string and pointer

This is a discussion on string and pointer within the C Programming forums, part of the General Programming Boards category; can I write like this? Code: string[1][0] = *pointer;...

  1. #1
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    string and pointer

    can I write like this?
    Code:
    string[1][0] = *pointer;

  2. #2
    Woof, woof! zacs7's Avatar
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    Depends how string is declared and what 'pointer' is.

    If it's a 2d array of characters, and pointer is a pointer to a character then yes.

    Why not compile and see for yourself!?

  3. #3
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    write to a Array of strings and print to the screen

    the program compile and run but got error message. Can anyone help me?

    Code:
    #include <stdio.h>
    #include <math.h>
    #include <string.h>
    #include <ctype.h>
    
    #define HISTORY_SIZE 10
    #define MAX_STRING_LEN 100
    #define  MAX_CMD_LEN 10
    
    void printHistory( char history[][MAX_CMD_LEN], int size);
    int processCommand( char *current, char *cmd, char *result); 
    void updateHistory( char history[][MAX_CMD_LEN], const char *cmd, int size);
    
    int main(void)
    {
    	int i;
    	char cmd[MAX_CMD_LEN]="";
    	char current[MAX_STRING_LEN]="";
    	char result[MAX_STRING_LEN]="";
    	char history_table[HISTORY_SIZE][MAX_CMD_LEN];
    	
    	int history_size = 0;
    	int flag;
    
    	printf("Enter command [help] for a list of commands\n");
    
    	do {
    		
    		printf("cmd> ");
    		gets(cmd); //get a command from user.
    
                            if(strcmp( cmd, "hist") == 0) {
    			printHistory( history_table, history_size);
    		}
    		else {
    			flag= processCommand(current, cmd, result);
    		}
                            if( cmd !=""){
    			updateHistory(history_table, cmd, history_size);
    			history_size++;
    		}
    	}while (flag!=1);
    	return 0;
    }
    
    void updateHistory( char history[][10], const char *cmd, int size)
    {
    	
    	if(cmd != '\0'){
    		history[size][0]=*cmd;
    		size=size+1;
    	}
    	
    }
    Last edited by phoebus; 05-01-2008 at 09:08 AM.

  4. #4
    Woof, woof! zacs7's Avatar
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    From your code it's evident you really have no idea what's going on.

    I suggest you start with the basics first, er yes... even more basic than that.
    Start by following the tutorials on this site, mainly the arrays & pointer sections.

  5. #5
    CSharpener vart's Avatar
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    And also read FAQ
    for example Why gets() is bad / Buffer Overflows
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  6. #6
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    for print the array of strings. I can use this code right?
    Code:
    for (i=0; i<=size; i++)
            puts (historytable[i]);
    and for copying *cmd to historytable[][10]
    i can use this code right?
    Code:
    for(i=0;i<=size;i++)
            historytable[size][0]=*cmd;
    Last edited by phoebus; 05-01-2008 at 06:13 AM.

  7. #7
    Woof, woof! zacs7's Avatar
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    Yes, except you want i < size Remember, arrays are 0 based not 1 based.

    ie, 5 1-based is
    1, 2, 3, 4, 5

    0 based
    0, 1, 2, 3, 4

    'size' specifies how many elements, not the end point.

  8. #8
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    Quote Originally Posted by zacs7 View Post
    Yes, except you want i < size Remember, arrays are 0 based not 1 based.

    ie, 5 1-based is
    1, 2, 3, 4, 5

    0 based
    0, 1, 2, 3, 4

    'size' specifies how many elements, not the end point.
    you are right.

    and back to the main code above

    Code:
    size++
    in the updatehistory function, it doesn't add one to the variable history_size. why is that?
    Last edited by phoebus; 05-01-2008 at 06:20 AM.

  9. #9
    Registered User nenpa8lo's Avatar
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    This copies first char of 'cmd' to first position of each string in historytable (which is wrong):
    Code:
    historytable[i][0]=*cmd;
    you have to do it like this:
    Code:
    for (i = 0; i < size; i++)
        historytable[0][i] = *cmd++;  /* copy one character from 'cmd' shift pointer etc. */
    this copies "whole" cmd into historytable[0], however you have to be aware to control where 'cmd' points - if 'cmd' and historytable[][] are the same size the you are safe.... hypothetically :-)

  10. #10
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    Quote Originally Posted by nenpa8lo View Post
    This copies first char of 'cmd' to first position of each string in historytable (which is wrong):
    Code:
    historytable[i][0]=*cmd;
    you have to do it like this:
    Code:
    for (i = 0; i < size; i++)
        historytable[0][i] = *cmd++;  /* copy one character from 'cmd' shift pointer etc. */
    this copies "whole" cmd into historytable[0], however you have to be aware to control where 'cmd' points - if 'cmd' and historytable[][] are the same size the you are safe.... hypothetically :-)
    i would do like this:
    [CODE]
    Code:
    for (i = 0; i <strlen(cmd); i++)
        historytable[size][i] = *cmd++;
    i need to copy 9 more strings
    Last edited by phoebus; 05-01-2008 at 06:57 AM.

  11. #11
    Registered User nenpa8lo's Avatar
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    this way strlen() is called at every iteration. So:
    Code:
    size=strlen(cmd);
    for(i = 0; i < size; i++)
        {...}

  12. #12
    CSharpener vart's Avatar
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    this way nul-char is not copied and there are possibilities of memory overrun

    there is function strcpy - used for copying C-strings

    and better check that strlen(cmd) < 10 before copying...

    of cource cmd buffer should be allocated somewhere... and fgets will prevent the memory overrun...
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  13. #13
    Registered User nenpa8lo's Avatar
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    First of all can you tell what are you going to do with this code? Secondly there is few mistakes:

    Quote Originally Posted by phoebus View Post
    the program compile and run but got error message. Can anyone help me?

    Code:
    int main(void)
    {
              char historytable[10][10];
               int history_size = 0;
                printf("cmd> ");
    	gets(cmd); //get a command from user.    where is declared 'cmd' ? 
    	updateHistory(historytable, cmd, history_size);
                printHistory( historytable, history_size);
               return 0;
    }
    void printhistory( char hist[][10], int size)
    {
    	int i;
    	printf("Size of history: %d\n", size);
    	for( i=0; i<=size; i++)  size is 0 so why you want to print it in loop ? 
    	{
    		puts(hist[i]);
    	}
    	printf("\n");
    }
    
    void updateHistory( char history[][10], const char *cmd, int size)
    {
    	
    	if(cmd != '\0'){
    		history[size][0]=*cmd;
    		size=size+1;    this makes no sense as 'size' is on the stack! If you want to make permanent changes to size you have to pass it like : updateHistory(...., &size); and then in function body cach it like updateHistory(...., int *size)
    	}
    	
    }

  14. #14
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    okay I fix my main function. Take a look at it. Sorry for the indent.

    Quote Originally Posted by vart View Post
    this way nul-char is not copied and there are possibilities of memory overrun

    there is function strcpy - used for copying C-strings

    and better check that strlen(cmd) < 10 before copying...

    of cource cmd buffer should be allocated somewhere... and fgets will prevent the memory overrun...
    If I want to use the strcpy function I have initalize
    Code:
    char history_table[HISTORY_SIZE][MAX_CMD_LEN];
    like
    Code:
    char string[10]="";
    but I don't know how to initialize a array of strings.
    can anyone help?

  15. #15
    CSharpener vart's Avatar
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    Do not edit your old posts - post new code version in the new post

    See comments
    Code:
    int main(void)
    {
    	int i;
    	char cmd[MAX_CMD_LEN]="";
    	char current[MAX_STRING_LEN]="";
    	char result[MAX_STRING_LEN]="";
    	char history_table[HISTORY_SIZE][MAX_CMD_LEN];
    	
    	int history_size = 0;
    	int flag; /* not initialized */
    	
    	printf("Enter command [help] for a list of commands\n");
    	
    	do {
    		
    		printf("cmd> "); /* add fflush(stdout); after this line */
    		gets(cmd); /* possible memory overrun */
    		
    		if(strcmp( cmd, "hist") == 0) {
    			printHistory( history_table, history_size); /* no prototype */
    		}
    		else {
    			flag= processCommand(current, cmd, result); /* no prototype */
    		}
    		if( cmd !="") /* incorrect operation - use strcmp */
    		{
    			updateHistory(history_table, cmd, history_size); /* no prototype */			history_size++;
    		}
    	}while (flag!=1); /* used not initialized var if command was hist */
    	return 0;
    }
    
    void updateHistory( char history[][10], const char *cmd, int size)
    {
    	
    	if(cmd != '\0'){
    		history[size][0]=*cmd; /* wrong - use strcpy */
    		size=size+1; /* has no effect */
    	}
    	
    }
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

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