C++ copy a part of a char array to another char array

Question

I'm trying to copy only a portion of a string (or char *) into another string (or another char *)

char * first_string = "Every morning I"
char * second_string = "go to the library, eat breakfast, swim."
char * final_string;

I would like to copy part of the second_string into the first_string.

For Example:

Every morning I eat breakfast.

What is the function that allows you to copy only a portion of a string, starting from a specific point in the string?

Note: I don't want to use string variables, but char *, or even char arrays, if possible.

I don't want to use string variables, but char *, or even char arrays, if possible. Why? — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Nov 26, 2012 at 18:39
These should all be char const*, btw. If your compiler isn't already warning you about that then you are not using the proper flags. — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Nov 26, 2012 at 18:40
I have bad memories with strings. I remember not being able to use strings in a win32 C++ file even after including the string header (<string.h>), so I try to avoid using them now. I'm still generally new to C++, so I use what I know how to use. — James Diaz
– James Diaz, Commented Nov 26, 2012 at 18:42
@JamesDiaz that's because you don't include <string.h>, but <string>. — Luchian Grigore
– Luchian Grigore, Commented Nov 26, 2012 at 18:43
@JamesDiaz: <string.h> has not been "the string header" since the mid-1990s, i.e. before C++ was even internationally standardised as a language. Strings are vastly and disproportionately easier, and if you were having trouble with them then you should focus on fixing those problems rather than creating myriad new ones for yourself. — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Nov 26, 2012 at 18:43

Lightness Races in Orbit · Accepted Answer · 2012-11-26 18:42:46Z

7

It's std::copy, but with your code it would result in undefined behavior, because you have pointers to string literals, which are illegal to modify.

You'll need something like

char first_string[256] = "Every morning I";
char second_string[256] = "go to the library, eat breakfast, swim.";

std::copy(
    &second_string[23],
    &second_string[36],
    &first_string[strlen(first_string)]
);

Indices might be off.

edited Nov 26, 2012 at 18:42

Lightness Races in Orbit

387k77 gold badges671 silver badges1.1k bronze badges

answered Nov 26, 2012 at 18:38

Luchian Grigore

260k68 gold badges465 silver badges634 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Lightness Races in Orbit Over a year ago

there's an analogy here somewhere about worms and being early?!

James Diaz Over a year ago

Your answer is totally right, but the person after you actually gave the me the function/and example I was looking for. If I could I would accept both answers, as both have helped me an equal amount.

Mike Yang · Accepted Answer · 2020-03-23 11:42:18Z

4

If you really want to, use strcat (if you don't use strings, you should be ok with the C-functions anyway):

const char * first_string = "Every morning I";
const char * second_string = "go to the library, eat breakfast, swim.";
char final_string[80]; // make sure it's big enough - I haven't counted ;-)

strcpy(final_string, first_string);     // copy to destination
strcat(final_string, second_string+18); // append part of the second string

edited Mar 23, 2020 at 11:42

Mike Yang

3,0023 gold badges27 silver badges29 bronze badges

answered Nov 26, 2012 at 18:48

Axel

14.2k6 gold badges55 silver badges80 bronze badges

2 Comments

Praetorian Over a year ago

But that gives you Every morning I eat breakfast, swim. You want strncat, not strcat

Axel Over a year ago

Oh yes, I missed the swimming. However you have to add the dot at the end yourself...

Guarita · Accepted Answer · 2012-11-26 19:03:27Z

1

I'd use 'strncat()' like this:

const char * first_string = "Every morning I";
const char * second_string = "go to the library, eat breakfast, swim.";

char final_string [200];

//Copies first string first
strcpy(final_string, first_string);

//Copies second string
strncat(final_string, second_string[ text_position ], text_length);

Replace text_position with the position of second_string from which you may want to start copying text and replace text_length with the length of the portion of text you want to copy.

This way you may copy separate portions of text and not necessarily from a point of the string to the end.

answered Nov 26, 2012 at 19:03

Guarita

1631 silver badge11 bronze badges

Comments

Sahil Goyal · Accepted Answer · 2021-10-22 19:12:46Z

1

you could use strtok with "," as a delimiter to separate the parts of the second string and then use strcat to append that part onto the first string

answered Oct 22, 2021 at 19:12

Sahil Goyal

354 bronze badges

Comments

Innkeeper · Accepted Answer · 2012-11-26 18:53:49Z

0

You can use strstr() to search for the begining of the part you want, then strcat() to concatenate.

char * first_string = "Every morning I";
char * second_string = "go to the library, eat breakfast, swim.";
char * final_string;

char* s = "eat";
char* r = strstr(second_string, s);

strcat(final_string, first_string);
strcat(final_string, " ");
strcat(final_string, r);

answered Nov 26, 2012 at 18:53

Innkeeper

6731 gold badge10 silver badges24 bronze badges

Collectives™ on Stack Overflow

C++ copy a part of a char array to another char array

5 Answers 5

2 Comments

2 Comments

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

2 Comments

2 Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related