Get a string and extract 2 substring in c

Question

I just want to read from input a string of the form "c4 d5" and save two substring :

str1 = "c4"

str2 = "d5"

I tried:

char action[5];
char str1[2];
char str2[2];
scanf("%s", action);
strncpy(str1, &action[0], 2);
strncpy(str2, &action[3], 2);

but it gives me strange behaviors....

Also as I am learning c, I'm looking for both solutions using only char * and only char[] (either in input and output).

`scanf` will stop reading at the space character. So `action` contains "c4". — user3386109, Mar 24 '18 at 17:37
The output is kind of weird and it depends on other factors in my code, for the first time is something like : input: ciaoo output: ci ooci — , Mar 24 '18 at 17:39
`strncpy` will *not* add a terminating 0 to your strings. And your target strings also cannot hold more than a single character (*plus* one position for aforementioned terminating 0). It doesn't add up well. — Jongware, Mar 24 '18 at 17:59

score 2 · Accepted Answer · answered Mar 24 '18 at 17:42

2

When you expire strange behaviour ALWAYS read the man pages of the functions you are using. A C string should with almost no exceptions end with a \0 because otherwise printf/puts will probably not only print what you want but also some random memory fragments. Also the index starts at 0 so in order to get the third char you need to use [2].

#include <stdio.h>

int main()
{
    char action[5];
    char str1[3];
    char str2[3];

    scanf("%s", action);

    strncpy(str1, action, 2);
    strncpy(str2, &action[2], 2);

    str1[2] = '\0';
    str2[2] = '\0';

    puts(str1);
    puts(str2);

    return 0;
}

Try it out

man page of strncpy

The strncpy() function is similar, except that at most n bytes of src are copied. Warning: If there is no null byte among the first n bytes of src, the string placed in dest will not be null-terminated.

answered Mar 24 '18 at 17:42

QuesterDesura

385
2
18

I still have problems with the " " character, is it related with scanf? – Mar 24 '18 at 17:47
What exactly do you mean? – QuesterDesura Mar 24 '18 at 17:49
try to use the string "d4 e3" for the test, only d4 will be saved – Mar 24 '18 at 17:50
I suggest you to look at this question: https://stackoverflow.com/questions/1247989/how-do-you-allow-spaces-to-be-entered-using-scanf – QuesterDesura Mar 24 '18 at 17:53

score 0 · Answer 2 · answered Mar 24 '18 at 17:54

Here is a substring function. Don't forget to free the result when needed.

/**
 * Substrings in a new string, handles \0
 * @param char* str , the string to substring
 * @param int from , the start index included
 * @param int count , the amount of characters kept
 */
char * strsubstr(char * str , int from, int count) {
    char * result;

    if(str == NULL) return NULL;

    result = malloc((count+1) * sizeof(char));

    if(result == NULL) return NULL;

    strncpy(result, str+from, count);
    result[count] = '\0';
    return result;
}

score 0 · Answer 3 · answered Mar 24 '18 at 17:57

At first you must understand some basic principles of c.
Scanf stops its %s "string-reader" if it's read a space
secondly you can read both of the strings in once. "%s %s"
at this case you do not need the action variable:

#include <stdio.h>
#include <string.h>
int main(int argc, char const *argv[])
{
    char str1[2];
    char str2[2];
    scanf("%s %s",str1,str2);
    return 0;
}

This way you take cover of the space termination and the pointers game..

thanks, it's a cool solution but i still have to save the string for other purposes — , Mar 24 '18 at 18:24

Get a string and extract 2 substring in c

3 Answers3