Thread: Warning C4700 uninitialized variables?

Hello All,
i am taking an engineering computer course which teaches c programming for beginners so i know almost nothing, i need to make a program that calculates x^n recursively without using the pow function. When i debug i get two warning messages saying (Warning C4700) that x and n are uninitialized local variables and i have to abort the program.

this is the code that produces the error;

Code:

#include <stdio.h>
#include <math.h>
int main()
{
	int x,n,ans;
	printf("input two positive values\n");
	scanf("%d" "%d",x,n);
	ans=x;
{
	while(n>0);
	ans=ans*x;
	n=n--;
}
	printf("%d\n",ans);
}

i think it should work but i don't understand why this error is coming up.
Thank you in advance!

We have a little bit of oopsidaisies. As laserlight said, you want to pass the address of x and n. (That let's scanf modify their values). I did the simplest fix on your loop (and then had to remove the semicolon because it was an infinite loop otherwise). I came up with

Code:

#include <stdio.h>
int main()
{
	int x,n,ans;
	printf("input two positive values\n");
	scanf("%d" "%d",&x,&n);
	ans=x;
	while(n>0)
	{

		ans=ans*x;
		n=n--;
	}
	printf("%d\n",ans);
}

And I ran it:

Code:

input two positive values
2 3
16

Oopsies. Change line 7 to read ans = 1;

Code:

#include <stdio.h>
int main()
{
	int x,n,ans;
	printf("input two positive values\n");
	scanf("%d" "%d",&x,&n);
	ans=1;
	while(n>0)
	{

		ans=ans*x;
		n=n--;
	}
	printf("%d\n",ans);
}

Prior to that your code was computing x^(n+1). I have some more thoughts. One, this isn't recursive, it's actually iterative. So when you turn it in it will compute correctly and still be wrong because it's not what they asked for. I won't post the code of the solution, but take it as a hint that there is an inductive definition of power(a, b):

power(a, 0) = 1
power(a, n) = a * power(a, n-1)

Second, do you understand now how scanf works? The idea is you put in a format string, then you follow that with a list of the references of the variables you would like to put the read values into. So let's say I'd like to read an int and then a string:

scanf("%d %s", &myInt, myStr); // myStr is a reference already, so &myStr would be wrong, in fact it will seg fault:

Code:

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

int main()
{
	int x;
	char str = malloc(sizeof(char)*16);
	scanf("%d %s", &x, &str); // Oops, wrong to take address of str as str is already and address
	printf("%d, %s", x, str);
	free(str);
}

Output:

Code:

./a.out
2 34lk
Segmentation fault

Reason why: scanf will attempt to write the string I enter into a pointer value which is wrong, because it's not in my address space:

Code:

valgrind ./a.out
==2378== Invalid read of size 1
==2378==    at 0x4E777AA: vfprintf (vfprintf.c:1614)
==2378==    by 0x4E7E6F9: printf (printf.c:35)
==2378==    by 0x400643: main (foo.c:9)
==2378==  Address 0x68 is not stack'd, malloc'd or (recently) free'd
==2378== 
==2378== 
==2378== Process terminating with default action of signal 11 (SIGSEGV)
==2378==  Access not within mapped region at address 0x68
==2378==    at 0x4E777AA: vfprintf (vfprintf.c:1614)
==2378==    by 0x4E7E6F9: printf (printf.c:35)
==2378==    by 0x400643: main (foo.c:9)

Thank you very much, now that i put in the &x and &n for scanf i can fix the programs mathematical issues. Thank you both so much, and thank you QuatdraticFighte for the scanf explanation.

Originally Posted by Elysia

QuadraticFighte:
You forgot to make str an actual pointer.

char str = malloc(sizeof(char)*16);
should really be
char* str = malloc(sizeof(char)*16);

But then again, you really don't need dynamic memory at all, so

char str[16];

would suffice.
I also urge you to read SourceForge.net: Scanf woes - cpwiki

My mistake, that was a bad copy-paste. The reason for malloc was that I wanted a contrived way to make sure I would go out of my address space. I don't know how to do that if I am on the stack.

Originally Posted by Elysia

"would go out of my address space"?
You mean don't go out of address space?

No, I don't. I wrote a program to seg fault on purpose to illustrate why if you are going to scanf("%s") you put it into a char* string, passing string rather than &string, because we don't want to pass the address of the string.

The purpose was to illustrate the difference between

Code:

char* myString;
scanf("%d", &myInt); // and the following...
scanf("%s", string); // Which is correct, versus
scanf("%s", &string); // Which is incorrect

What I did was intentional.