pointer to value in array?

Question

So I need to have a pointer to a value in a const char array. But I can't quite get it to work without errors. Here's the code.

int main (void)
{
    const char *alp = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

    char *ptr = &alp[3];

    printf("%s\n", ptr);

    return 0;
}

Edit- Sorry for not mentioning the errors. The thing is I get tons of different errors depending on where I put different asterisks and ampersands. There is no one particular error. One of the more frequent ones I get says "incompatible integer to pointer conversion assigning to 'char *' from 'const char';"

In the end I just want "ptr" to be equal to a pointer pointing to "D" in the array "alp".

Questions often come with question marks. What is your question? "What's wrong with my code?" isn't a valid question on SO. — Brian Roach
– Brian Roach, Commented Jun 7, 2013 at 19:59
What are the errors you are seeing? Can you not compile? Are they runtime errors, does it crash? What output do you expect? — Shafik Yaghmour
– Shafik Yaghmour, Commented Jun 7, 2013 at 19:59
You are trying to assign a pointer to a const char to a non-const char pointer. I'm guessing the compiler is complaining about that. — Gavin H
– Gavin H, Commented Jun 7, 2013 at 21:21

michael_wu · Accepted Answer · 2013-06-07 21:21:35Z

4

If you only want one character to print, change the %s to %c and dereference the pointer

printf("%c\n", *ptr);

It's true that you had a character pointer but %s tells printf to print from that pointer until it reads a null character. So we switch to %c which will print one character but it expects a value rather than a pointer to a value.

edited Jun 7, 2013 at 21:21

michael_wu

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answered Jun 7, 2013 at 20:04

axblount

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1 Comment

TimmayB Over a year ago

What do you mean to dereference the pointer, and which pointer? If I change to %c I get an ERROR "assigning to 'char *' from 'const char *' discards qualifiers" That's with the code >>char *ptr; ptr = &alp[3]<<

Gavin H · Accepted Answer · 2013-06-07 21:23:18Z

2

alp is a pointer to constant char.

ptr is a pointer to a non-const char.

If you want that to work out you would need to change ptr to be defined as:

char const * ptr = &alp[3];

answered Jun 7, 2013 at 21:23

Gavin H

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1 Comment

TimmayB Over a year ago

Thank you, this worked! I can't believe I didn't think to make it a const char. Anyways thank you!

asc99c · Accepted Answer · 2013-06-07 21:23:13Z

0

The const is saying that you're not going to change it, but then you're trying to copy the reference to a variable without that limitation. Instead, try:

const char *ptr = &alp[3];

But note that as other posters have already suggested, this code will give you a pointer to a string beginning with the D (e.g. DEFGHI...), not just the character D.

answered Jun 7, 2013 at 21:23

asc99c

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3 Comments

michael_wu Over a year ago

A pointer to a char and a pointer to a string (0-terminated sequence of chars) is exactly the same in C.

asc99c Over a year ago

Yes I understand that - the question asks for a pointer to the D, and I am not sure if that means they want the specific character D, or the string from that point onwards.

michael_wu Over a year ago

But there is no difference. If you do puts(ptr); the pointer will be treated as a string. With printf("%c\n", *ptr);, the same pointer will act as a pointer to a single char. It's the program logic which makes the difference.

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