for(i = 0; i < n; i++){
srand(time(NULL));
printf("%d ", time(NULL));
for(j = 0; j < (n-1); j++){
a[i][j] = rand();
}
}
I try to generate random numbers, but they are the same... I try srand(i * time(NULL)). No matter..
What should i do?
Array declaration:
int** a;
int i;
printf("Enter array size: ");
scanf("%d", &n);
a = (int**)calloc(n, sizeof(int));
for(i = 0; i < n; i++)
a[i] = (int*)calloc(n-1, sizeof(int));
Call srand() outside of the loop. You are reseeding it every iteration.
srand() seeds the random number generator so you get a different sequence of random numbers depending on the input. Your loop runs very fast, so the call to time(NULL) always returns the same value. You are resetting to the same random sequence with every iteration. As a general rule, only call srand() once in your program.
Don't call srand() every time through the loop - just do it once beforehand.
FAQs 13.15 to 13.20 will be of interest. And I am tempted to create a new tag for such questions.
srand is a function that "seeds" the random number generator. In case you don't know, random numbers in computers aren't really random. In effect, the computer just has a list of numbers that seem random in it, and you use srand to tell it where to start in that list, with each call to rand() returning the next item in the list.
The reason you write srand(time(NULL)) is to get the random numbers to start at some point that isn't going to be the same every time you run the program (unless the programs start at the same time).
So what you are doing here is repeatedly telling the program to restart the random number list at the same point (because the time is the same each time you go through the loop). Move the call to srand outside the loop and you will get the correct results.
srand(time(NULL));
for(i = 0; i < n; i++){
printf("%d ", time(NULL));
for(j = 0; j < (n-1); j++){
a[i,j] = rand();
}
}
Call srand once outside the loop.
You need to call srand() before entering the loop. srand() initializes the radnom number generator with the given seed and generates unique sequence of random numbers for this seed.
Your loop executes very fast so every call to time(NULL) yields the same time (measured in seconds) - hence you initialize random number generator with the same seed on every loop iteration.
srand(time(NULL));
for(i = 0; i < n; i++){
for(j = 0; j < (n-1); j++){
a[i,j] = rand();
}
}
No matter. The number are the same...
int** a;
int i;
printf("Enter array size: ");
scanf("%d", &n);
if( n < 1 ){
printf("Size should be > 0\n\n");
return NULL;
}
a = (int**)calloc(n, sizeof(int));
for(i = 0; i < n; i++)
a[i] = (int*)calloc(n-1, sizeof(int));
Here is my array...
Sergey, you did not get an error message with the a[i,j] version simply because this is a perfectly valid expression. The comma operator evaluates the sub-expressions from left to right and returns the value of the last expression. Thus, writing a[i,j] is identical to a[j]. What you received in the print was the value of the pointer to the j-th vector in your matrix.
Related
I'm trying to
Get rid of the info in an array with 10 "spots".
Fill the array with (10) random numbers
My code till time
int main()
{
int numbers[10] = { 0 };
int randNumber = 0;
int i = 0;
for (int i = 0; i <= 10; i++)
{
srand(time(NULL));
randNumber = rand() % 10 + 1;
printf("Random number saved in the array: %d\n", randNumber);
i++;
}
getchar();
getchar();
return 0;
}
First of all, you need to move the srand(time(NULL)); out of the loop.
Otherwise, because, time() has a time granularity of 1 second, in a second, if called multiple times in the loop (within a second, probably), it will re-initialize the PNRG with the same seed and all the next call to rand() will give you the same random number.
Now, once you have the random numbers, you need to assign it to the each array member like numbers[i] = randNumber; inside the loop, but there's more to it. Your loop, at present is off by one. You need to change
for (int i = 0; i <= 10; i++)
to
for (int i = 0; i < 10; i++)
to stay within bounds.
Your array's size is 10, and this loop runs 11 times, causing an overflow. This will solve it:
for (int i = 0; i < 10; i++)
Also remove the increasing of the loop's iterator, i, from inside the loop body. Remove the line:
i++;
Hello friends I need your help.
My program is such an array size 1000 where the numbers should be between 0-999. These numbers should be determined randomly (rand loop) and the number must not be repeated. Would be considered the main part, I have to count how many times I used rand().
My idea is that: one loop where it initializes all the 1000 numbers, and if in this loop they check whether the number appears twice, if the number appears twice is set it again until that not appear twice (maybe this is not the best way but ...)
It is my exercise (Here I need your help)-
#include <stdio.h>
#include <stdlib.h>
int main()
{
int const arr_size = 1000;
int i, j, c;
int arr[arr_size];
int loop = 0;
for(i = 0; i<arr_size; i++)
{
arr[i] = rand() % 1000;
loop++;
if (arr[i] == arr[i - 1])
{
arr[i] = rand() % 1000;
loop++;
}
}
printf("%d\n",loop);
}
So if anyone can give me advice on how I can make it work I appreciate your help.
Thanks.
As suggested, shuffling the set will work but other indirect statistical quantities might be of interest, such as the distribution of the loop variable as a function of the array index.
This seemed interesting so I went ahead and plotted the distribution of the loop as a function of the array index, which generally increases as i increases. Indeed, as we get near the end of the array, the chance of getting a new random number that is not already in the set decreases (and hence, the value of the loop variable increases; see the code below).
Specifically, for an array size = 1000, I recorded the non-zero values generated for loop (there were around 500 duplicates) and then made a plot vs the index.
The plot looks like this:
The code below will produce an array with the unique random values, and then calculate the value for loop. The loop values could be stored in another array and then saved for later analysis, but I didn't include that in the code below.
Again, I'm not exactly sure this fits the application, but it does return information that would not necessarily be available from an approach using a shuffle algorithm.
NOTE: some folks expressed concerns about how long this might take but it runs pretty quick, on my 2011 Macbook Pro it took a about a second for an array size of 1000. I didn't do a big-O analysis as a function of the array size, but that would be interesting too.
NOTE 2: its more elegant to use recursion for the numberInSet() function but it seemed best to keep simple.
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <stdbool.h> /* If C99 */
const int ARR_SIZE = 1000;
/* Check if the number is in the set up to the given position: */
bool numberInSet(int number, int* theSet, int position);
int main()
{
int* arr = malloc(sizeof(int)*ARR_SIZE);
srand((unsigned int)time(NULL));
/* Intialize array with rand entries, possibly duplicates: */
for(int i = 0; i < ARR_SIZE; i++)
arr[i] = rand() % ARR_SIZE;
/* Scan the array, look for duplicate values, replace if needed: */
for(int i = 0; i < ARR_SIZE; i++) {
int loop = 0;
while ( numberInSet(arr[i], arr, i-1) ) {
arr[i] = rand() % ARR_SIZE;
loop++;
}
/* could save the loop values here, e.g., loopVals[i] = loop; */
}
for(int i = 0; i < ARR_SIZE; i++)
printf("i = %d, %d\n",i,arr[i]);
/* Free the heap memory */
free(arr);
}
bool numberInSet(int number, int* theSet, int position) {
if (position < 0)
return false;
for(int i = 0; i <= position; i++)
if (number == theSet[i])
return true;
return false;
}
To make sure all random number you get in the same program are different, you must seed once the random generator:
srand (time(NULL)); //seed the random generator
//in the loop, rand will use the seeded value
rand() % 1000
I need to store random numbers between 500 and 600 to an array using a pointer and then print out those numbers. I get a segmentation error...core dump, I don't really understand what that means. The error happens after the printf statement ("%15d\n", aPtr[i]);
int main(){
int size;
int j, i;
int temp;
int sum = 0;
printf("Enter size of array");
scanf("%d", &size);
int array[size];
int *aPtr = malloc(sizeof(int) * size);
for (i = 0; i <= size; i++){
srand(time(NULL));
aPtr[i] = rand() % 500 + 100;
printf("%15d\n", aPtr[i]);
i <= size; should be i < size;
If you have an array of 50 items, the valid indices are [0,49].
you need to call srand ( which initializes the random number generator ) only once. Move it out of the for. And if you want random numbers between 500 and 600, you need to generate them between 0 and and 100 ( rand() % 101) and then add 500.
You need to call srand before you enter the loop.
As it stands now, you'll get a long sequence of identical numbers because you're resetting the random number generator to the same seed each time (assuming the time doesn't change, which is likely). In addition, if you want numbers between 500 and 600 inclusive, your formula is wrong. Try this snippet:
srand(time(NULL));
for (i = 0; i < size; i++){
aPtr[i] = (rand() % 101) + 500;
printf("%d\n", aPtr[i]);
}
Let us say I have a double for loop.
/*Just a double for loop
*/
for(int i = 0; i<IMAX; i++){
for(int j = 0; j<JMAX; j++){
count++;
recover_loop_indices(count,IMAX,JMAX); /*this is not real world code.Just to illustrate what I mean*/
}
}
My question is precisely, given count, IMAX and JMAX, is it possible to recover the unique loop indices, i and j?
Yes, based on count:
i = floor(count / JMAX);
j = count % JMAX;
You don't need IMAX at all. In fact, this is often how one can reconstruct an image from a serial stream of pixels, given only the width of the stream.
Edit:
I am assuming you want to recover the values of i and j before the count++. To recover it after the count++, use (count-1).
Ok so I get this code to do the averaging : (written in C )
.
.
int sum[3];
int j;
int avg;
for(;;) //infinite loop
{
for(j=0;j<3;j++){
i = ReadSensor(); // function that keeps saving sensor values as int i
sum[j]=i;
}
avg=sum[0]+sum[1]+sum[2]+sum[3];
printf("Sonar: %d \r \n", avg >> 2);
}
.
.
Is this correct ? im shifting by 2 to divide by avg / 2^(2) which is 4
The problem is im expecting a value of about 15, however I get about 8--9 .. Im not sure why this is happening ?
Basically the sensor's readings fluctuate between 15-17, I want to get an average instead of printing noise values. Is my code correct ? Then why do I get wrong outputs !?
Looks like your script only captures three values (j=0, j=1, j=2), then divides by four.
You have a few problems, here are some suggestions:
You're iterating through the inside loop 3 times, however you're saying you have 4 sensors, you should change your for loop to: for (j = 0; j < 4; j++).
sum is an array of 3 elements, yet you're accessing an element 1 past the end of the array when calculating avg (sum[3]). This will cause undefined behaviour. sum should be declared as char sum[4] for this reason and the one above.
(Optional) sum does not need to be an array in the above example, it can simply be an int.
(Optional) If you want to divide an int by 4, use the division operator. The compiler should be better at optimizing the code for your particular architecture than you.
This is how your code could now look, depending on whether you need to keep an array or not:
int sum[4];
int total, j;
for (;;)
{
total = 0; /* reset at every iteration of the outside loop */
for (j = 0; j < 4; j++) {
sum[i] = ReadSensor();
total += sum[i];
}
printf("Sonar: %d \r \n", total / 4);
}
OR
int total, j;
for (;;)
{
total = 0; /* reset at every iteration of the outside loop */
for (j = 0; j < 4; j++)
total += ReadSensor();
printf("Sonar: %d \r \n", total / 4);
}
Isn't this
avg=sum[0]+sum[1]+sum[2]+sum[3];
should be
avg=sum[0]+sum[1]+sum[2];
as the loop as well declaration int sum[3]; means we are trying to store only 3 values.
Now if you want 4 and ok with divide operator. There are the new code which should replace the mentioned lines
int sum[4];
for(j=0;j<4;j++)
avg=sum[0]+sum[1]+sum[2]+sum[3]; // this part stays the same
The number of values read from sensor is required twice. First, to control the number of iterations of for loop. Second, as the divisor of sum. Introduce a variable (say, N) to capture that.
Also, the division by shifting does not sound right, because that restricts the number of readings from the sensor to power of two.
enum { N = 4 };
sum = 0;
for( j = 0; j < N; j++) {
i = ReadSensor(); // function that keeps saving sensor values as int i
sum += i;
}
avg = sum / N;
printf( "Sonar average: %d\n", avg );