how do i randomly select an element in an array?

Question

So I've been trying to do this for an embarrassingly long time now. First of all here is my (very bad) code:

char firstInfection() {            
     char cities[] = {"aa", "bb", "cc", "dd", "ee", "ff"};            
     srand(time(NULL));            
     int randnum = rand() % 5;            
     char firstCity = cities[randnum];            
     return firstCity; 
}

I didn't include the preprocessor stuff but I'm also using string, math and time (obviously stdio and stdlib too)

Line 2 errors out here, saying: error: too many initializers for ‘char []’ I'm not really sure what's going on here.

I'm having trouble understanding some of the things I need to do. I was told on tutorialspoint that I have to initialize random with line 3: srand(time(NULL)) I'm not sure why lol.

I tried shortening the names of the elements in cities[]

I'm not really sure what else to do try. I'm guessing that the whole thing is completely wrong but I really don't know.

you need `char* firstCity`. also this has undefined behavior as you are returning off the stack — Daniel A. White, Mar 27 '22 at 01:41
you are not working with character literals correctly in C. in C, character literals require single-quotes (') instead of double quotes and must contain a single character. if you intended to work with "strings", so char-arrays, you should define and initialize your array as `char* cities[] = {"aa", "bb", "cc", "dd", "ee", "ff"};` — D3PSI, Mar 27 '22 at 01:45
it looks like the only difference here is the pointer... im not really sure how that fixes it but i'll still try that! — C3A1, Mar 27 '22 at 01:47
check out "strings" in C: https://www.tutorialspoint.com/cprogramming/c_strings.htm, and array as pointer in C: https://www.tutorialspoint.com/cprogramming/c_pointer_to_an_array.htm — D3PSI, Mar 27 '22 at 01:49

abelenky · Answer 1 · 2022-03-27T01:54:19.503

0

#include <stdio.h>

const char* firstInfection() {            
     static const char* cities[] = {"aa", "bb", "cc", "dd", "ee", "ff"};            
     srand(time(NULL));            
     int randnum = rand() % 6;  // Select value 0 to 5 (inclusive)            
     const char* firstCity = cities[randnum];            
     return firstCity; 
}

int main(void) {
    
    printf("Random String: %s\n", firstInfection());
    return 0;
}

edited Mar 27 '22 at 01:54

answered Mar 27 '22 at 01:41

abelenky

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thats still undefined behavior! – Daniel A. White Mar 27 '22 at 01:41
ohhh pointers xD im not really sure what the difference here is though? Like in what situation should I use a pointer and when shouldnt i? – C3A1 Mar 27 '22 at 01:45
the use of `char*` as type for the elements of the array is nothing more as C's way of referring to a multi-character string as there does not exist a thing such as a "string" in C. In C, strings are represented as an array of `char`'s. An array type can be referred to by its base pointer, which is why we can use `char*` instead of `char[]` – D3PSI Mar 27 '22 at 01:52
oh and also this solution errors out again. ISO C++ forbids converting a string constant to ‘char*’ [-Wwrite-strings] 4 | char* cities[] = {"aa", "bb", "cc", "dd", "ee", "ff"}; and it does this for every element in the array. – C3A1 Mar 27 '22 at 01:53

score 0 · Accepted Answer · answered Mar 27 '22 at 04:35

Answer given by @abelenky it not that good as he/she is calling srand() function inside a function which is not main(). NOTE: srand() should be called only once in any program within the main() function.

Some Improvements:

Instead to using bare return 0;, use return EXIT_SUCCESS;, which is defined in the header file stdlib.h.
Data type char means a single character which is enclosed within single quotes '', use char * (c-string) to store multiple characters.
Always use size_t to store indexes of any arrays, instead of int
Only C If your function is not taking any arguments then write it like this int func(void) { }, here's why.
Instead of using bare rand() % 5, get the size of the array stored on stack memory using sizeof() operator
Mark your local variable cities as static because you are returning it too.
Your function firstInfection() should be declared with static linkage.
You are calling time() function which is declared in time.h header file, so include that too

Final Code:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

static const char *firstInfection(void)
{
    static const char *cities[] = {"aa", "bb", "cc", "dd", "ee", "ff"};
    size_t rnd_index = rand() % (sizeof(cities) / sizeof(*cities));
    return cities[rnd_index];
}

int main(void)
{
    srand(time(NULL));
    printf("%s\n", firstInfection());
    return EXIT_SUCCESS;
}

Output:

[tushar@tushar-arch Desktop]$ ./main
aa
[tushar@tushar-arch Desktop]$ ./main
ee
[tushar@tushar-arch Desktop]$ ./main
cc

how do i randomly select an element in an array?

2 Answers2

Some Improvements:

Final Code:

Output: