Pointer arithmetic - counting dots - c

Question

Can't get any of the if statements to trigger and print something in the main. I am confused on what to do from here.
Can you please find where I have gone wrong for this problem. I need to count the number of dots and dashes (I've simplified it to work on the dots first).
The question must be solved using pointer arithmetic and the function has been provided
void analyse(char* code, int* p_dots, int* p_dashes);
^
#include <stdio.h>
#include <string.h>
void analyse(char* code, int* p_dots)
{
char *current = code;
int k = 0;
if ((*current + k) == '.')
{
p_dots++;
k++;
}
if ((*current + k) == '-')
{
//p_dashes++;
k++;
}
if ((*current + k) ==' ')
{
//p_dashes++;
k++;
}
}
int main(void)
{
char* morse[] =
{
"... --- ..." // SOS
// "-- --- .-. ... .", // MORSE
// "-.-. --- -.. .", // CODE
// "-.-. --- -- .--. ..... ----- -----", // COMP500
// ". -. ... . ..... ----- .----" // ENSE501
};
char* code = *morse;
int*p_dots=0;
//int*p_dashes = 0;
analyse(code, p_dots);
printf("Line 1 has %d dots", *p_dots);
return 0;
}

Answer 1

Here's another approach:
#include <stdio.h>
void analyse(char *code, int *p_dots, int *p_dashes)
{
for( ; *code ; ++code)
{
if (*code == '.')
*p_dots += 1;
if (*code == '-')
*p_dashes += 1;
}
}
int main(void)
{
int dots;
int dashes;
char *morse[] =
{
"... --- ...", // SOS
"-- --- .-. ... .", // MORSE
"-.-. --- -.. .", // CODE
"-.-. --- -- .--. ..... ----- -----", // COMP500
". -. ... . ..... ----- .----" // ENSE501
};
for(int i = 0 ; i < sizeof(morse) / sizeof(char *) ; ++i)
{
dots = 0;
dashes = 0;
analyse(morse[i], &dots, &dashes);
printf("Line %d ('%s') has %d dots and %d dashes\n", i, morse[i], dots, dashes);
}
return 0;
}
A pointer is a variable which contains the address of something. Using a pointer you can look at and/or change the thing it points to. If you increment the pointer, you're changing the address in the pointer, and thus you're changing what it points to.
In this code all of the pointer arithmetic is done in the for loop for( ; *code ; ++code) in the analyse function. What we're saying here is that A) there's no initialization section (there's no code before the first ; in the for statement); B) we want to continue as long as what's being pointed at the the code pointer is not zero *code in the "test" portion of the for statement; and C) after each pass through the loop we want to increment the code pointer (++code in the "increment" section of the for statement).
If you wanted to you could replace the for loop in analyse with:
while(*code <> 0)
{
if (*code == '.')
*p_dots += 1;
if (*code == '-')
*p_dashes += 1;
code += 1;
}
When looking at code like this I find it useful to mentally say "object of pointer" whenever I see an asterisk used with a pointer variable - so I'd read the first line of the code above as
While object of pointer "code" is not equal to zero...
then the next lines read as
if object of pointer "code" is equal to the character "period"
then add one to object of pointer "p_dots"
if object of pointer "code" is equal to the character "dash"
then add one to object of pointer "p_dashes"
add one to the variable "code"
Instead of "object of pointer" maybe you could read it as "target of pointer" to remind yourself that the pointer is pointing to something, and you're manipulating what the pointer points to, or in other words you're manipulating the pointers "target".
I find this kind of thing helps me understand pointer-based code a little bit more. Perhaps that'll help you too.

Answer 2

Change all occurrences of this:
(*current + k) == '-'
to this:
*(current + k) == '-'
(which means the same as the following:)
current[k] = '-'
Also in main() your code needs to update the value of an int, so just create a regular int, then pass it's address:
int p_dots = 0;
...
analyse(code, &p_dots);//sends the address so the value can be updated
Also, to get all of the values summed properly in the void analyse(char* code, int* p_dots) analyze function, a while loop is necessary: The following is your code with a while loop, and some simplifications: (it only tracks p_dots)
void analyse(char* code, int* p_dots)
{
char *current = code;
int k = 0;
while(*current)//check for NULL character
{
if (*current == '.')
{
(*p_dots)++;
}
if (*current == '-')
{
k++;
}
if (*current ==' ')
{
k++;
}
current++; //increment pointer to next position
}
}
By the way, one way to track each of the symbols ( ., -. ) is to create a struct, then pass it's address to update the member values. The example code for this is below:
typedef struct {
int dot;
int dash;
int space;
} count_s;
void analyse(char* code, count_s *c)//new prototype to track `.`, `-` and ` `
{
char *current = code;
int k = 0;
while(*current)
{
if (*current == '.')
{
(*c).dot++;
}
if (*current == '-')
{
(*c).dash++;
}
if (*current ==' ')
{
(*c).space++;
}
current++;
}
}
int main(void)
{
char* morse[] =
{
"... --- ..." // SOS
// "-- --- .-. ... .", // MORSE
// "-.-. --- -.. .", // CODE
// "-.-. --- -- .--. ..... ----- -----", // COMP500
// ". -. ... . ..... ----- .----" // ENSE501
};
char* code = *morse;
//int p_dots;
count_s count;
//int*p_dashes = 0;
analyse(code, &count);
printf("Line 1 has %d dots\n", count.dot);
printf("Line 1 has %d dashes\n", count.dash);
printf("Line 1 has %d spaces\n", count.space);
return 0;
}
EDIT to address question in comments: How would I go about doing it for the following lines that I've commented out?
Create an array of string literals
const char *morse[] = {{"... --- ..."},
{"-- --- .-. ... ."},
{"-.-. --- -.. ."},
{"-.-. --- -- .--. ..... ----- -----"},
{". -. ... . ..... ----- .----"}};
New main()...
int main(void)
{
char* code = NULL;//will be used to point to each array line.
count_s count = {0};//instance of struct with accumulators
// place declaration of const char *morse[] here as illustrated above
for(int i=0;i<sizeof(morse)/sizeof(morse[0]);i++)
{
code = morse[i];//set pointer to successive array elements 0-4
analyse(code, &count);//count will accumulate values as loop progresses.
}
printf(" %d dots\n", count.dot);
printf(" %d dashes\n", count.dash);
printf(" %d spaces\n", count.space);
return 0;
}

Answer 3

You have issue on the increment of dots and on scanning the string, see p_dots update on main.
I change p_dots to an int to have it on the stack (should be renamed I think). On your code you need to allocate some memory.
To scan the string:
Just increment the pointer to read char by char in an infinite loop, use a switch for the future of scanning '-', stop on null char.
code is pointing to the beginning of the string, switch (*code++) will read the current char and move pointer to the next one.
Start:
"... --- ..."
^
code
Read . and move code
"... --- ..."
^
code
and so on
#include <stdio.h>
#include <string.h>
void analyse(char* code, int* p_dots)
{
while (42) {
switch (*code++) {
case '.':
(*p_dots)++;
break;
case '-':
break;
case '\0':
return;
default:
break;
}
}
}
int main(void)
{
char* morse[] =
{
"... --- ..." // SOS
// "-- --- .-. ... .", // MORSE
// "-.-. --- -.. .", // CODE
// "-.-. --- -- .--. ..... ----- -----", // COMP500
// ". -. ... . ..... ----- .----" // ENSE501
};
char* code = *morse;
int p_dots=0;
//int*p_dashes = 0;
analyse(code, &p_dots);
printf("Line 1 has %d dots\n", p_dots);
return 0;
}