Thread: Understanding Pointers

I'm creating a simple project to better understand pointers.

I'm trying to scanf some values into an array from a function and then printing them ...
but when i do, i get the correct words printed except for some symbols tact on to the end of the string... here is the code:

Code:

void get_colors(char scanned[][], int n); 

int main(void){
 int n = 6, j, i;  
 char scanned[6][7];
 char color[12][7];
 char *pcolor;
 char *pscanned;

 /* Declare pointers to their arrays */
 pcolor = &color[0][0];
 pscanned = &scanned[0][0];
 
 get_colors(pscanned, n);
 
for(j = 0; j<6; j++){
      for(i = 0; i<7; i++){
         printf("%c", scanned[j][i]);
         } printf("\n");
}

 getch();
 return 0;   
}

void get_colors(char scanned[6][7], int n){
     printf("Please enter 6 resistor colors: \n");
     scanf("%s %s %s %s %s %s", &scanned[0], &scanned[1], &scanned[2], &scanned[3], &scanned[4], &scanned[5]) ;
}

but when i enter this for instance:
brown green orange yellow black green

the output looks like:
brown
green á <--- that ...?
orange
yellow
black
green

and any useful tips to getting my head around pointers would be AMAZING! Pointers are going to be the death of me! lol...

thanks in advance

I thought c knows where to terminate a string based on the space in between the scanf ... thats how we have been using them all year.

but how do i apply the \0 thing mentioned to a multiple input scanf like in the project above?

And as for the compile warning, I dont know what I did wrong with the different types. I'm just learning pointers. What is this error being produced from? and how do i fix it?

Originally Posted by Oonej

And as for the compile warning, I dont know what I did wrong with the different types. I'm just learning pointers. What is this error being produced from? and how do i fix it?

You're passing a "pointer to an array" to the function get_colors(). But the argument list of that function has an array declared (not a pointer to an array).

I would recommend reading up on referencing an array with a pointer. It's not that difficult when you get the hang of it. Here's the gist (hope it's clear):

Code:

// this is not actual code, just my attempt to describe the relationship between
//   an array index and a pointer to that array

char ch_array[10];
char *pointer_to_ch_array = &ch_array[0];

//------

*pointer_to_ch_array		// points to data at the address of the first element in the array
//points to data in
ch_array[0]

*(pointer_to_ch_array + 1)	// points to data at the address of the second element in the array,
//points to data in		//   which is the address of the first element PLUS 1
ch_array[1]

*(pointer_to_ch_array + 2)	// and so on.  Note how the dereference operator is being used
//points to data in		//   around the modified address
ch_array[2]

There's a tad more math involved with more than one dimension in the array, but it's pretty much the same idea.

Code:

$ gcc -Wall colors.c
colors.c:3: error: array type has incomplete element type
colors.c: In function ‘main’:
colors.c:16: error: type of formal parameter 1 is incomplete
colors.c:24: warning: implicit declaration of function ‘getch’
colors.c: In function ‘get_colors’:
colors.c:30: warning: format ‘%s’ expects type ‘char *’, but argument 2 has type ‘char (*)[7]’
colors.c:30: warning: format ‘%s’ expects type ‘char *’, but argument 3 has type ‘char (*)[7]’
colors.c:30: warning: format ‘%s’ expects type ‘char *’, but argument 4 has type ‘char (*)[7]’
colors.c:30: warning: format ‘%s’ expects type ‘char *’, but argument 5 has type ‘char (*)[7]’
colors.c:30: warning: format ‘%s’ expects type ‘char *’, but argument 6 has type ‘char (*)[7]’
colors.c:30: warning: format ‘%s’ expects type ‘char *’, but argument 7 has type ‘char (*)[7]’

You have a few problems:

1. Your prototype, like your declaration needs some sizes for the array. Technically, you can leave the leftmost [] empty, but you might as well make it match the function.
2. You need to remove the & from each of the scanned[] elements in your scanf. scanned[0] is the name of the array starting at scanned[0][0], and the name of an array serves as a pointer to it in this case.
3. You can ditch color, pcolor and pscanned since you don't need them. Just pass scanned directly to get_colors.
4. You don't need or use n in get_colors, so drop it from the param list and the prototype.
5. Now the big one. You're getting that junk because you print all 7 characters for each color, even if there's only 5 of them. If the color name has 5 letters, like brown, you have 'b', 'r', 'o', 'w', 'n', '\0' put in there by scanf. It doesn't fill the whole array with null characters, it only puts one at the end of what was entered. That leaves one junk character after the nul terminator that you print out. Just do printf("%s\n", scanned[i]) and you should be fine.
6. Not a problem per se, but getch() isn't a standard function. Try using getchar() instead, so it's easier for people to paste your code into their editor and compile.

I stand corrected, thank you.

- Matt

Made some of the recommended changes...

also figured out a loop to replace random characters!

Code:

#include <stdio.h>

void get_colors(char *pointer); 

int main(void){
 int n = 6, j, i;  
 char scanned[6][7];
 char color[12][7];
 char *pcolor;
 char *pscanned;

 /* Declare pointers to their arrays */
 pcolor = &color[0][0];
 pscanned = &scanned[0][0];
 
 get_colors(pscanned);
 
for(j = 0; j<6; j++){
      for(i = 0; i<7; i++){
         printf("%c", scanned[j][i]);
         } printf("\n");
}

 getchar();
 return 0;   
}

void get_colors(char *pointer){
     int i, j, k;
     char x, y;
     int check;
     x = '\0';
     y = ' ';
     
     
     printf("Please enter 6 resistor colors: \n");
     scanf("%s %s %s %s %s %s", pointer, (pointer+7), (pointer+14), (pointer+21), (pointer+28), (pointer+35)) ;
     
     /* Replace characters after the first \0 with empty spaces */
        for(i = 0; i < 42; i++){
           if(*(pointer+i) == x) { 
              for(j = i%7; j<7; j++){
                    k = 0;
                    *(pointer+i) = y;
                    i++; 
              }
           }
   
     }
}

thoughts / opinions?

i got rid of all the weird errors

Ok i'm starting to understand how to use pointers with multi-d arrays now how does this look?

Code:

void get_colors(char (*scanned)[7]);


int main(void){
 int j, i;  
 char scanned[6][7];
 
 get_colors(scanned);

for(j = 0; j<6; j++){
      for(i = 0; i<7; i++){
         printf("%c", scanned[j][i]);
         } printf("\n");
}

 getch();
 return 0;   
}

/* Get colors from user */
void get_colors(char (*scanned)[7]){
     int i, j, k;  
     
     printf("Please enter 6 resistor colors: \n");
     scanf("%6s %6s %6s %6s %6s %6s", scanned, scanned+1, scanned+2, scanned+3, scanned+4, scanned+5);
     
     /* Replace characters after the first \0 with empty spaces */
        for(j=0; j<6; j++){
           for(i=0; i<7; i++){ 
             if(scanned[j][i] == '\0') { 
               for(k = i; k<7; k++){
                    scanned[j][k] = ' ';
               }
             }
           }
        }
         
     /* Make all lower case letters */
     for(j=0; j<6; j++){
        for(i=0; i<7; i++){ 
           scanned[j][i] = tolower(scanned[j][i]); 
        }
     }
}

Originally Posted by Oonej

how does this look?

It looks like quzah and King Mir are having some kind of code show-down.

As far as your code; I'm sure we could all find something to nit-pick over, but the important thing is that you seem to have gotten it do to what you want. Take the knowledge you have gained and expand on it.