How to remove first three characters from string with C?

Question

How would I remove the first three letters of a string with C?

`str = str + 3;` since `str+=3;` is too short for a comment! — David Heffernan, Jan 21 '11 at 17:23

score 35 · Answer 1 · answered Jan 21 '11 at 17:24

35

Add 3 to the pointer:

char *foo = "abcdef";
foo += 3;
printf("%s", foo);

will print "def"

answered Jan 21 '11 at 17:24

BlackBear

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Need to check to make sure it's at least three characters long first! – kindall Jan 21 '11 at 17:26

Jonathan Leffler · Accepted Answer · 2011-01-21T19:10:50.977

18

void chopN(char *str, size_t n)
{
    assert(n != 0 && str != 0);
    size_t len = strlen(str);
    if (n > len)
        return;  // Or: n = len;
    memmove(str, str+n, len - n + 1);
}

An alternative design:

size_t chopN(char *str, size_t n)
{
    assert(n != 0 && str != 0);
    size_t len = strlen(str);
    if (n > len)
        n = len;
    memmove(str, str+n, len - n + 1);
    return(len - n);
}

edited Jan 21 '11 at 19:10

answered Jan 21 '11 at 17:26

Jonathan Leffler

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+1, but wouldn't it be better as int, which prints the # of chars actually remaining (or -1 instead of the assertion) ? – Tim Post Jan 21 '11 at 18:35
@Tim: there are all sorts of possible designs; this is a roughly minimal implementation - on the whole, I think the assignment is better than the early return. As to the return value - I'd be fine with returning the reduced length - that would be `size_t` like the input, most likely. – Jonathan Leffler Jan 21 '11 at 19:09

Martin Babacaev · Answer 3 · 2011-01-21T18:39:17.757

11

For example, if you have

char a[] = "123456";

the simplest way to remove the first 3 characters will be:

char *b = a + 3;  // the same as to write `char *b = &a[3]`

b will contain "456"

But in general case you should also make sure that string length not exceeded

edited Jan 21 '11 at 18:39

answered Jan 21 '11 at 17:29

Martin Babacaev

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Not only is `sizeof(char)` **useless**, it's also **wrong** for other types. Pointer arithmetic takes place in units of elements, not bytes. – R.. GitHub STOP HELPING ICE Jan 21 '11 at 18:25

Mahesh · Answer 4 · 2011-01-21T17:33:23.280

0

In C, string is an array of characters in continuous locations. We can't either increase or decrease the size of the array. But make a new char array of size of original size minus 3 and copy characters into new array.

edited Jan 21 '11 at 17:33

answered Jan 21 '11 at 17:23

Mahesh

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That'll copy the first three characters of the string. The question asks how to *remove* the first three characters. – mipadi Jan 21 '11 at 17:30
Yeah Just noticed.Changed it. Thanks. – Mahesh Jan 21 '11 at 17:34

score 0 · Answer 5 · answered Jan 21 '11 at 17:25

0

Well, learn about string copy (http://en.wikipedia.org/wiki/Strcpy), indexing into a string (http://pw1.netcom.com/~tjensen/ptr/pointers.htm) and try again. In pseudocode:

find the pointer into the string where you want to start copying from
copy from that point to end of string into a new string.

answered Jan 21 '11 at 17:25

I82Much

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-1. Code without explanation is better than links to references and completely obvious "pseudocode". – darda Jan 30 '14 at 18:21

How to remove first three characters from string with C?

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