall yield 1 if the specified relation is true and 0 if it is false. The result has type int.
Click to expand...

B
 
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