dynamically allocated strings??

CS_Student8337 · 03-19-2009, 12:49 AM

Ok, part of my assignment for school is to create a struct with 2 char pointers in it. These pointers are supposed to point to a couple of dynamically allocated strings and I'm supposed to create a little function that prints the contents of the struct out. So far I have the struct:

Code:

typedef struct
{
	char *name;
	char *number;
}myStruct;

And I have the print function:

Code:

void printMyStruct(myStruct *a, myStruct *b)
{
     printf("%s: %s\n", *(a).name, *(b).number);
}

I think those two parts are right. My question is about the dynamically allocated strings that the pointers in my struct are supposed to point to. I'm not quite sure of how to do that..

Nightowl · 03-19-2009, 12:56 AM

Use the malloc() function.

For example:

Code:

char *string = malloc(sizeof(char) * 100);

. . . for a string of a size of 100 characters.

EDIT: don't forget to free the memory when you're done with it . . .

vart · 03-19-2009, 12:58 AM

second part is wrong . has precedence over the * so it should not even compile

should be

printf("%s: %s\n", a->name, b->number);

to allocate string use malloc

Code:

a->name = malloc(strlen(temp)+1);
if(a->name)
    strcpy(a->name,temp);

CS_Student8337 · 03-19-2009, 01:12 AM

I thought that's what it was but I wasn't sure because the instructions say that the strings shouldn't have any wasted space.. So, is it ok to do something like this?

Code:

int capacity = 1;
int size = 0;
int c;
char *string = malloc(sizeof(char) * capacity);
while((c = fegetc(stdin) != EOF)
{
     string[size++] = c;
     if(size == capacity)
     {
          string = realloc ( string ,capacity * sizeof (char));
     }

Nightowl · 03-19-2009, 01:15 AM

Yes, it is, but be careful, because realloc() and malloc() can fail . . .

CS_Student8337 · 03-19-2009, 01:17 AM

Right. I didn't include any code to check if they return NULL. I know I should for my program though. In the code I just posted I used fgetc(). Would it be possible to substitute fgets() instead?

Nightowl · 03-19-2009, 01:19 AM

Hmm. Yes, it is possible, but it would be quite useless, as you'd be using it in the same position as fgetc() unless you read into a huge temporary buffer.

CS_Student8337 · 03-19-2009, 01:20 AM

Cool. Thank you. I'll post again if I run into any problems

cyberfish · 03-19-2009, 01:26 AM

Quote:

Code:

char *string = malloc(sizeof(char) * 100);

. . . for a string of a size of 100 characters.

You meant 99?

Nightowl · 03-19-2009, 01:39 AM

Yes, yes, I'm sorry. My brain's on holidays tonight.

A string of 100 including the NULL, is what I meant . . .

CS_Student8337 · 03-19-2009, 01:45 AM

Ok. One quick, little question. How do I get one of the pointers in my struct to point to the string??

Nightowl · 03-19-2009, 01:47 AM

Like so . . .

Code:

a->name = localstring;

. . . for example. localstring is the malloc'd char *.

CS_Student8337 · 03-19-2009, 01:55 AM

Hmm.. that's how I was trying to do it but I keep getting this error when I try to compile:

error: invalid type argument of ‘->’

vart · 03-19-2009, 02:05 AM

Quote:

Originally Posted by CS_Student8337

I thought that's what it was but I wasn't sure because the instructions say that the strings shouldn't have any wasted space..

in this case - use temporary static buffer to enter string and after that use strlen to detrmine the needed space to be allocated like in my example

vart · 03-19-2009, 02:06 AM

Quote:

Originally Posted by CS_Student8337

Hmm.. that's how I was trying to do it but I keep getting this error when I try to compile:

error: invalid type argument of ‘->’

is a pointer or struct?

03-19-2009, 12:49 AM	#1
CS_Student8337 Registered User Join Date: Feb 2009 Posts: 19	dynamically allocated strings?? Ok, part of my assignment for school is to create a struct with 2 char pointers in it. These pointers are supposed to point to a couple of dynamically allocated strings and I'm supposed to create a little function that prints the contents of the struct out. So far I have the struct: Code: typedef struct { char name; char number; }myStruct; And I have the print function: Code: void printMyStruct(myStruct a, myStruct b) { printf("%s: %s\n", (a).name, (b).number); } I think those two parts are right. My question is about the dynamically allocated strings that the pointers in my struct are supposed to point to. I'm not quite sure of how to do that..
CS_Student8337 is offline

03-19-2009, 12:56 AM	#2
Nightowl Resu Deretsiger Join Date: Nov 2008 Location: /dev/null Posts: 185	Use the malloc() function. For example: Code: char string = malloc(sizeof(char) 100); . . . for a string of a size of 100 characters. EDIT: don't forget to free the memory when you're done with it . . . __________________ Do as I say, not as I do . . . Experimentation is the essence of programming. Just remember to make a backup first. "I'm a firm believer that <SomeGod> gave us two ears and one mouth for a reason - we are supposed to listen, twice as much as we talk." - LEAF Questions posted by these guidelines are more likely to be answered. Debian GNU/Linux user, with the awesome window manager, the git version control system, and the cmake buildsystem generator.
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03-19-2009, 12:58 AM	#3
vart CSharpener Join Date: Oct 2006 Posts: 5,314	second part is wrong . has precedence over the * so it should not even compile should be printf("%s: %s\n", a->name, b->number); to allocate string use malloc Code: a->name = malloc(strlen(temp)+1); if(a->name) strcpy(a->name,temp); __________________ If I have eight hours for cutting wood, I spend six sharpening my axe.
vart is offline

03-19-2009, 01:12 AM	#4
CS_Student8337 Registered User Join Date: Feb 2009 Posts: 19	I thought that's what it was but I wasn't sure because the instructions say that the strings shouldn't have any wasted space.. So, is it ok to do something like this? Code: int capacity = 1; int size = 0; int c; char string = malloc(sizeof(char) capacity); while((c = fegetc(stdin) != EOF) { string[size++] = c; if(size == capacity) { string = realloc ( string ,capacity * sizeof (char)); }
CS_Student8337 is offline

03-19-2009, 01:15 AM	#5
Nightowl Resu Deretsiger Join Date: Nov 2008 Location: /dev/null Posts: 185	Yes, it is, but be careful, because realloc() and malloc() can fail . . . __________________ Do as I say, not as I do . . . Experimentation is the essence of programming. Just remember to make a backup first. "I'm a firm believer that <SomeGod> gave us two ears and one mouth for a reason - we are supposed to listen, twice as much as we talk." - LEAF Questions posted by these guidelines are more likely to be answered. Debian GNU/Linux user, with the awesome window manager, the git version control system, and the cmake buildsystem generator.
Nightowl is offline

03-19-2009, 01:17 AM	#6
CS_Student8337 Registered User Join Date: Feb 2009 Posts: 19	Right. I didn't include any code to check if they return NULL. I know I should for my program though. In the code I just posted I used fgetc(). Would it be possible to substitute fgets() instead?
CS_Student8337 is offline

03-19-2009, 01:19 AM	#7
Nightowl Resu Deretsiger Join Date: Nov 2008 Location: /dev/null Posts: 185	Hmm. Yes, it is possible, but it would be quite useless, as you'd be using it in the same position as fgetc() unless you read into a huge temporary buffer. __________________ Do as I say, not as I do . . . Experimentation is the essence of programming. Just remember to make a backup first. "I'm a firm believer that <SomeGod> gave us two ears and one mouth for a reason - we are supposed to listen, twice as much as we talk." - LEAF Questions posted by these guidelines are more likely to be answered. Debian GNU/Linux user, with the awesome window manager, the git version control system, and the cmake buildsystem generator.
Nightowl is offline

03-19-2009, 01:20 AM	#8
CS_Student8337 Registered User Join Date: Feb 2009 Posts: 19	Cool. Thank you. I'll post again if I run into any problems
CS_Student8337 is offline

03-19-2009, 01:39 AM	#10
Nightowl Resu Deretsiger Join Date: Nov 2008 Location: /dev/null Posts: 185	Yes, yes, I'm sorry. My brain's on holidays tonight. A string of 100 including the NULL, is what I meant . . . __________________ Do as I say, not as I do . . . Experimentation is the essence of programming. Just remember to make a backup first. "I'm a firm believer that <SomeGod> gave us two ears and one mouth for a reason - we are supposed to listen, twice as much as we talk." - LEAF Questions posted by these guidelines are more likely to be answered. Debian GNU/Linux user, with the awesome window manager, the git version control system, and the cmake buildsystem generator.
Nightowl is offline

03-19-2009, 01:45 AM	#11
CS_Student8337 Registered User Join Date: Feb 2009 Posts: 19	Ok. One quick, little question. How do I get one of the pointers in my struct to point to the string??
CS_Student8337 is offline

03-19-2009, 01:47 AM	#12
Nightowl Resu Deretsiger Join Date: Nov 2008 Location: /dev/null Posts: 185	Like so . . . Code: a->name = localstring; . . . for example. localstring is the malloc'd char *. __________________ Do as I say, not as I do . . . Experimentation is the essence of programming. Just remember to make a backup first. "I'm a firm believer that <SomeGod> gave us two ears and one mouth for a reason - we are supposed to listen, twice as much as we talk." - LEAF Questions posted by these guidelines are more likely to be answered. Debian GNU/Linux user, with the awesome window manager, the git version control system, and the cmake buildsystem generator.
Nightowl is offline

03-19-2009, 01:55 AM	#13
CS_Student8337 Registered User Join Date: Feb 2009 Posts: 19	Hmm.. that's how I was trying to do it but I keep getting this error when I try to compile: error: invalid type argument of ‘->’
CS_Student8337 is offline

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