switch strings without using any array notation in C

Question

Hi I am kind of new to pointers and I am trying write a function that switches one string with another without using any array notation, so completely with pointers.

I have got this function, which works, but in this one, I use one array notation.

Is there an easier, better way to do this? Thank you.

void stringtausch(char *eins, char *zwei) {
    char first[50], *philfe = first, *one = eins, *two = zwei;
    while(*one != '\0') {
        *philfe++ = *one++;
    }
    *philfe = '\0';
    while(*two != '\0') {
        *eins++ = *two++;
    }
    *eins = '\0';
    philfe = first;
    while(*philfe != '\0') {
        *zwei++ = *philfe++;
    }
    *zwei = '\0';
}

It depends on what you are trying to do, in your code if the strings are not of equal lengths you will have undefined behavior trying to write past the shortest. — Iharob Al Asimi
– Iharob Al Asimi, Commented Jan 4, 2015 at 19:03

Iharob Al Asimi · Accepted Answer · 2015-01-04 19:13:29Z

1

If both strings have equal length

void stringtausch(char *eins, char *zwei) {
    while ((*eins != '\0') && (*zwei != '\0')) {
        char swp;
        swp = *zwei;
        *zwei++ = *eins;
        *eins++ = swp;
    }
    *zwei = '\0';
    *eins = '\0';
}

if they don't it's not possible unless you allocate enough space before passing the pointers. And I don't see much benefit in that since you can

void stringtausch(char **eins, char **zwei) {
    char *swp;

    swp   = *zwei;
    *zwei = *eins;
    *eins = swp;
}

and call stringtausch

stringtausch(&s1, &s2);

edited Jan 4, 2015 at 19:13

answered Jan 4, 2015 at 19:07

Iharob Al Asimi

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1 Comment

Iharob Al Asimi Over a year ago

Downvote? why? can you explain what is wrong? may be I don't know that, may be I just missed it...

Iharob Al Asimi · Accepted Answer · 2015-01-04 19:07:28Z

0

Well an easier way is to use strcpy, but if you want to do your way, it's much easier. A pointer points to the first cell address of an array or single cell. You can merely just do this:

char * eins = "dogslovecats";
char * zwei = "cats";

stringtausch(&eins, &zwei);

void stringtausch(char ** eins, char ** zwei) {

    char * temp;

    temp = *eins;

    *eins = *zwei;

    *zwei = *temp;
}

edited Jan 4, 2015 at 19:07

Iharob Al Asimi

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answered Jan 4, 2015 at 19:05

jake

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1 Comment

Michiel uit het Broek Over a year ago

To be picky, first define the function and then call it. Just to avoid that one who read this will try to compile it and we will ruin his day ;)

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