More elegant way of searching for pattern in this uint8 array

Question

I am using C language in an embedded system. I have this uint8 array.

static uint8_t data_array[20];

Content of data_array is terminated with '\n'.

I want to check whether the first 3 bytes are "ABC". This is how I do it.

if (data_array[0]=='A' && data_array[1]=='B' && data_array[2]=='C')
{
    printf("pattern found\n");
}

Are there more elegant ways to detect the pattern? My method can be cumbersome if the pattern consists of 10 bytes.

Does the code rely on a specific character set, ASCII, for example, or does it need be able to handle others? — alk, Jul 21 '16 at 09:45
@alk Good point. But as long as characters are used consistently, the results will be valid. I'm pretty sure they must fit into range 0-127. Edit: it must fit into a `char`. So that range isn't even valid. — 2501, Jul 21 '16 at 09:53
My concern is more about the implicit conversion from `int` (the `char` literals) to an 8-bit `unsigned`. Imagine an implementation for a platform with `CHAR_BIT == 9` and `'A' == 0b1 0100 0001`. @2501 — alk, Jul 21 '16 at 09:59
@alk Yeah, see the comment edit above. ( uint8_t cannot exist with CHAR_BIT ==9 , but still, uint8_t may have trap values ) — 2501, Jul 21 '16 at 09:59
Ok, an explicity question: Are you after detecting any possible encoding of `char[3]={'A', 'B', 'C'}` or are you "just" after `char[3]={65, 66, 67}`? — alk, Jul 21 '16 at 10:03
Funny, how a deceptively simple question like this, requires several standard citations and/or assumptions to make a correct answer. — 2501, Jul 21 '16 at 10:03
@2501: Well yes, fair enough. So then I see no more issues with the OP's code. — alk, Jul 21 '16 at 10:09

Ohad Eytan · Accepted Answer · 2016-07-21T09:56:27.377

0

Just use a loop:

static uint8_t data_array[20] = "ABC";
static uint8_t s[4] = "ABC";
static uint8_t length = 3;

uint8_t bool = 1;
for (int i = 0; i < length; i++) {
    if (s[i] != data_array[i]) {
        bool = 0;
        break;
    }
}
if (bool) {
    printf("pattern found\n");
}

Live code here

edited Jul 21 '16 at 09:56

answered Jul 21 '16 at 09:47

Ohad Eytan

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This `uint8_t s[4] = "ABC";` makes assumption which not necessarily are met on any implementation/platform. – alk Jul 21 '16 at 10:01

score 0 · Answer 2 · edited Jul 21 '16 at 10:16

0

What about strncmp()?

static uint8_t data_array[20];

if(strncmp(data_array, "ABC", 3) == 0)
{
   printf("pattern found\n");
}

edited Jul 21 '16 at 10:16

alk

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answered Jul 21 '16 at 09:53

Batuu

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Does not need to be 0-terminated, because it's strNcmp. – Batuu Jul 21 '16 at 10:01
2

`strncmp` doesn't require `0`-termination, only that (in this case) there are 3 bytes which can be read – Markus Laire Jul 21 '16 at 10:01
[link](http://www.tutorialspoint.com/compile_c_online.php?PID=0Bw_CjBb95KQMdjdZcWhEaFo0V1k) Works perfectly fine! – Batuu Jul 21 '16 at 10:08

score -1 · Answer 3 · answered Jul 21 '16 at 09:45

-1

char small='a' ,big='A';
int i=0;
char arr[10];

//To check 

for(i; i<10 ; i++)
{


    if(arr[i]==small || arr[i]==big)
    {
          //what you want to do 
    }
    small++;
    big++;



}

answered Jul 21 '16 at 09:45

tryKuldeepTanwar

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You are talking about `char`, the OP is using `uint8_t`. – alk Jul 21 '16 at 09:52
ohh sorry I'm not familiar with uint8_t . – tryKuldeepTanwar Jul 21 '16 at 09:55

score -1 · Answer 4 · answered Jul 21 '16 at 10:33

You can utilize macros to generate code similar to your:

#include <stdio.h>


#define TEST_FIRST_1(arr, c0)       ((c0) == *(arr))
#define TEST_FIRST_2(arr, c0, ...)  (TEST_FIRST_1(arr, c0) && TEST_FIRST_1((arr) + 1, __VA_ARGS__))
#define TEST_FIRST_3(arr, c0, ...)  (TEST_FIRST_1(arr, c0) && TEST_FIRST_2((arr) + 1, __VA_ARGS__))
#define TEST_FIRST_4(arr, c0, ...)  (TEST_FIRST_1(arr, c0) && TEST_FIRST_3((arr) + 1, __VA_ARGS__))
#define TEST_FIRST_5(arr, c0, ...)  (TEST_FIRST_1(arr, c0) && TEST_FIRST_4((arr) + 1, __VA_ARGS__))
/* continue, if need */


static unsigned char data[30] = {
    1,2,3,4,5,6,7,8,9,10,11,12,13,
};

int main(int argc, char **argv)
{
    /* true cases */
    printf("%d\n", TEST_FIRST_1(data, 1));
    printf("%d\n", TEST_FIRST_2(data, 1, 2));
    printf("%d\n", TEST_FIRST_3(data, 1, 2, 3));
    printf("%d\n", TEST_FIRST_4(data, 1, 2, 3, 4));
    printf("%d\n", TEST_FIRST_5(data, 1, 2, 3, 4, 5));

    /* false cases */
    printf("%d\n", TEST_FIRST_5(data, 0, 0, 0, 0, 0));
    printf("%d\n", TEST_FIRST_4(data, 0, 0, 0, 0));
    printf("%d\n", TEST_FIRST_3(data, 0, 0, 0));

    return 0;
}

It may appear to be not clean at first look, but anyway doesn't lead to creating of an extra string literals for pattern to compare with.

More elegant way of searching for pattern in this uint8 array

4 Answers4