Thread: pointer issues

hey guys, i'm pretty new to C, worked with java before.
I thought those two languages are very similar, but the pointers are killing me:

i have to write a c-program which recognizes 5 different operations und executes them.
add, sub, mul, gcd and isprime.

ie: input string "12*6" should result in "72".

its working fine for add,sub and mul, but there is a conflict between gcd and prime.

Like i told, im pretty new to C, dont laught at me if the code is that bad

Sorry for the monster box, i dont know how to create a scroll box

Code:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>


long int isnumcheck (char *string)
{	
	char res[20]; 
	
	char *rest=res;
	
		
	strtol(string, &rest, 0);
	
	if (strcmp(rest, "")==0)
		
	{	
		return 1;

	}
	
	else 
		
		return 0;
}



int ggt (int a, int b) {
	
	if (a == 0) return b;
	
    else while (b != 0) {
        if (a > b) 
		{
			a = a - b;
		}
        else b = b - a;
	}
	return a;
	
}

int isprime (int n) {
	
	for(int i=2;i<=(n/2);i++)
		if(n%i==0) return 0;
	
	return 1 ;
}


int arithmexe (char* zahl1, char* zahl2, int operation)
// operation 0 = PLUS ; 1 = MINUS ; 2 = MAL ; 3 = Primzahl; 4 = GGTeiler


{
	int erste=atoi(zahl1);
	int	zweite=atoi(zahl2);
	int ausgabe;
		
	if (operation==0)
	{
		ausgabe=erste + zweite;
		
	}
	if (operation==1) 
	{
		ausgabe=erste-zweite;
	}
	if (operation==2) 
	{
		ausgabe=erste*zweite;
	}
	if (operation==3)
	{
		ausgabe= isprime(*zahl1);
	}
	if (operation==4) {
		ausgabe==ggt(*zahl1, *zahl2);
	}
	return ausgabe;


}

//MAIN

int main()
{
	
	char befehl[20]="", plus[20]="", minus[20]="", mal[30]="", klammer1[20]=""; //
	int		eins=0, zwei=0;
	char  *plustok=0,*minustok=0, *maltok, *ggttok=0, *primtok=0, *plustokh=0, *minustokh=0, *maltokh=0, *ggttokh=0, *klammertok1=0, *klammertok2=0;
	
	
	printf("Folgendes Eingabeformat erforderlich:\n");
	printf("a+b / a-b / a*b / ggt(a,b) / isprim(a)\n");
	printf("Zum Beenden: q!\n");
	printf ("Befehl eingeben: \n");
	scanf ("%s", befehl);
	printf("Eingabe: %s\n", befehl);
	
	
	
// LOOP	
	
		
	while (strcmp(befehl, "q!")) {
	
		
		strcpy(plus,befehl);
		strcpy(minus, befehl);
		strcpy(mal, befehl);
		strcpy(klammer1, befehl);

		plustok=plus;
		minustok=minus;
		maltok=mal;
		klammertok1=klammer1;
		
	
		plustok		=	strtok(plus, "+");
		plustokh	=	strtok(NULL, " ");
		minustok	=	strtok(minus, "-");
		minustokh	=	strtok(NULL, " ");
		maltok		=	strtok(mal, "*");
		maltokh		=	strtok(NULL, " ");
		/*
		klammertok1	=	strtok(klammer1, "(");
		klammertok1	=	strtok(NULL, " ");
		eins		=	strcmp(klammer1, "ggt");
		zwei		=	strcmp(klammer1, "isprim");

		
	

		if (eins==0) 
			{	
				ggttok		=	strtok(klammertok1, ",");
				ggttokh		=	strtok(NULL, ")");	
			}
		if (zwei==0) 
			{
				primtok		=	strtok(klammertok1, ")");
			}
		
		
		
		 printf ("plus: %s\n", plustok);
		 printf ("plus2: %s\n", plustokh);
		 printf ("Minus: %s\n", minustok);
		 printf ("Minus2: %s\n", minustokh);
		 printf ("mal: %s\n", maltok);
		 printf ("mal2: %s\n", maltokh);
		 printf ("klammer: %s\n", klammertok1);
		 printf ("prim: %s\n", primtok);
		 printf ("ggt: %s\n", ggttok);
		 printf ("ggt2: %s\n", ggttokh);
		*/
		
		
		if (isnumcheck(plustok) && isnumcheck(plustokh))
			
			{	
				printf("%d\n",arithmexe(plustok, plustokh, 0));

			}
		else if (isnumcheck(minustok) && isnumcheck(minustokh))
				{	
					printf("%d\n",arithmexe(minustok, minustokh, 1));
				}
		else if (isnumcheck(maltok) && isnumcheck(maltokh))
				{
					printf("%d\n", arithmexe(maltok, maltokh, 2));
				}
				
		/*else if (isnumcheck(primtok)) 
			{
					printf(" %d\n",arithmexe(primtok,"1",3));
				printf ("plus: %s\n", plustok);
				printf ("plus2: %s\n", plustokh);
				printf ("Minus: %s\n", minustok);
				printf ("Minus2: %s\n", minustokh);
				printf ("mal: %s\n", maltok);
				printf ("mal2: %s\n", maltokh);
				printf ("klammer: %s\n", klammertok2);
				printf ("prim: %s\n", primtok);
				printf ("ggt: %s\n", ggttok);
				printf ("ggt2: %s\n", ggttokh);
			}
	
		else if (isnumcheck(ggttok) && isnumcheck(ggttokh))
			{
				
				printf("%d\n",arithmexe(ggttok, ggttokh, 4));
			}
		*/
		else {
			
			printf("falsche Eingabe\n");
		}

			
			
		
		maltok		=	0;
		maltokh		=	0;
		eins		=	0;
		zwei		=	0;
		klammertok2	=	0;
		klammertok1	=	0;
		ggttok		=	0;
		ggttokh		=	0;
		primtok		=	0;
		
		
		printf("Befehl eingeben\n");
		scanf ("%s", befehl);
		printf("Eingabe: %s\n", befehl);


	} // LOOPEND

}

What conflict? nuclear , human rights ?

I'll look into it. In the meantime, here are some things:

Code:

	if (operation==4) {
		ausgabe==ggt(*zahl1, *zahl2);
	}

You probably meant assignment.
Also consider reading SourceForge.net: Scanf woes - cpwiki

EDIT:
The code is pretty confusing as it is now, with all german (I believe) names and whatnot all over the place and a naive approach to dealing with this.
If you have time, I would suggest another approach usually used by things like calculators.

ohh you're right! thx, one issue solved

edit:
any tips, how making it less "naive"?

The common approach to a calculator is reading an identifier, interpreting it, storing it, then moving on.
Say, you're going to do a calculation. Read a number, push it onto a stack.
Read operation. Store it.
Read the next number. Pop the number off the stack. Perform the operation and push the new result onto the stack.
If it's a function, then it's just a little different order. First read and interpret function, then read arguments and perform action.

Anyway, regarding your code, I believe I've figured out everything.
First off:
else if (isnumcheck(primtok))
The problem here is that unless you actually type isprim, primtok will be NULL and isnumcheck never checks for NULL.
So either:

• Initialize primtok to a valid string.
• Make isnumcheck check for NULL and fail if it is.

Then, in your operation function, you are passing the first character of the passed string into the operation functions. You must understand that's simply not going to work. One character is not a number. Furthermore, a number can take up more than one character.
You want to pass in the actual numbers that you've extracted:

Code:

	if (operation==3)
	{
		ausgabe= isprime(erste);
	}
	if (operation==4) {
		ausgabe=ggt(erste, zweite);
	}

And then there was the comparison instead of the assignment bug.

You need to learn to use a debugger. These were very simple issues that could have been found with just a small amount of experience instead of evil printf statements.

my "isnumcheck" function does check if the string is NULL, it differs between 0 and NULL, and fails if string=NULL, because "rest" is in that case not empty.

Then, in your operation function, you are passing the first character of the passed string into the operation functions. You must understand that's simply not going to work. One character is not a number. Furthermore, a number can take up more than one character.

I didn't understand this, but if I leave out ggt prime works fine and vice versa.

im trying to work with the debugger, but does show up anything, unless i try ggt, than it crashes:

Originally Posted by Bobbel

my "isnumcheck" function does check if the string is NULL, it differs between 0 and NULL, and fails if string=NULL, because "rest" is in that case not empty.

No, it does not. It invokes undefined behavior by trying to pass it off to strtol which expects a string and not a NULL pointer.
And I don't see what you mean by "it differs between 0 and NULL".

I didn't understand this, but if I leave out ggt prime works fine and vice versa.

Take a look at the strings
"100" and "200"
The pointers point to the first character in the strings, '1' and '2' respectively.
By dereferencing the pointers, you get the first character, '1' and '2' respectively.
Now, a character is basically an integer. All characters are represented as numbers and simply "mapped" to some graphic when they're printed.
So now you pass '1' and '2' to the functions, which is not the same as 1 and 2. Furthermore, this is not 100 and 200 which the real numbers were.

Yes, but you aren't passing those converted numbers to the functions. Also better to use strtol.
Be sure to fix scanf too (see link).

scanf_s isn't enough; see my link.

Code:

	if (string == NULL)
		return 0;

Checking to make sure string isn't NULL should be enough.

but doesnt

Code:

	if (strcmp(rest, "")==0)

	{
		return 1;

	}

	else

		return 0;

do the same?

Yes, but you aren't passing those converted numbers to the functions.

Why not?

edit:

thanks a lot for your help

Originally Posted by Bobbel

but doesnt

Code:

	if (strcmp(rest, "")==0)
	{
		return 1;
	}
	else
		return 0;

do the same?

Absolutely not. NULL is not a string!

Why not?

Code:

ggt(*zahl1, *zahl2);

What does this do, do you think?

sends the pointers of the integers to the function

edit:
lol now i see it, hell im wondering why it even worked.

i picked the strings, instead of the integers.

But ggt still doesn't work!