How to find small string in big string in C?

Question

trying to create a code in C. First of all, I can't use the string.h library or any other that helps similarly. The input will be from command line like: ./program pattern file, I already succeed to open the file and write line of the file to an array, but now I have to compare the pattern a get with the line in file. If there is written the pattern, I have to printf the whole line.

For the input I use:

char array[1000]; // hope 1000 will be enough

char *pattern= argv[1];

For the scanning I use:

while (fscanf(f,"%c", &temp)!= EOF){
        if (temp=='\n'){
             Algoritm...
        }
        add to array( array[i++]=temp) - already works`

And If I get end of line, I do not When the '\n' ocures, I need to start the algorithm to find out if the small string is in the line.

My questions:

How can I compare these "strings"? efficiently?

How to get know the size of pattern I get as input? (I think I need that to do the algorithm)

There are [many functions in standard C dealing with characters and strings](https://en.cppreference.com/w/c/string/byte). I'm sure you'll be able to find a function that [finds a substring](https://en.cppreference.com/w/c/string/byte/strstr). — Some programmer dude, Mar 20 '19 at 09:06
as for the "I can't use ``"... you *know* what function you could use *if* you could use it, i.e. `strstr`, then google for *"strstr c implementation site:stackoverflow.com"* — Antti Haapala -- Слава Україні, Mar 20 '19 at 09:06
If you know which function *could* be used, then it's very easy to use its name to find tutorials and examples on how to use it. For example searching for `strstr c example` or `strstr c tutorial`. — Some programmer dude, Mar 20 '19 at 09:14
I understand you, and your functions, but I can't use any library in this work, so any of the funtions you sent me cannot be used.. — Martin Indrych, Mar 20 '19 at 09:19
I guess, you don't really understand what is suggested. If you search for the implementation of a given function you can add your own version of that function to your project. No library involved. — Gerhardh, Mar 20 '19 at 09:23

score 0 · Answer 1 · answered Mar 20 '19 at 09:32

I suppose you are working on linux.

For your first question. memmem() should find the needle in your haystack;

From the memmem() manpage:

The memmem() function finds the start of the first occurrence of the substring needle of length needlelen in the memory area haystack of length haystacklen.

memmem returns:

The memmem() function returns a pointer to the beginning of the substring, or NULL if the substring is not found.

http://man7.org/linux/man-pages/man3/memmem.3.html

For your second question: How to find the length of argv[] in C

This isn't an answer. The question states *"I can't use the string.h library or any other that helps similarly."* And your second part directs the reader to another question, with no actual solution proposed here. — Weather Vane, Mar 20 '19 at 10:19

score 0 · Answer 2 · answered Mar 20 '19 at 12:18

You can write your own find function.

int find(char *array, char *pattern) {
    char *ap, *aap, *pp;
    for(ap=array; *ap; ap++) {
        for(pp=pattern, aap=ap; *aap && *pp && *pp==*aap; aap++, pp++); 
        if(*pp=='\0') return ap-array;
    }
    return -1;
}

The outer loop loops over your line 1 char a loop. It's the start point for the compare in the inner loop. The inner loop loops over your pattern and the line while

pattern not at end
line not at end
pattern char equals line char

without loop body.

If the pattern end is reached it was found and the function returns the found position in the line.

How to find small string in big string in C?

2 Answers2