Append a character to a list n times - c

Question

I want to create random data for testing. I want to fill an array with 100 strings of random length with the letter 'A'.
example:
array[0] = "AAAAA"
array[1] = "AAAAAAAA"
array[2] = "A"
...
char **create_string()
{
char **array = malloc(sizeof(**array));
srand((unsigned int)time(NULL));
int random = 0;
int i, j;
for(int i=0; i<100; i++)
{
random = rand() % 100;
for(j=0; j < random; j++)
{
array[i] = // some sort of string append that would be cheap.
}
}
}
I was looking at this C string append and they use strcat. Is there a better way to solve my problem? Since I will be running in a loop to create those random size strings.

Answer 1

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
char **create_string(size_t n) {
char **array = malloc(sizeof(char*) * n);
int i, j;
for(i=0; i<100; i++)
{
size_t sz = rand() % 100;
array[i] = malloc(sz + 1);
for(j=0; j < sz; j++) {
array[i][j] = 'A';
}
array[i][sz] = 0;
}
return array;
}
int main() {
char **array;
size_t i;
srand((unsigned int)time(NULL));
array = create_string(100);
for (i = 0; i < 100; i++)
printf("%s\n", array[i]);
return 0;
}
Alternatively, you can create a template string and copy required number of characters into each random string:
char **create_string(size_t n) {
char template[101];
char **array = malloc(sizeof(char*) * n);
int i;
for (i = 0; i < 100; i++)
template[i] = 'A';
template[100] = 0;
for(i = 0; i < n; i++) {
size_t sz = rand() % 100;
array[i] = malloc(sz + 1);
strncpy(array[i], template, sz);
array[i][sz] = 0;
}
return array;
}

Answer 2

This will depend on the distribution of string lengths that you want. This is a uniform distribution of string lengths, from 0 to 200.
int n = rand() % 200 * sizeof(*array);
array[i] = malloc(n + 1);
memset(array[i], 'A', n);
array[i][n] = '\0';
But you could have a Gaussian distribution, a Poisson distribution, etc.

Answer 3

char **create_string()
{
char **array = malloc(sizeof(char *) * 100);
srand((unisgned int)time(NULL));
int i;
for (i = 0; i <100;i++)
{
int random = rand() % 100;
array[i] = malloc(random);
memset(array[i],'A',random-1);
array[random-1] = '\0';
}
return array;
}
Problem for you to fix: what happens if random is 0? Also the random numbers will not be equaly distributed. Only modulo by a power of 2 will achieve that.

Answer 4

Here's an approach that doesn't put an upper bound on any individual string, but does put an exact bound on the total length of all strings. It also only calls malloc twice.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define TOT_SIZE (5000) /* adjust to taste */
#define TOT_STRS (100)
char **create_strings()
{
char **array = (char**) malloc(TOT_STRS * sizeof(char *));
char *str = (char*) malloc(TOT_SIZE);
int zeros = 1;
int i;
memset(str, 'A', TOT_SIZE - 1);
str[TOT_SIZE - 1] = 0;
while (zeros < TOT_STRS)
{
int pos = rand() % TOT_SIZE;
if (str[pos] != 0)
{
str[pos] = 0;
zeros++;
}
}
array[0] = str;
for (i = 1; i < TOT_STRS; i++)
{
array[i] = array[i - 1] + strlen(array[i - 1]) + 1;
}
return array;
}
And a short test program:
int main()
{
char **a = create_strings();
int i;
for (i = 0; i < TOT_STRS; i++)
{
printf("%3d: %s\n", i, a[i]);
}
return 0;
}
This code assumes that all the random strings need to be non-overlapping. If they can overlap in memory, you only need one string, and an array of pointer to different starting points in that one string.

Answer 5

You do not have to allocate the real 100 strings. Firstly you just declare a long enough array char long_array[100]. and then you use random = rand() % 100; get the random. Secondly you just pass the long_array and the random to your function. Then your problem is solved.

Answer 6

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
char **create_string(const size_t array_size,
const size_t string_size,
const unsigned char chr)
{
srand((unsigned)time(NULL));
char ** array = malloc(array_size * sizeof (char *));
size_t t;
for (t = 0; t < array_size; ++t) {
array[t] = malloc(string_size * sizeof(char));
array[t][string_size] = '\0';
memset(array[t], chr, (rand() % string_size) + 1);
}
return array;
}
main() {
char ** randstring = create_string(10, 7, 'A');
int t = 0;
for (; t < 10; ++t)
printf("%s\n", randstring[t]);
return 0;
}
Possible output
AAAAAA
AAAAA
AAAAAA
AAA
AAAAA
AAAAAA
AAAAAA
AAAAAAA
AAAA
AAAA