Thread: My understanding of pointing is so disappointing: Exercise 5b Chapter 13

Hey Guys, I'm working on what for me has been the best chapter so far in this book and it is kicking my butt but, I'm learning a ton. This code finds letters in a string and capitalizes them. It skips what it has to. The updated string is copied into a char array called 'new'.

Can someone please help me understand why I can't print out the first letter or the entire string at the end of this code?

BTW: I have to use the pointer arithmetic.

Code:

#include <stdio.h>

void capitalize(char str[], int n);
int n=20;

int main(){
    char ch[]="ljS4e8sjHd67ds";
    int size = sizeof(ch)/sizeof(ch[0]);
    capitalize(ch, size);
    return (0);
}

void capitalize(char str[], int n){
    printf("%s\n",str);         // prints entire string
    char new[n];
    char *p;
    p=str;
    printf("p=%s\n",p);         // prints entire string
    for(p=str; p<str+n; p++){
        if(*p>64 && *p<91){
            *new=*p;
        }else if(*p>96 && *p<123){
            *new=toupper(*p);
        }else{
            *new=*p;
        }
        printf("%c\n", *new); // *q and *new both work
    }
    printf("%c", new[0]);   // not working: I expected L
    printf("%s", new);      // not working: I expected LJS4E8SJHD67DS
}

Thank you.

You are all the time overwriting the first element in your "new" array. The last character you're writing is the terminating null from the original string. Thus, the resulting string in "new" is a zero-length string.

Oh my gosh. This is so obvious now that you pointed this out. Thank you so much. My code now works.

Code:

#include <stdio.h>
// http://knking.com/books/c2/answers/c13.html


void capitalize(char str[], int n);
int n=20;


int main(){
    char ch[]="ljS4e8sjHd67ds";
    int size = sizeof(ch)/sizeof(ch[0]);
    capitalize(ch, size);
    return (0);
}


void capitalize(char str[], int n){
    char new[n];
    int i=0;
    char *p;
    p=str;
    for(p=str; p<str+n; p++){
        if(*p>96 && *p<123){
            new[i]=toupper(*p);
        }else{
            new[i]=*p;
        }
        i++;
    }
    printf("%s", new);      // working as expected LJS4E8SJHD67DS
}

Now that you're fiddling with pointers, what about using a pointer variable to fill your "new" array? Hint: The array designator is also a pointer to the first element of the array. That's the reason why the indirection operator (*) worked in your first attempt.

Unrelated side notes:
- The n parameter of your capitalize() function shadows the (unused) global n variable.
- One day you may come across the term Variable-length array - Wikipedia. The more you read about it the less you probably want to use VLAs.