CODEWITHSUNDEEP
cstring
Co_Co
Co_Co

Reputation: 73

How to generate random sentences in C using arrays?

I`m trying to generate random sentences but sometimes i got an error or it prints random characters. Below is what i done. (the sentences should have the folowing format: article , noun , verb, preposition, article and noun).

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


int main() {
srand(time(NULL));
    int r1, r2, r3, r4, r5, r6;
    char *article[] = {"the" , "a" , "one" , "some", "any"};
    char *noun[] = {"boy" , "girl" , "dog" , "town", "car"};
    char *verb[] = {"drove" , "jumped" , "ran" , "walked", "skipped"};
    char *preposition[] = {"to" , "from" , "over" , "under", "on"};

    r1 = rand() % 6; r2 = rand() % 6; r3 = rand() % 6;  r4 = rand() % 6; r5 = rand() % 6; r6 = rand() % 6;
    printf("%s %s %s %s  %s %s\n", article[r1], noun[r2], verb[r3], preposition[r4], article[r5], noun[r6]);
return 0;
}

Upvotes: 1

Views: 2387

Answers (2)

Jean-Fran&#231;ois Fabre
Jean-Fran&#231;ois Fabre

Reputation: 140168

All your lists have less than 6 elements, so from time to time, the random routine returns a value outside the list bounds: read a garbage pointer => undefined behaviour. So now the "random" bug is explained.

Now what happens when you (or another developper) add or remove elements to one list? you have to adjust sizes again.

I'd create a small macro to avoid doing that:

#define RANDSTRING(s) s[rand() % (sizeof(s)/sizeof(*(s)))]

now:

const char *r = RANDSTRING(noun);

picks a value in the noun list, guaranteed to be in bounds.

it works on lists of strings and computes the size correctly. It won't work on pointers on pointers like char **, though.

It also doesn't work on empty lists (% 0 is illegal, and it makes no sense anyway)

Upvotes: 2

user2736738
user2736738

Reputation: 30926

r1 = rand() % 6;

have values 0,1,2,3,4,5

But the first array has 5 elements. So you were accessing array index out of bound and were having undefined behavior.

Instead 

r1 = rand() % arraysize;

This works.(Same you have to do for other arrays also).

So for example in the first case arraysize = 5 so r1= rand()%5 and for r2=rand()%5 etc. (Here you have array size = 5 for all the arrays).

Getting the array size will be like this in this case

r2 = rand()%( sizeof noun/ sizeof noun[0] );

Upvotes: 4

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