random_sample
Generates a simple pseudorandom sample from a finite population.
Synopsis
#include <imsls.h>
float *imsls_f_random_sample(int nrow, int nvar, float population[], int nsamp, …, 0)
The type double function is imsls_d_random_sample.
Required Arguments
int nrow (Input)
Number of rows of data in population.
int nvar (Input)
Number of variables in the population and in the sample.
float population[] (Input)
nrow by nvar matrix containing the population to be sampled. If either of the optional arguments IMSLS_FIRST_CALL or IMSLS_ADDITIONAL_CALL are specified, then population contains a different part of the population on each invocation, otherwise population contains the entire population.
int nsamp (Input)
The sample size desired.
Return Value
nsamp by nvar matrix containing the sample. To release this space, use imsls_free.
Synopsis with Optional Arguments
#include <imsls.h>
float *imsls_f_random_sample (int nrow, int nvar, float population[], int nsamp,
IMSLS_FIRST_CALL, int **index, int *npop,
IMSLS_FIRST_CALL_USER, int index[], int *npop,
IMSLS_ADDITIONAL_CALL, int *index, int *npop, float *samp,
IMSLS_POPULATION_COL_DIM, int population_col_dim,
IMSLS_RETURN_USER, int samp[],
0)
Optional Arguments
IMSLS_FIRST_CALL, int **index, int *npop (Output)
This is the first invocation with this data; additional calls to imsls_f_random_sample may be made to add to the population. Additional calls should be made using the optional argument IMSLS_ADDITIONAL_CALL. Argument index is the address of a pointer to an internally allocated array of length nsamp containing the indices of the sample in the population. Argument npop returns the number of items in the population. If the population is input a few items at a time, the first call to imsls_f_random_sample should use IMSLS_FIRST_CALL, and subsequent calls should use IMSLS_ADDITIONAL_CALL. See example 2.
IMSLS_FIRST_CALL_USER, int index[], int *npop (Output)
Storage for index is provided by the user. See IMSLS_FIRST_CALL.
IMSLS_ADDITIONAL_CALL,
int *index,
int *npop,
float *samp (Input/Output)
This is an additional invocation of
imsls_f_random_sample, and updating for the subpopulation in
population is performed. Argument
index is a pointer to an array of length
nsamp containing the indices of the sample in the population, as returned using optional argument
IMSLS_FIRST_CALL. Argument
npop, also obtained using optional argument
IMSLS_FIRST_CALL, returns the number of items in the population. It is not necessary to know the number of items in the population in advance.
npop is used to cumulate the population size and should not be changed between calls to
imsls_f_random_sample. Argument
samp is a pointer to the array of size
nsamp by
nvar containing the sample.
samp is the result of calling
imsls_f_random_sample with optional argument
IMSLS_FIRST_CALL. See
Example 2.
IMSLS_POPULATION_COL_DIM, int population_col_dim (Input)
Column dimension of the matrix population.
Default: x_col_dim = nvar
IMSLS_RETURN_USER, int samp[] (Output)
User-supplied array of size nrow by nvar containing the sample. This option should not be used if IMSLS_ADDITIONAL_CALL is used.
Description
Function imsls_f_random_sample generates a pseudorandom sample from a given population, without replacement, using an algorithm due to McLeod and Bellhouse (1983).
The first nsamp items in the population are included in the sample. Then, for each successive item from the population, a random item in the sample is replaced by that item from the population with probability equal to the sample size divided by the number of population items that have been encountered at that time.
Examples
Example 1
In this example, imsls_f_random_sample is used to generate a sample of size 5 from a population stored in the matrix population.
#include <imsls.h>
int main()
{
int nrow = 176, nvar = 2, nsamp = 5;
float *population;
float *sample;
population = imsls_f_data_sets(2,
0);
imsls_random_seed_set(123457);
sample = imsls_f_random_sample(nrow, nvar, population, nsamp,
0);
imsls_f_write_matrix("The sample", nsamp, nvar, sample,
IMSLS_NO_ROW_LABELS,
IMSLS_NO_COL_LABELS,
0);
}
Output
The sample
1764 36
1828 62
1923 6
1773 35
1769 106
Example 2
Function imsls_f_random_sample is now used to generate a sample of size 5 from the same population as in the example above except the data are input to imsls_f_random_sample one observation at a time. This is the way imsls_f_random_sample may be used to sample from a large data file. Notice that the number of records need not be known in advance.
#include <stdio.h>
#include <imsls.h>
int main()
{
int i, nrow = 176, nvar = 2, nsamp = 5;
int *index, npop;
float *population;
float *sample;
population = imsls_f_data_sets(2, 0);
imsls_random_seed_set(123457);
sample = imsls_f_random_sample(1, 2, population, nsamp,
IMSLS_FIRST_CALL, &index, &npop,
0);
for (i = 1; i < 176; i++) {
imsls_f_random_sample(1, 2, &population[2*i], nsamp,
IMSLS_ADDITIONAL_CALL, index, &npop, sample,
0);
}
printf("The population size is %d\n", npop);
imsls_i_write_matrix("Indices of random sample", 5, 1, index, 0);
imsls_f_write_matrix("The sample", nsamp, nvar, sample,
IMSLS_NO_ROW_LABELS,
IMSLS_NO_COL_LABELS,
0);
}
Output
The population size is 176
Indices of random sample
1 16
2 80
3 175
4 25
5 21
The sample
1764 36
1828 62
1923 6
1773 35
1769 106
