CODEWITHSUNDEEP
carrayspointerswhile-loop
Manda K
Manda K

Reputation: 3

C pointers and arrays

In this code I have a function besides main which finds a specific character in a string (sentence) and returns a pointer to it and otherwise it returns NULL:

char *position (char letter, char *sentence){
int i=0;
while (*(sentence+i) != '\0') {
    if (toupper(*(sentence+i)) == toupper (letter))
      return (sentence+i);
    ++i;
}
return NULL;}

It's all clear to me here.

And in the main function i have to use the previous function (position) to find how many times does a specific letter appear in a specific sentence.

int main (void){
char sentence [200+1];
char letter;
char *pos = sentence;
int counter=0;
gets(sentence);
scanf("%c", &letter);
while (pos != NULL){
    pos = position (letter, pos);
    if (pos != NULL){
        ++counter;
        pos +=1;
    }
}
return 0;   
}

What is unclear to me is what is exactly in the pos variable once it is set and how exactly the while loop at the end works?

edit: forgot to initialise variable i in function(), thanks for pointing it out

Upvotes: 0

Views: 93

Answers (3)

morgelo
morgelo

Reputation: 146

Lets do a small example: you've got the String "abaab" and your looking for the Letter b. As you know a C-String always ends with '\0'. So at beginning your pos represents the whole string.

abaab\0 //pos represents ="abaab"
| 
pos

After that you call your function: postiton(pos, 'b'). Pos now points on the seccond letter. And with this pos only represents the smaller String "baab".

abaab\0 //pos represents="baab"
 | 
pos

After ++pos; you got this situation: (pos represents a smaller String)

abaab\0 //pos represents ="aab"
  | 
 pos

When you now call the position function again. pos will point on the next 'b'. And of course the String is shorter again.

abaab\0 //pos represents ="b"
    | 
   pos

Increment pos:

abaab\0 //pos represents =""
     | 
    pos

Calling the position function again will return NULL, because there is no 'b'. When this function returns NULL the while function is finished.

Upvotes: 2

Michele d'Amico
Michele d'Amico

Reputation: 23711

What is unclear to me is what is exactly in the pos variable once it is set and how exactly the while loop at the end works?

Your code contains lot of bugs but if your question is just what I quote a simple answer is:

pos is a pointer in sentence string. Start to point in where sentence point and move away to reach the end of string.

position() take a start point end return the point where letter is found, otherwise it return NULL: so when you write pos = position (letter, pos); pos is moved to letter position. After that by pos +=1; you move away to the next element (move the pointer of one char). When there are any letter position() return NULL and while cycle terminate.

Upvotes: 1

Peter - Reinstate Monica
Peter - Reinstate Monica

Reputation: 16017

pos is initialized with char *pos = sentence; which lets it point to the first character in sentence, due to arrays "decaying" to pointers in most expressions, like this one. After that, it points to elements inside sentence because position() returns sentence+i which is the address of the first element of sentence plus the offset i.

Upvotes: 0

Related Questions