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

In order for an array of characters to be considered a string (ie: to work with string functions or functions that assume you are passing a string), you need to nul terminate the array. That means that if you have "dog", you really have "dog\0", where \0 is the terminating nul character. Without that, C doesn't know where your string stops.

You should also compile with warnings on, because pscanned is not the same as char [6][7] (what your function is expecting).


Quzah.

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?

Quote:

---

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.

Quote:

---

Originally Posted by Matticus

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).

---

No. He's passing a pointer to a character. A pointer to an array would be:

Code:

---

type (*ptr)[X][Y];

---

ptr is a pointer to an array of X by Y of type.

pointer_to_ch_array is in fact a pointer to a character, not a pointer to an array of characters.


Quzah.

I stand corrected, thank you.

- Matt

Quote:

---

Originally Posted by quzah

No. He's passing a pointer to a character. A pointer to an array would be:

Code:

---

type (*ptr)[X][Y];

---

ptr is a pointer to an array of X by Y of type.

pointer_to_ch_array is in fact a pointer to a character, not a pointer to an array of characters.


Quzah.

---

This is a confusing thing to say, because pointer_to_ch_array does indeed point to an array of characters.

I'm not disagreeing with you about the C, but your english could do more to confuse than clarify.

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 :D

Quote:

---

Originally Posted by King Mir

This is a confusing thing to say, because pointer_to_ch_array does indeed point to an array of characters.

---

No it doesn't. It points to a single character. If that happens to be part of an array, that's fine, but it doesn't actually point to an array. It points to a single character:

Code:

---

type *foo;

---

foo is a pointer to type.
foo is not a pointer to an array off type.

You can say that because given just a single pointer, you cannot tell how much memory it points to.

Code:

---

void foo( type *bar )
{
    ...how much memory is bar pointed at?
}

---

Did you pass it an array?

Code:

---

type bar[ 10 ];
foo( bar );

---

Or did you pass it a single character?

Code:

---

type bar;
foo( &bar );

---

In either case, bar in function foo is a pointer to a single type.


Quzah.

Anyone writing foo would be aware of how much memory that foo is pointing to, and whether it points to an array or a single element.

But Oonej's new code is ridiculous, and for precisely the reason that it voilates the convention of passing arrays as pointers correctly. If a pointer to type T is passed to a function, any reasonable person would expect that either it serves as pass by reference of a value of type T, or an array (or the middle of one) of type T. Passing a char pointer as what is logically a 2D array or chars is unconventional and therefore confusing.

Here's a more reasonable version of that function:

Code:

---

void get_colors(char scanned[6][7], int n){/*alternatively:char (*scanned)[7]*/
    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) ;
}

//called as:
char scanned[6][7];
get_colors(scanned);

---

Quote:

---

Originally Posted by King Mir

Anyone writing foo would be aware of how much memory that foo is pointing to, and whether it points to an array or a single element.

---

Not necessarily so.

The writers of strcat aren't aware of if I pass it a single nul character, and I'm free to do so, because a single nul character is safely considered a string.

Code:

---

char buf[BUFSIZ] = {0}, n = 0;
strcat( buf, &n );

---

Perfectly legal.


Quzah.

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]);
        }
    }
}

---

Quote:

---

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.

Code:

---

    /* 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] = ' ';
              }
            }
          }
        }

---

Your code and comment do not agree. Setting k = i is a mistake because you're also erasing the ith character, the \0 you should be keeping.

You can prove it by taking your array of arrays and printing out all the colors like this:

Code:

---

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

---

It's undefined, so expect cuts from broken glass.

It doesn't really matter what any left over characters might be, so I would either erase that bit the comment refers to, or fill the whole matrix with '\0' before you do input.

Quote:

---

Originally Posted by quzah

Not necessarily so.

The writers of strcat aren't aware of if I pass it a single nul character, and I'm free to do so, because a single nul character is safely considered a string.

Code:

---

char buf[BUFSIZ] = {0}, n = 0;
strcat( buf, &n );

---

Perfectly legal.


Quzah.

---

Sure. But the arguments to strcat are still logically arrays, even when they are of size 1. The type system isn't aware of it, as you rightly point out, but it is useful to distinguish these two uses of pointers. And distinguish them both from abuses of pointers. A pointer to type T can be expected to point to either a single value of type T, or an array of type T.

Having more issues with the near finished program i'm making...

I found out about strncmp and it simplified my project a lot.

But now i'm getting weird errors when transferring my arrays as pointers to functions.

Code:

Code:

---

#include <stdio.h>
#include <stdbool.h>
#include <ctype.h>
#include <string.h>

void get_colors(char (*scanned)[7]);
bool colors_validate(const char (*scanned)[7], const char (*colors)[7]);
void rvalue(const char (*scanned)[7], const char (*colors)[7]);
void tvalue(const char (*scanned)[7], const char (*colors)[7]);
void rtcvalue(const char (*scanned)[7], const char (*colors)[7]);

int main(void){
 char scanned[6][7], answer;
 char colors[12][7] = {"silver", "gold", "black", "brown", "red", "orange", "yellow", "green", "blue", "violet", "grey", "white"};
 int i;

    get_colors(scanned);
    if(colors_validate(scanned, colors)){
        rvalue(scanned, colors);
        tvalue(scanned, colors);
        rtcvalue(scanned, colors)
    }
   
    getch();
 return 0; 
}

void get_colors(char (*scanned)[7]){
    int i, j;
         
    printf("\nPlease enter 6 resistor colors: \n");
    scanf("%s %s %s %s %s %s", scanned[0], scanned[1], scanned[2], scanned[3], scanned[4], scanned[5]);
    while(getchar() != '\n');
   
    for(j=0; j<6; j++){
        for(i=0; i<7; i++){
          scanned[j][i] = tolower(scanned[j][i]);
        }
    }
}

bool colors_validate(const char (*scanned)[7], const char (*colors)[7]){
    int i, j;
   
    for(i = 0; i < 12; i++){
          for(j = 0; j < 6; j++){
              if(strncmp(scanned[j], colors[i], 7) != 0){ return false; }
          }
    }
   
    if(strncmp(scanned[0], colors[2], 7) == 0 || strncmp(scanned[0], colors[1], 7) == 0 || strncmp(scanned[0], colors[0], 7) == 0){ printf("Wrong color. Try again\n"); return false; }
    else if(strncmp(scanned[1], colors[1], 7) == 0 || strncmp(scanned[1], colors[0], 7) == 0 || strncmp(scanned[2], colors[1], 7) == 0 || strncmp(scanned[2], colors[0], 7) == 0){ printf("Wrong color. Try again\n"); return false; }
    else if(strncmp(scanned[4], colors[11], 7) == 0) { printf("Wrong color. Try again\n"); return false; }
    else if(strncmp(scanned[5], colors[7], 7) == 0 || strncmp(scanned[5], colors[10], 7) == 0 || strncmp(scanned[5], colors[11], 7) == 0 || strncmp(scanned[5], colors[1], 7) == 0 || strncmp(scanned[5], colors[0], 7) == 0) { printf("Wrong color. Try again\n"); return false; }
    else return true;   
}

/* Find the Resistance value */
void rvalue(const char (*scanned)[7], const char (*colors)[7]){
    int i;
    double rv, rval, power;
   
    for(i=0; i<4; i++){
            if ( strncmp(scanned[i], colors[0], 7) == 0 ) { rv = -2; power = .01; }
      else if ( strncmp(scanned[i], colors[1], 7) == 0 ) { rv = -1; power = .1;}
      else if ( strncmp(scanned[i], colors[2], 7) == 0 ) { rv = 0; power = 1;}
      else if ( strncmp(scanned[i], colors[3], 7) == 0 ) { rv = 1; power = 10;}
      else if ( strncmp(scanned[i], colors[4], 7) == 0 ) { rv = 2; power = 100;}
      else if ( strncmp(scanned[i], colors[5], 7) == 0 ) { rv = 3; power = 1000;}
      else if ( strncmp(scanned[i], colors[6], 7) == 0 ) { rv = 4; power = 10000;}
      else if ( strncmp(scanned[i], colors[7], 7) == 0 ) { rv = 5; power = 100000;}
      else if ( strncmp(scanned[i], colors[8], 7) == 0 ) { rv = 6; power = 1000000;}
      else if ( strncmp(scanned[i], colors[9], 7) == 0 ) { rv = 7; power = 10000000;}
      else if ( strncmp(scanned[i], colors[10], 7) == 0 ){ rv = 8; power = 100000000;}
      else if ( strncmp(scanned[i], colors[11], 7) == 0 ){ rv = 9; power = 1000000000;}
     
      if(i == 0){ rval = rv * 100; }
      if(i == 1){ rval += rv * 10; }
      if(i == 2){ rval += rv; }
      if(i == 3){ rval = rval * power; }
    }
   
  if(rval < 1000000){ printf("Resistance = %.2f kOhms\n", rval/1000); }
  else { printf("Resistance = %.2f MOhms\n", rval/1000000); }

}

/* Find the resistance tolerance value */
void tvalue(const char (*scanned)[7], const char (*colors)[7]){
    double tv;
   
          if( strncmp(scanned[4], colors[3], 7) == 0){ tv = 1; }
    else if( strncmp(scanned[4], colors[4], 7) == 0){ tv = 2;  }
    else if( strncmp(scanned[4], colors[5], 7) == 0){ tv = 3;  }
    else if( strncmp(scanned[4], colors[6], 7) == 0){ tv = 4;  }
    else if( strncmp(scanned[4], colors[7], 7) == 0){ tv = .5;  }
    else if( strncmp(scanned[4], colors[8], 7) == 0){ tv = .25; }
    else if( strncmp(scanned[4], colors[9], 7) == 0){ tv = .1;  }
    else if( strncmp(scanned[4], colors[10],7) == 0){ tv = .05; }
    else if( strncmp(scanned[4], colors[1], 7) == 0){ tv = 5;  }
    else if( strncmp(scanned[4], colors[0], 7) == 0){ tv = 10;  }
   
    if(tv > 0){ printf("Tolerance = +/- %.2f%%\n", tv); }
    else { printf("Tolerance = +/- %.2f%%\n", tv); }
   
 
}

/* Find the resistance temperature coefficient */
void rtcvalue(const char (*scanned)[7], const char (*colors)[7]){
  int rtc;
 
          if(strncmp(scanned[5], colors[3], 7) == 0) { rtc = 100; }
    else if(strncmp(scanned[5], colors[4], 7) == 0) { rtc = 50; }
    else if(strncmp(scanned[5], colors[5], 7) == 0) { rtc = 15; }
    else if(strncmp(scanned[5], colors[6], 7) == 0) { rtc = 25; }
    else if(strncmp(scanned[5], colors[8], 7) == 0) { rtc = 10; }
    else if(strncmp(scanned[5], colors[9], 7) == 0) { rtc = 5; }
   
    printf("Tempco = %d ppm/degreeC\n", rtc);

}

---

Error output:
for all my functions ; incompatible pointer type.... but i dont see what i did different..

Code:

---

    for(i=0; i<4; i++){
            if ( strncmp(scanned[i], colors[0], 7) == 0 ) { rv = -2; power = .01; }
      else if ( strncmp(scanned[i], colors[1], 7) == 0 ) { rv = -1; power = .1;}
      else if ( strncmp(scanned[i], colors[2], 7) == 0 ) { rv = 0; power = 1;}
      else if ( strncmp(scanned[i], colors[3], 7) == 0 ) { rv = 1; power = 10;}
      else if ( strncmp(scanned[i], colors[4], 7) == 0 ) { rv = 2; power = 100;}
      else if ( strncmp(scanned[i], colors[5], 7) == 0 ) { rv = 3; power = 1000;}
      else if ( strncmp(scanned[i], colors[6], 7) == 0 ) { rv = 4; power = 10000;}
      else if ( strncmp(scanned[i], colors[7], 7) == 0 ) { rv = 5; power = 100000;}
      else if ( strncmp(scanned[i], colors[8], 7) == 0 ) { rv = 6; power = 1000000;}
      else if ( strncmp(scanned[i], colors[9], 7) == 0 ) { rv = 7; power = 10000000;}
      else if ( strncmp(scanned[i], colors[10], 7) == 0 ){ rv = 8; power = 100000000;}
      else if ( strncmp(scanned[i], colors[11], 7) == 0 ){ rv = 9; power = 1000000000;}
     
      if(i == 0){ rval = rv * 100; }
      if(i == 1){ rval += rv * 10; }
      if(i == 2){ rval += rv; }
      if(i == 3){ rval = rval * power; }
    }

---

Shouldn't you be breaking here if you get a match, instead of ignoring it and continuing to loop three more times?

edit - I read that last block as your tolower, ignore that. But I think the word 'const' in there is what is throwing you off. What is it you are trying to make the constant? The pointer, or the characters being pointed to?


Quzah.

I just want the chars to be constant, not the pointer. The constant characters are not to be changed.. thus being a const.

I can remove them, but it was advised by my teacher to make info that will not be edited to be made into a const.

Code:

---

rtcvalue(scanned, colors)

---

Missing a semicolon here.

yeah i caught the missing semicolon, but still getting for all my functions ; incompatible pointer type.

Well you can make colors a const char array without any issues, so that part's fine. You can't make scanned const, since that starts out uninitialized and changes at some point. So that means that for any parameter where you intend to use scanned, it cannot be const. const is not a function thing -- you can't use const to say "this function won't change this parameter". const means "this object can never change, by anybody anytime".

1. In some instances, your functions are expecting "const char" but you're passing them "char"s.
2. You appear to be using "scanned" as both a "char" and a "const char."

There's a few more things, but start with that.