Hey, i'm new here and hoping to get a few noobish questions answered

Hi everyone. Basically i want to be good at c programming. But everytime i start to get into it. I get frustrated that i cant do something i want to do and end up leaving it. Hopefully with your help i can get these questions answered.

So just for a start ive got a few random questions.

1) if you have a struct with a variable like char *name; in it. Can you assign it a value by doing structname->name = "myname"; or is that wrong? Do you have to malloc it some space first?

2) Some source code i look at keeps talking about HANDLE - what is this and what is it used for?

3) How can you use strtok to split a string into an array? ive seen different example but i cant understand most of them.

A program im trying to make at the minute is a gym program. Which uses a weight lifting routine of 3 exercises

bench press, squat and power clean.

I wanted the program when it was first run to detect the date and store it somewhere permanent that it could access when its run again.

I was looking it to realise when it was run a second time, that it had run before and compare the date now, to the date when it was first run. Then count how many weeks the program had started and add the right amount of weights to the lift

so to summerise.

Make the program know its been run before.
Make it be able to calculate how many weeks since it was first run - using difftime().

Also wanted to store the previous lifts in a file. But ive not idea how to effectively pick correct data from a file. i.e how to find the number and how to save it as an int.

sorry if this is too many questions or unclear. But these things have been bugging me and i really want to understand it all.

Thanks.

Quote:

Originally Posted by mushy

Hi everyone. Basically i want to be good at c programming. But everytime i start to get into it. I get frustrated that i cant do something i want to do and end up leaving it. Hopefully with your help i can get these questions answered.

So just for a start ive got a few random questions.

1) if you have a struct with a variable like char *name; in it. Can you assign it a value by doing structname->name = "myname"; or is that wrong? Do you have to malloc it some space first?

Depends on what you want to achieve. Essentially, you have to choose EITHER assign it with a constant, or malloc memory (and use strcpy() to fill it in).

Quote:

2) Some source code i look at keeps talking about HANDLE - what is this and what is it used for?

The term handle is used frequently in computer systems - it is a generic term for "something that can be used to access a resource without the user function needing to know what it represents". In general, it is an integer type that you get back from the OS or some runtime functions when you open or create something (a graphical window, file or such), and can be passed back to the OS/runtime functions when you want to use that something (e.g. draw some pixels in the window or read/write a file).

Quote:

3) How can you use strtok to split a string into an array? ive seen different example but i cant understand most of them.

This is a bit more complex to just answer - if you post an example of something that you want explained, I'm sure someone can explain it.

Quote:

A program im trying to make at the minute is a gym program. Which uses a weight lifting routine of 3 exercises

bench press, squat and power clean.

I wanted the program when it was first run to detect the date and store it somewhere permanent that it could access when its run again.

I was looking it to realise when it was run a second time, that it had run before and compare the date now, to the date when it was first run. Then count how many weeks the program had started and add the right amount of weights to the lift

so to summerise.

Make the program know its been run before.
Make it be able to calculate how many weeks since it was first run - using difftime().

Also wanted to store the previous lifts in a file. But ive not idea how to effectively pick correct data from a file. i.e how to find the number and how to save it as an int.

sorry if this is too many questions or unclear. But these things have been bugging me and i really want to understand it all.

Thanks.

All of your above ideas seem quite doable.

Since your data format is entirely up to you, it is just a normal "read data from a file and make it into a number". There are many different ways, but one of those is to use fgets() to fetch a line of data, and then use for example sscanf() and/or strtok() to get the data out of the string. This is better than using fscanf(), as if there's something wrong with the input, fscanf() can easily be confused in such a way that you don't get anything useful out of it ever again [until you stop the program and start it over again, that is - it's not a "permanent forever failure"].

--
Mats

Quote:

Originally Posted by mushy

1) if you have a struct with a variable like char *name; in it. Can you assign it a value by doing structname->name = "myname"; or is that wrong? Do you have to malloc it some space first?

Now, while it certainly works, I don't know if I would recommend it for char*.
It's best to allocate using malloc and strcpy the string into there.
If it was const char*, however, then it should be perfectly fine to assign it as you mention.

Quote:

Make the program know its been run before.
Make it be able to calculate how many weeks since it was first run - using difftime().

To do such a thing, one way would be to open a file, if it doesn't exist, it's the first time the program is run, so open the file and write the date, then close.
All other times the program is run, it will successfully open the file and read that date. So then you can compare the dates and output accordingly.

Hey matsp. Thanks for the info.

Regarding strtok. i was looking to use it in this way.

Code:

#include <stdio.h> #include <string.h> typedef struct{ char *word; }s; int main(){ int wordcount; char str[] = "This is a string to split"; for(int i = 0; i < strlen(str); i++) { if(str[i] == ' ') { wordcount++; //find out how many words there are. } } wordcount +=1; // add one to account for the last word s splitter[wordcount]; // create a struct for every word in string

This is where im stuck, how can i assign each word in the string to the variable splitter[0]->word in each of the array?

Hey elysia.

can you explain why its different for a const char* and an ordinary char *?

Also i reading in the cplusplus c reference library about the time.h header file. But there dosent seem to be a function that just gives you the current date and time?

also why do you need to use size-t and time_t sometimes as opposed it an int?

Ok, using variables to create array is not allowed in standard C, so perhaps you may want to rethink that one.

Also, aside from the fact that you would have to know from the beginning how many words you have, you don't actually benefit from using strtok() if you are already walking through the string itself.

Something like this would do:

Code:

#include <stdio.h> #include <string.h> #define MAXWORDS 100 // Something large enough typedef struct{ char *word; }s; int main(){ s splitter[MAXWORDS] int wordcountn = 0; int len; char str[] = "This is a string to split"; char *pword = str; len = strlen(str); for(int i = 0; i < len; i++) { if(str[i] == ' ') { splitter[wordcount].word = pword; str[i] = 0; p = &str[i+1]; wordcount++; //find out how many words there are. } } wordcount +=1; // add one to account for the last word

You'll have to fix up the last word yourself.

--
Mats

Quote:

Originally Posted by mushy

can you explain why its different for a const char* and an ordinary char *?

Char* is a pointer to char. An array of chars, typically.
Const char* is a pointer to const char. So an array of const chars, typically. Since it's const, the contents of the value or array pointed to cannot change.
A string literal, such as "MY string" is a const char*, because you cannot change the contents of that string. Therefore, it should be assigned to const char*.
Assigning it to char* should be a mistake since if you try to modify the contents later, you'll get a seg fault. So you should declare it as const char* from the beginning. That way, you can't accidentally assign or change something later.

Hey matsp i was doing it this way because i assumed it was the only way?

Also i dont have a clue how this works?

Code:

if(str[i] == ' ') { splitter[wordcount].word = pword; str[i] = 0; p = &str[i+1];

ok thanks Elysia. So what if you tried to assign char *name a value from an inputed variable? is it ok then?

matsp just to elaborate further on why i dont get this

Code:

#include <stdio.h> #include <string.h> #define MAXWORDS 100 // Something large enough typedef struct{ char *word; }s; int main(){ s splitter[MAXWORDS] int wordcountn = 0; int len; char str[] = "This is a string to split"; char *pword = str; len = strlen(str); for(int i = 0; i < len; i++) { if(str[i] == ' ') { splitter[wordcount].word = pword; // assigning the value pword to the word variable - earlier you assigned pword to the value of the whole string - so the whole string is stored in splitter[0].word? str[i] = 0; // putting i back to the start of the string p = &str[i+1]; // dont know how this splits it wordcount++; //find out how many words there are. } } wordcount +=1; // add one to account for the last word

OK, what you must understand is that strings are just an array of chars, and at the end, there's an extra char with the value '\0' (or 0).
So when function read the '\0', they know they're found the end of the string.
So what mats is doing is truncating the string but replacing the ' ' with '\0', so it really marks the end of the string. Or so functions will believe.
Then just set a new pointer to begin pointing at the character after the '\0'.
The string will look like:
"This\0is\0a\0string\0to\0split"
And the word member of the struct will point at:
"This\0is\0a\0string\0to\0split" -> Essentially "This"
"is\0a\0string\0to\0split" -> Essentially "is"
"a\0string\0to\0split" -> Essentially "a"
"string\0to\0split" -> Essentially "string"
"to\0split" -> Essentially "to"
"split"

As for the char* question... if you have a char* pointer, then it's best to allocate memory and use strcpy to copy the string in there. Thus, you can modify the string if you want.

ok i think i understand elysia. So basically.

p = &str[i+1]; // was the p ment to be pword? i can see that this points to the space next char after the space!

I still dont understand two things though.

Why does the full string not get assined to splitter[wordcount].word ? why does it stop reading when it gets to the first space. i mean if you done

char s1[] = "this is a string";
char s2[100] = s1;

unless im wrong it wouldnt just copy the first word across! also it wouldnt leave s1 equaling "is a string" with the first word missing

i realise this is wrong. Because obviously the code matsp posted will work, but im just trying to find out how it works. Sorry for being an idiot.

Quote:

Originally Posted by mushy;711867 p = &str[i+1

; // was the p ment to be pword? i can see that this points to the space next char after the space!

I assume so, yes.

Quote:

Why does the full string not get assined to splitter[wordcount].word ? why does it stop reading when it gets to the first space.

Because mats's code replace the space with a '\0', basically marking the end of the string.

Quote:

i mean if you done

char s1[] = "this is a string";
char s2[100] = s1;

unless im wrong it wouldnt just copy the first word across! also it wouldnt leave s1 equaling "is a string" with the first word missing

I'm pretty sure that won't compile at all. Anyway, the point is that we're working with pointers, not the full data itself.
The data is stored somewhere in memory and all we do is pass around the address of where the string begins. The '\0' char marks the end of the string.
So if you just pass around a pointer without modifying it, all pointers will show the same data. And if you modify the data itself, all pointers will change.
In your sample, you're trying to copy the data from one char array to another, I'm guessing. This should be done via strcpy. But we're not modifying the data itself, as I explained. We're passing around pointers.

ok. i think i understand now. Were not actually doing anything to the original string. Were just using pointers to pick from different points of the string.

str[i] = 0; -- i get what this does now, it replaces spaces with 0. I thought originally that it was trying to go back to the first character of the string.

I get it now. Thanks for the help. Now that i know this it sould make reading and editing the data in the file easier.

Quote:

Originally Posted by mushy

ok. i think i understand now. Were not actually doing anything to the original string. Were just using pointers to pick from different points of the string.

Although you could say we're modifying the data by replacing the spaces, but other than that, we're not modifying it anything.

ok. Cool Thanks. a few more silly questions.

Code:

char str[] = "This is a string to split"; char *pword = str;

when assigning a pointer to str why do you not use the & operator? i.e char *pword = &str;

Code:

splitter[wordcount].word = pword;

i thought that with pointers in structs you had to use the -> as opposed to . or is it different if you are assiging a pointer to another pointer?

Quote:

Originally Posted by mushy

Code:

char str[] = "This is a string to split"; char *pword = str;

when assigning a pointer to str why do you not use the & operator? i.e char *pword = &str;

An array is automatically interpreted as array*, so char[] would become char* when you try to assign it somewhere. So this example is fine.
If str was only char, then you have to use &.

Quote:

Code:

splitter[wordcount].word = pword;

i thought that with pointers in structs you had to use the -> as opposed to . or is it different if you are assiging a pointer to another pointer?

x->y is a shortcut for (*x).y.
However, the [] operator also dereferences the pointer.
So...

Code:

const char* x = "My string"; char c1 = x[0]; /* Is the same as *x */ char c2 = x[1]; /* Is the same as *x + 1 */ char c3 = *x[2]; /* Compile error, but it's the same as **x + 2 */

Thanks again. I'm sure you will be glad to know thats all the questions i have. ........ For now lol

Thanks.

Hey ok, so i got a compiler installed and im trying to run this code

Code:

#include <stdio.h> #include <stdlib.h> #define MAXWORDS 100 typedef struct splitter{ char *word; }s; int main(){ s words[MAXWORDS]; int wordcount = 0; int len; int i; char str[] = "This is some string"; char *ptr = str; len = strlen(str); for(i = 0; i < len; i++) { if(str[i] == ' ') { words[wordcount].word = ptr; words[wordcount].word = (char *) malloc(sizeof(len)); printf("%s",words[wordcount].word); str[i] = 0; ptr = &str[i +1]; wordcount++; } } return 0; }

So i knows it assigns a pointer to each word. So how do i use the pointer to print each word to screen. It compliles but just prints jibberish to screen. I must be printing the address. But not sure how to make it work.

Code:

words[wordcount].word = ptr;

This will assign the pointer to the struct.

Code:

words[wordcount].word = (char *) malloc(sizeof(len));

...But this will replace the previous pointer with a pointer to newly acquired memory which you then print on your next line.

Btw, 'tis your code?

edit My bad

elysia, well its basically the same as my code with the changes matsp and you made to it earlier and then i just tried to make it print out which messed it up again.

Well, no, not like you want.
First it should be

Code:

strcpy(words[wordcount].word, str);

Remember: destination, source is the argument for strcpy, in order.
Then you have to remember to truncate the string first, or you'll get the entire string copied.

So it needs to be:

Code:

str[i] = 0; words[wordcount].word = (char *) malloc(sizeof(len)); strcpy(words[wordcount].word, str); printf("%s",words[wordcount].word); wordcount++;

---OR---

Code:

str[i] = 0; words[wordcount].word = str; printf("%s",words[wordcount].word); wordcount++;

Indentation can use some work too.
Here's an excellent guide: http://cpwiki.sf.net/User:Elyisa/Indentation

dont this i understand fully your way elysia i have this now

Code:

#include <stdio.h> #include <stdlib.h> #define MAXWORDS 100 typedef struct splitter{ char *word; }s; int main(){ s words[MAXWORDS]; int wordcount = 0; int len; int i; char str[] = "This is some string"; char *ptr = str; len = strlen(str); for(i = 0; i < len; i++) { if(str[i] == ' ') { str[i] = 0; strcpy(words[wordcount].word, str); words[wordcount].word = (char *) malloc(sizeof(len)); strcpy(words[wordcount].word,str); printf("%s",words[wordcount].word); ptr = &str[i +1]; wordcount++; } } return 0; }

My question to you is: Do you want to copy the pointers or the data?
Mats's example is to copy the pointers.

hey. I just want to split a string, using this basic setup. I know and understand that the pointer are pointing to the seperate words.

so i was just wondering now i have a pointer to the data. How i can go about manipulating the data like a string. so i have words[0].word

if i wanted to check using strcmp if that word was equal too "hello". How could i do that.
basically physically break them up and give them their own space. so i can do

printf("%s", words[0].word); and it will display the first word in the string.

First you must allocate space using malloc or calloc.
Then you can truncate the string so as to only copy a part of it.
Then you copy the string using strcpy.
The pointers example is more efficient, but it's also more error prone. If you would change the data somehow, it would reflect in all strings. And if the original string buffer goes out of scope, so does the other string pointers.
The good thing about copying the data is that it won't go out of scope and won't affect the other strings. But it takes longer time to perform.

hey. Thanks for all you help. Got it working how i wanted. like this.

Code:

for(i = 0; i < len; i++) { if(str[i] == ' ') { str[i] = 0; words[wordcount].word = (char *) malloc(sizeof(len)); strcpy(words[wordcount].word,ptr); printf("%s \n",words[wordcount].word); ptr = &str[i +1]; wordcount++; } }

So as far as im aware what ive done here is
1) ran through the string replacing all spaces with 0 to signal the end of the string.
2) i assign each pointer in the struct some space using malloc
3)i use strcpy to tell that space to use the address of ptr which is pointing to the first word
4) i assign pointer a new value which will be the start of the new string.

Hope thats right

Quote:

Originally Posted by mushy

3)i use strcpy to tell that space to use the address of ptr which is pointing to the first word
4) i assign pointer a new value which will be the start of the new string.

Not exactly.
Strcpy copies a string from one buffer to another. From the original buffer to your newly allocated buffer allocated by malloc.
Btw, you're also allocating insufficient space.
Sizeof takes the size of a variable. In your case, len is an int, thus you allocate 4 bytes. That's not right. You should allocate the length of your string (len) + 1 (for the ending '\0').
Also make sure to free all that space using free when you're finished with it, before the program ends.

I thought ptr was only an address and had no space assigned to it? How can i use strcpy to if that only takes two buffers as arguments?

A buffer is stored in memory somewhere.
Ptr is a pointer to an address that specifies a region within that buffer to copy.
And the word is a pointer to a buffer that malloc created for you.

ok! phewwwww. well ive taken most of that in. Must seem simple to you. But im finding it hard to work it out to be honest. Thanks for all your help

Sure, when you've programmed several years, it seems a piece of cake to you.

lol i just realised. that i could just print out the words with a simple printf("%s\n",ptr); didnt even realise i could do it that simple. lol i'll know for next time. Well bed time here in ireland night night.

Yes, night.

FWIW, a strtok version:

Code:

char ** tokenize ( const char * split_it, const char * delim, char ** string_ar, const unsigned long AR_LENGTH ) { /**shouldn't assume that no one will want 'split_it' as-is later, so **create a copy in memory:**/ char * split_me = duplicate( split_it ); char * found = NULL; if( split_me != NULL ) { unsigned long wordcount = 0; for( found = strtok( split_me, delim ); found != NULL && wordcount < AR_LENGTH; found = strtok( NULL /**more matches?**/, delim ) ) { string_ar[wordcount] = duplicate( found ); if( string_ar[wordcount] == NULL ) return NULL; /**failure return**/ ++wordcount; } string_ar[wordcount] = NULL; /**like argv**/ free( split_me ); /**free( found ); is not nexessary because it simply points **to a part of the recent 'split_me', and since we copied the **words to 'string_ar' we can safely free 'split_me'**/ } return string_ar; }