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convert int to string

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Join Date: Sep 2004

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cortiknee

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convert int to string

Sep 18th, 2004

I need to figure out how to append an int to a string, so basically i need to convert the int to a string 1st, but I cant figure out how. :rolleyes: any help would be greatly appreciated. thank you, courtney

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Re: convert int to string

Sep 18th, 2004

Greetings,

This process is not hard since the default libraries included a function to do this. It's called itoa().

» char *itoa(int value, char *buffer, int radix);
Converts an integer value to a null-terminated string using the specified radix and stores the result in the given buffer.
If radix is 10 and value is negative the string is preceded by the minus sign (-). With any other radix, value is always considered unsigned.
buffer should be large enough to contain any possible value: (sizeof(int)*8+1) for radix=2, i.e. 17 bytes in 16-bits platforms and 33 in 32-bits platforms.
Further information can be found here.

Here is an example of how to use this function:

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#include <stdlib.h>	// for itoa() call
#include <stdio.h>	// for printf() call

int main() {
	int num = 123;
	char buf[5];

	// convert 123 to string [buf]
	itoa(num, buf, 10);

	// print our string
	printf("%s\n", buf);

	return 0;
}

I hope this helps,
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Re: convert int to string

Sep 18th, 2004

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Originally Posted by Stack Overflow

This process is not hard since the default libraries included a function to do this. It's called itoa().

And itoa is nonstandard. A standard function is sprintf.

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#include <stdio.h>
   
   int main(void)
   {
      const char base[] = "filename";
      char filename [ FILENAME_MAX ];
      int number = 42;
      sprintf(filename, "%s%d", base, number);
      printf("filename = \"%s\"\n", filename);
      return 0;
   }
   
   /* my output
  filename = "filename42"
  */

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Re: convert int to string

Sep 18th, 2004

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Originally Posted by Dave Sinkula

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sprintf(filename, "%s%d", base, number);
sprintf(filename, "%s%d", base, number);

Ah, yes. Though remember sprintf() does not calculate for buffer overflow. sprintf() doesn't flush any file buffer, in fact it doesn't have any buffer associated with it. sprintf() operates on a character buffer (not quite the same thing). If you overflow that buffer you can crash the system.

A safer version of sprintf() is snprintf() which unfortunately isn't very standard yet. It appears in various forms from many compiler vendors and was finally standardized in C99 and will hopefully make it as part of standard C++ in the next standard of C++. Until then you just have to dig up the unstandard version of it if you have one.

If you have no unstandard version of snprintf() you're sort of out of luck. You can of course always write your own version of sprintf() but it can be quite tricky unless you have your compiler's version of sprintf().

Here is an open-source version of snprintf() and others if interested:
snprintf.c - a portable implementation snprintf (etc...)

Writing your own version of itoa() on the contrary is quite simple. Just for reference, here is an example of how itoa() works:

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

// Function declarations
// typedef __w64 unsigned int size_t
size_t strlen(const char *);
char *strrev(char *);
char *itoa(int, char *, int);

int main() {
	int num = 123;
	char buf[5];

	itoa(num, buf, 10);

	printf("%s\n", buf);

	return 0;
}

size_t strlen(const char *string) {
	const char *s;

	s = string;
	while (*s)
		s++;
	return s - string;
}

char *strrev(char *str) {
	char *p1, *p2;

	if (!str || !*str)
		return str;

	for (p1 = str, p2 = str + strlen(str) - 1; p2 > p1; ++p1, --p2) {
		*p1 ^= *p2;
		*p2 ^= *p1;
		*p1 ^= *p2;
	}

	return str;
}

char *itoa(int n, char *s, int b) {
	static char digits[] = "0123456789abcdefghijklmnopqrstuvwxyz";
	int i=0, sign;
    
	if ((sign = n) < 0)
		n = -n;

	do {
		s[i++] = digits[n % b];
	} while ((n /= b) > 0);

	if (sign < 0)
		s[i++] = '-';
	s[i] = '\0';

	return strrev(s);
}

It isn't perfect, but it gets the job done.

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Narue

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Re: convert int to string

Sep 18th, 2004

>Though remember sprintf() does not calculate for buffer overflow.
Neither does itoa.

>A safer version of sprintf() is snprintf() which unfortunately isn't very standard yet.
It is standard, but the standard just isn't widely implemented yet. As it is, C89 is the dominant C standard, and snprintf is nothing more than an extension on many compilers.

>Writing your own version of itoa() on the contrary is quite simple.
Yes, but doing it correctly is not as trivial as some would have you believe.

>*p1 ^= *p2;
>*p2 ^= *p1;
>*p1 ^= *p2;
It's beyond me why people still use this.

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Re: convert int to string

Sep 18th, 2004

» Neither does itoa.
Exactly, though now there is a working prototype. It wouldn't be hard for someone to modify it and make it safer. It is alot better off than re-writing sprintf().

» It is standard, but the standard just isn't widely implemented yet.
Standard only in C99.

» It's beyond me why people still use this.
Bitwise XOR Operator ^
a^b: 1 if both bits are different. 3 ^ 5 is 6.

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Re: convert int to string

Sep 18th, 2004

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Originally Posted by Stack Overflow

» It's beyond me why people still use this.
Bitwise XOR Operator ^
a^b: 1 if both bits are different. 3 ^ 5 is 6.

Yes, I think several of us know this swap trick. To others it may be confusing. It might even be confusing to the compiler such that it doesn't optimize as well as the straightforward approach. Other hacks too.

>>I need to figure out how to append an int to a string
>Ah, yes. Though remember sprintf() does not calculate for buffer overflow.

You must be on a platform with some really huge ints :lol: (whose decimal representation size, by the way, can be accounted for at compile time).

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Narue

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Re: convert int to string

Sep 19th, 2004

>It wouldn't be hard for someone to modify it and make it safer.
Don't forget that someone would also have to modify it to make it correct as well as safer.

I was implying previously that your code has a bug.

» It is standard, but the standard just isn't widely implemented yet.
>Standard only in C99.
Yes, that would be the standard that isn't widely implemented yet. Unless you know of any others that is.

>Bitwise XOR Operator ^
>a^b: 1 if both bits are different. 3 ^ 5 is 6.
I'm well aware of what it does and how it does it. What confuses me is why people still think that it's a neat trick. The XOR swap is difficult to get right because of obscurity, and when it is right, it isn't very flexible. On top of that, it may not be as efficient as you think.

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Re: convert int to string

Mar 7th, 2007

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Originally Posted by cortiknee

I need to figure out how to append an int to a string, so basically i need to convert the int to a string 1st, but I cant figure out how. :rolleyes: any help would be greatly appreciated. thank you, courtney

hope this helps :

 Help with Code Tags 
 C Syntax (Toggle Plain Text) 
 
int i = 42; // number to convert
char buffer[3]; // 3 digit buffer
char* output; //output string (already filled with something)
 
sprintf(buffer, "%d", i);
strcat(output, buffer);
 
int i = 42; // number to convert
char buffer[3]; // 3 digit buffer
char* output; //output string (already filled with something)
 
sprintf(buffer, "%d", i);
strcat(output, buffer);