Get a substring of a char*

Question

For example, I have this

char *buff = "this is a test string";

and want to get "test". How can I do that?

Solution 1

char subbuff[5];
memcpy( subbuff, &buff[10], 4 );
subbuff[4] = '\0';

Job done :)

Solution 2

Assuming you know the position and the length of the substring:

char *buff = "this is a test string";
printf("%.*s", 4, buff + 10);

You could achieve the same thing by copying the substring to another memory destination, but it's not reasonable since you already have it in memory.

This is a good example of avoiding unnecessary copying by using pointers.

Solution 3

Use char* strncpy(char* dest, char* src, int n) from <cstring>. In your case you will need to use the following code:

char* substr = malloc(4);
strncpy(substr, buff+10, 4);

Full documentation on the strncpy function here.

Solution 4

You can use strstr. Example code here.

Note that the returned result is not null terminated.

Solution 5

You can just use strstr() from <string.h>

$ man strstr

View more solutions

Comments

[evandrix]

don't forget to add the null-termination character '\0' if you do this.

[Grault]

This is kind of the reverse of a normal substring operation - more like indexOf.

[alexandernst]

This will waste memory. You have been warned! :P

[Goz]

@alexandernst: How exactly? 5 bytes allocated on the stack (which will get freed when it drops out of scope) is hardly wasting memory ...

[alexandernst]

Still more than what you need, but yeah, they'll get free-ed as soon as the code leaves the current stack.

[Goz]

@alexandernst: The problem is that you can't do much with that string data without copying it. If you take a pointer to the section from the original string you need to know the length as it will have no null terminator (Without affecting the original string).

[Juan Pablo Rinaldi]

Don't forget to #include <string.h>.

[FUD]

don't forget to malloc the substr large enough if you do this.

[Blagovest Buyukliev]

@procrastinator: Not really only for printing. This is a viable approach in case the substring is not going to be modified. This way you simply keep a pair of pointers instead of allocating a variable-length buffer.

[Admin]

N̶i̶c̶e̶ ̶o̶n̶e̶ ̶f̶o̶r̶ ̶p̶r̶i̶n̶t̶i̶n̶g̶ ̶o̶n̶l̶̶̶̶y̶ Sorry if you got me wrong, I mean that your solution doesn't require an extra string, which is perfect when you only need to print :-)

[Parnab Sanyal]

use sprintf to store the above formatted string

[adrian]

See Grault's comment down below

[Verma Aman]

@ParnabSanyal how can I use sprintf to store the substring? Do I need to pass substring length as third parameter?

[Parnab Sanyal]

@VermaAman no you don't. Your first argument would be the resulting string, second argument would be the source string and if you have placeholders inside second argument like %c, %i etc then from third argument to upto the number of placeholders, would be the actual values of the placeholder( comma separated). Just like printf

[TNT]

@Juampi: Is string.h really needed? You're only using arrays and memcpy.

[TNT]

@alexandernst: Couldn't you reuse the same buff? memcpy(buff, &buff[10], 4); buff[4] = '\0';

[Ekrem Dinçel]

@TNT writing to pointer to string literal is undefined behavior. Your code crashes the program on my machine. See this.

[Good man]

subbuff[4] = '\0'; this is very important, thanks for answer.

[VimNing]

@Barmar: So in your definition what is substring?

[Barmar]

@Rainning A substring is any contiguous portion of a string. E.g. def is a substring of abcdefghi

[VimNing]

@Barmar: Then since strstr() return the first index of the substring it found, but you said it's not just the substring.

[Barmar]

The question asked how to get test from this is a test string. This answer would return test string, not just test.

[Barmar]

Yes it is a substring, but it's limited to substrings that reach the end, it doesn't work if you want shorter substrings as in the question.