Thread: extracting char from a string

I am trying to extract a number of characters from a string specified by the position where it has to begin and the number of characters to be extracted. The function seems to return some garbage value and it prints some funny characters. I was debugging it but unable to figure out the reason for it.


[insert]

Code:

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

char* getsub(char *a, int loc, int len);

int main(void)
{
	
	char str[20] = "Rohan is a good boy";
	char str1[20] = "Roh";
	char str2[20] = "Rohan is a good boy";
	char str3[20] = "Sohan";
	char *sub;
	char s;
	int result;	

	sub = getsub(str, 2, 4);
	printf("\n%s", sub);

	return 0;
}

char* getsub(char *a, int loc, int len)
{
	int lenstr;
	int pos= 0, i,j;
	char str[20];

	lenstr = strlen(a);
	for(pos = 0;pos< loc;pos++)
	{
		a++;
	}

	for(i =0; i<len;i++)
	{
		str[i]= *a;
		a++;
	}
	
	str[++i] = '\0';
	return str;

}

You are returning a local pointer -- once the function finishes, str ceases to exist, but you have a pointer to it in your main program. You also increment i one too many times before writing your \0 character.

Does that imply that char str[20] in function getsub() has to be declared outside main?

Or is there a way to get around this using some other thing?

If you declare it outside main, it will "work" in the sense that you will get a valid value out. The fun part will then be that you can only call the function once -- every time you call the function you'll get the same pointer back, because each function call is using the same "scratch area" to give the answer. That is, you'll get something like:

Code:

char *a, *b;
a = getsub(str, 2, 4);
printf("\"%s\"\n", a);  /* prints "han " */
b = getsub(str, 4, 2);
printf("\"%s\"\n", a); /* now prints "n ", even though a was not "changed" */

You're left with two choices:

1. Pass in the array that you want to be filled with your substring
2. Use dynamic memory allocation to continually give you new pieces of memory to hand out

The fun part will then be that you can only call the function once -- every time you call the function you'll get the same pointer back, because each function call is using the same "scratch area" to give the answer. That is, you'll get something like:
Code:

char *a, *b;
a = getsub(str, 2, 4);
printf("\"%s\"\n", a); /* prints "han " */
b = getsub(str, 4, 2);
printf("\"%s\"\n", a); /* now prints "n ", even though a was not "changed"

But if we are making successive calls to the function as you showed in the above case its a valid output. I mean its correct thats what the output should be when you call it for b.
It works fine even the second time.

That's what the output should be when I call it for b, yes. Unfortunately, I called it with a instead.

Okay but if i make the declaration of

char str[20]

outside main it gives me instead of "han " it gives NOPQ

I dont see anything wrong that i am doing.

Originally Posted by roaan

Okay but if i make the declaration of

char str[20]

outside main it gives me instead of "han " it gives NOPQ

I dont see anything wrong that i am doing.

Well, you're doing something wrong. You'll have to post the new code.

Here it is:

[insert]

Code:

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

int search (char *a, char b);
int isequals(char *a, char *b);
int issmaller(char *a, char *b);
int isgreater(char *a, char *b);
char atpos(char *a, int loc);
char* getsub(char *a, int loc, int len);

char str4[20];

int main(void)
{
	
	char str[20] = "Rohan is a good boy";
	char str1[20] = "Roh";
	char str2[20] = "Rohan is a good boy";
	char str3[20] = "Sohan";
	char *sub;
	char s;
	int result;	
	char *a, *b;

	//printf("\nEnter a character to search");
	//scanf("%c", &s);

	/*result = search(str, s);
	if(result == 1)
		printf("\n Char found");
	else
		printf("\n Char not found");
	*/

	/*result = isequals(str, str1);
	if(result == 1)
		printf("\n Char found");
	else
		printf("\n Char not found");
	*/

	/*
	result = issmaller(str, str1);
	if(result == 1)
		printf("\n 1st string smaller");
	else
		printf("\n Not smaller");

	
	result = isgreater(str3, str1);
	if(result == 1)
		printf("\n 1st string greater");
	else
		printf("\n 1st string not greater");
	*/
	
	/*
	s = atpos(str, 0);
	if((s >= 65 && s < 91) || (s >= 97 && s < 122)) 
	printf("\nChar is %c",s);
	else
		printf("\nYou are trying more than the lenght");
	*/

	sub = getsub(str, 2, 4);
	printf("\n%s", sub);
	
	return 0;
}

char* getsub(char *a, int loc, int len)
{
	int lenstr;
	int pos= 0, i,j;

	for(i = 0;i< 20;i++)
		str4[i] = NULL;

	lenstr = strlen(a);
	for(pos = 0;pos< loc;pos++)
	{
		a++;
	}

	for(i =0; i<len;i++)
	{
		str4[i]= a;
		a++;
	}
	
	str4[++i] = '\0';
	return str4;
}

What you have here doesn't compile.
Leaving aside the unused variable warnings, you still have this:

Code:

temp.c: In function `char* getsub(char*, int, int)':
temp.c:78: warning: converting to non-pointer type `char' from NULL
temp.c:88: error: invalid conversion from `char*' to `char'

This compiles


[insert]

Code:

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

int search (char *a, char b);
int isequals(char *a, char *b);
int issmaller(char *a, char *b);
int isgreater(char *a, char *b);
char atpos(char *a, int loc);
char* getsub(char *a, int loc, int len);

char str4[20];

int main(void)
{
	
	char str[20] = "Rohan is a good boy";
	char str1[20] = "Roh";
	char str2[20] = "Rohan is a good boy";
	char str3[20] = "Sohan";
	char *sub;
	char s;
	int result;	
	char *a, *b;

	//printf("\nEnter a character to search");
	//scanf("%c", &s);

	/*result = search(str, s);
	if(result == 1)
		printf("\n Char found");
	else
		printf("\n Char not found");
	*/

	/*result = isequals(str, str1);
	if(result == 1)
		printf("\n Char found");
	else
		printf("\n Char not found");
	*/

	/*
	result = issmaller(str, str1);
	if(result == 1)
		printf("\n 1st string smaller");
	else
		printf("\n Not smaller");

	
	result = isgreater(str3, str1);
	if(result == 1)
		printf("\n 1st string greater");
	else
		printf("\n 1st string not greater");
	*/
	
	/*
	s = atpos(str, 0);
	if((s >= 65 && s < 91) || (s >= 97 && s < 122)) 
	printf("\nChar is %c",s);
	else
		printf("\nYou are trying more than the lenght");
	*/

	sub = getsub(str, 2, 4);
	printf("\n%s", sub);
	
	return 0;
}

char* getsub(char *a, int loc, int len)
{
	int lenstr;
	int pos= 0, i,j;

	for(i = 0;i< 20;i++)
		str4[i] = NULL;

	lenstr = strlen(a);
	for(pos = 0;pos< loc;pos++)
	{
		a++;
	}

	for(i =0; i<len;i++)
	{
		str4[i]= a;
		a++;
	}
	
	str4[++i] = '\0';
	return str4;
}

Originally Posted by roaan

This compiles


[insert]

Code:

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

int search (char *a, char b);
int isequals(char *a, char *b);
int issmaller(char *a, char *b);
int isgreater(char *a, char *b);
char atpos(char *a, int loc);
char* getsub(char *a, int loc, int len);

char str4[20];

int main(void)
{
	
	char str[20] = "Rohan is a good boy";
	char str1[20] = "Roh";
	char str2[20] = "Rohan is a good boy";
	char str3[20] = "Sohan";
	char *sub;
	char s;
	int result;	
	char *a, *b;

	//printf("\nEnter a character to search");
	//scanf("%c", &s);

	/*result = search(str, s);
	if(result == 1)
		printf("\n Char found");
	else
		printf("\n Char not found");
	*/

	/*result = isequals(str, str1);
	if(result == 1)
		printf("\n Char found");
	else
		printf("\n Char not found");
	*/

	/*
	result = issmaller(str, str1);
	if(result == 1)
		printf("\n 1st string smaller");
	else
		printf("\n Not smaller");

	
	result = isgreater(str3, str1);
	if(result == 1)
		printf("\n 1st string greater");
	else
		printf("\n 1st string not greater");
	*/
	
	/*
	s = atpos(str, 0);
	if((s >= 65 && s < 91) || (s >= 97 && s < 122)) 
	printf("\nChar is %c",s);
	else
		printf("\nYou are trying more than the lenght");
	*/

	sub = getsub(str, 2, 4);
	printf("\n%s", sub);
	
	return 0;
}

char* getsub(char *a, int loc, int len)
{
	int lenstr;
	int pos= 0, i,j;

	for(i = 0;i< 20;i++)
		str4[i] = NULL;

	lenstr = strlen(a);
	for(pos = 0;pos< loc;pos++)
	{
		a++;
	}

	for(i =0; i<len;i++)
	{
		str4[i]= a;
		a++;
	}
	
	str4[++i] = '\0';
	return str4;
}

No. No, it does not. The same thing as before -- str4[i] = a is so amazingly illegal. Either you are not understanding what your compiler is telling you, or your compiler is broken.

Oh i got it as you pointed out

str4[i]= *a

I suppose thats why it was giving NOPQ instead of "han " Now it works fine. But i dont know why the code was not compiling becasue everytime i ran the code it compiled and gave NOPQ.

Thanks :-)