I tried creating an array of ten random strings that would print directions randomly. Such as first time "up down right ... rot_x" and second time "forward rot_y up ... down" etc. I tried using a char* pc and allocating memory for it with memset but that didn't work so I tried the following code but I'm getting weird output. How can I fix this?
int main()
{
int r_num;
char r_arr[10][10];
for (int i = 0; i < 10; i++)
{
r_num = rand() % 10;
switch(r_num)
{
case 0: strcpy(r_arr[0], "up");
break;
case 1: strcpy(r_arr[1], "down");
break;
case 2: strcpy(r_arr[2], "left");
break;
case 3: strcpy(r_arr[3], "right");
break;
case 4: strcpy(r_arr[4], "rot_x");
break;
case 5: strcpy(r_arr[5], "rot_y");
break;
case 6: strcpy(r_arr[6], "rot_z");
break;
case 7: strcpy(r_arr[7], "forward");
break;
case 8: strcpy(r_arr[8], "back");
break;
case 9: strcpy(r_arr[9], "reset");
break;
default:
fprintf(stderr, "Cannot process input/n");
}
}
for (int i = 0; i < 10; i++)
{
printf("%s ", r_arr[i]);
}
return 0;
}
here's my output:
up ?V? left right rot_x ?V? forward back reset
A few problems with your code are:
You aren't seeding rand(), so every run of your program will generate identical output. You need to use srand() first with a seed. Traditionally one uses time().
Secondly despite the randomness you are unrandomly (is that a word?) filling r_arr. "up" will always be first, "down" will always be second etc.... Instead you should do something like
for (int = 0; i< 10; i++) {
r_num = rand() % 10;
strcpy(r_arr[i], getDirection(r_num));
}
where getDirection() will return a string based on an integer input (e.g.: via a case statement that associates 0 with "up").
Your r_arr needs to be initialized. There is no guarantee in your current code that each entry in the array will be populated with chars before being accessed. If you implement suggestion 2 then you wont have a problem. Today however your code is accessing potentially uninitialized memory.
As noted by others above, your issue is that you're not indexing your array with the iteration number of your loop. You had:
case 0: strcpy(r_arr[0], "up");
Whereas you should have had:
case 0: strcpy(r_arr[i], "up");
The additional thing that I wanted to point out is that rand() uses a linear equation (at least on many systems) so it will be impossible for you to ever get two even numbers in a row or two odd numbers in a row, which is not very random. Hence I suggest something like:
r_num = (rand() >> 8) % 10;
As the commenters pointed out, you are randomizing not what value you put in each position but which positions get filled with their preset value. Also, your use of a switch statement here is just odd. Try something like:
char value_arr[10][10]={"up", "down", "left", "right", "rot_x", "rot_y", "rot_z", "forward", "back", "reset"}
for (int i = 0; i < 10; i++)
{
r_num = rand() % 10;
strcpy(r_arr[i], value_arr[r_num]);
}
Print the strings inside the switch instead of the for-loop at the end.
Maybe you'll also need something like:
srand (time(NULL));
here is a code that fits exactly to your need :
#include <stdio.h>
#include <time.h>
#include <stdlib.h>
#include <string.h>
int main()
{
// we use this instruction to get a random result each time we run the program
srand(time(NULL));
int r_num;
char r_arr[10][10];
// the dictionary array will be used to take from it the possible strings
char dictionary[10][10]={"up","down","left","right","rot_x","rot_x","rot_x","forward","back","reset"};
int i=0;
for(i=0; i<10; i++)
{
// r_num will be assigned randomly an index from dictionary tab
// the general equation is (rand()%(max-min+1))+min to get a random value
// between max and min inclusive
r_num=(rand()%(9-0+1))+0;
// we will put the random string chosen in the array r_num each time
strcpy(r_arr[i],dictionary[r_num]);
}
// this loop will print the result
for(i=0; i<10; i++)
{
printf("r_arr[%d]=%s \n",i,r_arr[i]);
}
return 0;
}
by looking at your output i noticed some strange values like ?v?,
the problem is that not all numbers between 0 and 9 will be generated by
the rand() function which mean that the corresponding array element(to those numbers) will not be initialized and therefor it contain garbage values from what ever was stored in that memory address.
i hope that explain why you are getting those strange values.
Related
I am sorry for the vague title, but I am having a very hard time figuring out how to describe this issue. I am trying to add data to an array and somehow it is all getting written to index 0 despite my explicitly indicating otherwise. I have created a minimal sample.
Expected Behavior
With command line arguments as -u rwx -g rw -o r:
bins[0] == "111"
bins[1] == "110"
bins[2] == "100"
Actual Behavior
Indices 0, 1 and 2 all end up as "100". If you put a printf() to check their value inside the various case statements, you will find that, for instance, when case 'g': runs, bins[1] == "110" AND bins[0] == "110". When case 'o': runs, all three indices will will hold the value "100".
Minimal functional sample
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
void processPerms(char *targets, char **perms, char **bins, const size_t arrSize) {
for(int i = 0; i < arrSize; i++) {
//This string will be modified
//so that the right bits will
//be set.
char binDig[] = "000";
//We decide which, if any, bit
//to set based upon the current
//letter being considered.
for(int k = 0; k < strlen(perms[i]); k++) {
switch(perms[i][k]) {
case 'r':
binDig[0] = '1';
break;
case 'w':
binDig[1] = '1';
break;
case 'x':
binDig[2] = '1';
break;
}
}
//Here, we check the target
//whose index corresponds to
//the index of the permissions
//string we just accessed.
//They will always be in
//an order where the related
//target and perm str are in
//the same array position.
switch(targets[i]) {
case 'u':
bins[0] = binDig;
//bins[0] == "111"
break;
case 'g':
bins[1] = binDig;
//bins[0] == "110"
break;
case 'o':
bins[2] = binDig;
//bins[0] == "100" && bins[1] == "100"
break;
}
}
}
int main(int argc, char *argv[]) {
const size_t arrSize = (argc-1)/2;
char *targets = (char*)calloc(arrSize, sizeof(char));
char **perms = (char**)calloc(arrSize, sizeof(char*));
//Copying just the letters
//of the args into these
//arrays.
for(int i = 0; i < arrSize; i++) {
targets[i] = argv[i+(i+1)][1];
}
for(int i = 0; i < arrSize; i++) {
perms[i] = argv[i*2+2];
}
//This array should hold three
//strings which shall be binary
//representations of the
//permissions
char **bins = (char**)calloc(3, sizeof(char*));
processPerms(targets, perms, bins, arrSize);
return 0;
}
bins[2] = binDig;
This will make you point to a variable which has automatic storage duration. This is a gateway to undefined behavior when you are returning it from the function. The life time of the variable is over as in this case and accessing it is UB.(Dereferencing a pointer to variable beyond its lifetime is Undefine d behavior).
A easy solution would be to use (This is part of POSIX standard but it is common enough) (In case you don't have this, you can allocate memory and copy to it the content of the array binDig).
bins[2] = strdup(binDig);
(Make the same changes for bins[0] and bins[1]).
Making this change as mentioned gives the expected behavior of being bins[0] equal to "111" and so on. The only thing is that, you need to free the dynamically allocated memory (including as that of returned by strdup
when you are done working with it). Similarly, don't cast the return value of malloc,calloc etc(because void* to char* is an implicit conversion). And check the return value of malloc/calloc.
The lifetime of you binDig array is limited by one iteration of the outer for cycle. This binDig array gets destroyed at the end of each iteration and created anew at the beginning of the next iteration.
This means that everything you do during each iteration of the outer cycle is lost when that iteration ends. The value of bins[] that you assigned during that iteration begins to point to some indeterminate location in memory where nothing exists anymore. Any attempts to access data through the corresponding bins[] entries lead to undefined behavior.
In real life each iteration of the cycle will typically recreate binDig at exactly the same location in memory, which creates an illusion of all of your bins[] pointers remaining valid but pointing to the same value. But this is a mere illusion. The behavior is already undefined at that point.
When the outer cycle ends the binDig array disappears forever and all your bins[] pointers become hopelessly invalid for good. But that's just the last straw. Your program died well before that moment.
I'm writing a program to take items from a given .txt file and put them into an array, then from that array, depending on the number, print it out as a character. For some reason it continually prints 3 spaces, then 8 at symbols, creates a new line and repeats, infinitely. I've tried a switch statement (which is commented out) but it did the same thing. cloudberry is the array in which it reads from, and it contains all the correct numbers.
while (read.hasNextLine()) {
int nutmeg = 0;
for (int x = 0; x < cloudberry.length;x++ ){
for (int y = 0; y < cloudberry[x].length;y++){
nutmeg = cloudberry[x][y];
switch (nutmeg){
case 1:
System.out.print("#");
break;
case 2:
System.out.print("+");
break;
case 3:
System.out.print(" ");
break;
}
}
}
}
Your code checks whether there is a new line but you never actually read it, therefore there is always the same line to read and the outer loop is iterating infinitely.
The code should be probably something like this:
while (read.hasNextLine()) {
String line = read.nextLine();
...
}
I have some problem with that. I am trying to learn C programming. Please help me
#include<stdio.h>
int main()
{
int a, factorial;
printf("Please enter a value :" );
scanf("%d", &a);
for (int i = 1; i<=a; i++)
{
a = (a - 1)*a;
}
printf("%d", factorial);
return 0;
}
Well in your code line a = (a - 1)*a; you actually changed your input for getting the factorial. It also will blow your loop. See your for loop will continue as long as your i is less than a, lets say you choose a=3 after first iteration the a itself will become 6, so the for loop will continue until it reach the integer limit and you will get overflow error.
What you should do?
First of all you should use a second variable to store the factorial result, you introduced it as factorial, the way that #danielku97 said is a good way to write a factorial since if you present 0 as input it will also give the correct result of 1. so a good code is:
factorial = 1;
for (int i = 1; i<=a; i++)
{
factorial *= i;
}
But lets say you insist of subtraction, the way you just tried to use, then you need to change the code like:
scanf("%d", &a);
if (a==1 || a==0){
printf("1");
return 0;
}
factorial = a;
for (int i = 1; i<a; i++)
{
factorial *= (a - i)*factorial;
}
You can see that the code just got unnecessarily longer. An if included to correct the results for 1 and 0. Also you need to make sure that i never become like i =a since in that case a-i will be equal to zero and will make the factorial result equal to zero.
I hope the explanations can help you on learning C and Algorithm faster.
Your for loop is using your variable 'a' instead of the factorial variable and i, try something like this
factorial = 1;
for (int i = 1; i<=a; i++)
{
factorial *= i;
}
You must initialize your factorial to 1, and then the for loop will keep multiplying it by 'i' until 'i' is greater than 'a'.
You are modifying the input a rather than factorial and also wrong (undefined behaviour) because you are using factorial uninitialized. You simply need to use the factorial variable you declared.
int factorial = 1;
...
for (int i = 1; i<=a; i++) {
factorial = i*factorial;
}
EDIT:
Also, be aware that C's int can only hold limited values. So, beyond a certain number (roughly after 13! if sizeof(int) is 4 bytes), you'll cause integer overflow.
You may want to look at GNU bugnum library for handling large factorial values.
I am (re-)learning C and in the book I am following we are covering arrays, and the book gives an algorithm for finding the first n primes; myself being a mathematician and a decently skilled programmer in a few languages I decided to use a different algorithm (using the sieve of Eratosthenes) to get the first n primes. Well making the algorithm went well, what I have works, and even for moderately large inputs, i.e. the first 50,000 primes take a bit to run as you would expect, but no issues. However when you get to say 80,000 primes pretty much as soon as it begins a window pops up saying the program is not responding and will need to quit, I made sure to make the variables that take on the primes were unsigned long long int, so I should still be in the acceptable range for their values. I did some cursory browsing online and other people that had issues with large inputs received the recommendation to create the variables outside of main, to make them global variables. I tried this for some of the variables that I could immediately put outside, but that didn't fix the issue. Possibly I need to put my arrays isPrime or primes outside of main as well? But I couldn't really see how to do that since all of my work is in main.
I realize I should have done this with separate functions, but I was just writing it as I went, but if I moved everything into separate functions, my arrays still wouldn't be global, so I wasn't sure how to fix this issue.
I tried making them either static or extern, to try and get them out of the stack memory, but naturally that didn't work since they arrays change size depending on input, and change over time.
the code is:
#include <math.h>
#include <stdbool.h>
#include <stdio.h>
unsigned long long int i,j;
unsigned long long int numPrimes,numPlaces;
int main(void)
{
bool DEBUG=false;
printf("How many primes would you like to generate? ");
scanf("%llu",&numPrimes);
// the nth prime is bounded by n*ln(n)+n*ln(ln(n)), for n >=6
// so we need to check that far out for the nth prime
if (numPrimes>= 6)
numPlaces = (int) numPrimes*log(numPrimes)+
numPrimes*log(log(numPrimes));
else
numPlaces = numPrimes*numPrimes;
if(DEBUG)
printf("numPlaces: %llu\n\n", numPlaces);
// we will need to check each of these for being prime
// add one so that we can just ignore starting at 0
bool isPrime[numPlaces+1];
// only need numPrimes places, since that is all we are looking for
// but numbers can and will get large
unsigned long long int primes[numPrimes];
for (i=2; i<numPlaces+1;i++)
isPrime[i] = true; // everything is prime until it isn't
i=2; // represents current prime
while (i < numPlaces + 1)
{
for (j=i+1;j<numPlaces+1;j++)
{
if (isPrime[j] && j%i ==0) // only need to check if we haven't already
{
isPrime[j] = false;// j is divisibly by i, so not prime
if(DEBUG)
{
printf("j that is not prime: %llu\n",j);
printf("i that eliminated it: %llu\n\n",i);
}//DEBUG if
}//if
}//for
// ruled out everything that was divisible by i, need to choose
// the next i now.
for (j=i+1;j<numPlaces+2;j++)// here j is just a counter
{
if (j == numPlaces +1)// this is to break out of while
{
i = j;
break;
}// if j = numPlaces+1 then we are done
else if (isPrime[j]==true)
{
i = j;
if (DEBUG)
{
printf("next prime: %llu\n\n",i);
}//DEBUG if
break;
}//else if
}// for to decide i
}//while
// now we have which are prime and which are not, now to just get
// the first numPrimes of them.
primes[0]=2;
for (i=1;i<numPrimes;i++)// i is now a counter
{
// need to determine what the ith prime is, i.e. the ith true
// entry in isPrime, 2 is taken care of
// first we determine the starting value for j
// the idea here is we only need to check odd numbers of being
// prime after two, so I don't need to check everything
if (i<3)
j=3;
else if (i % 2 ==0)
j = i+1;
else
j = i;
for (;j<numPlaces+1;j+=2)// only need to consider odd nums
{
// check for primality, but we don't care if we already knew
// it was prime
if (isPrime[j] && j>primes[i-1])
{
primes[i]=j;
break;
}//if, determined the ith prime
}//for to find the ith prime
}//for to fill in primes
// at this point we have all the primes in 'primes' and now we just
// need to print them
printf(" n\t\t prime\n");
printf("___\t\t_______\n");
for(i=0;i<numPrimes;i++)
{
printf("%llu\t\t%llu\n",i+1,primes[i]);
}//for
return 0;
}//main
I suppose I could just avoid the primes array and just use the index of isPrime, if that would help? Any ideas would help thanks!
Your problem is here, in the definition of the VLA ("Variable Length Array", not "Very Large Array")
bool isPrime[numPlaces+1];
The program does not have enough space in the area for local variables for the array isPrime when numPlaces is large.
You have two options:
declare the array with a "big enough" size outside of the main function and ignore the extra space
use another area for storing the array with malloc() and friends
option 1
#include <stdio.h>
unsigned long long int i,j;
bool isPrime[5000000]; /* waste memory */
int main(void)
option 2
int main(void)
{
bool *isPrime;
// ...
printf("How many primes would you like to generate? ");
scanf("%llu",&numPrimes);
// ...
// we will need to check each of these for being prime
// add one so that we can just ignore starting at 0
isPrime = malloc(numPrimes * sizeof *isPrime);
// ... use the pointer exactly as if it was an array
// ... with the same syntax as you already have
free(isPrime);
return 0;
}
The array you allocate is a stack variable (by all likelihood), and stack size is limited, so you are probably overwriting something important as soon as you hit a certain size threshold, causing the program to crash. Try using a dynamic array, allocated with malloc, to store the sieve.
I am trying to write a program that will assign 5 random integers to an array but so far it is filling the array with zeroes. Also, the first number can't be a zero. What am I doing wrong?
#include <stdio.h>
#include <stdlib.h>
int number[5];
int main() {
int i;
int t=0;
for (int t=0;t<5;t++) {
if (i=0)
{number[i] = rand()%8+1;}
else
{number[i] = rand()%10;}
i++;
printf("%d", number[i]);
}
return (0);
}
if (i=0)
That's assignment, not equality comparison.
int i;
Also, i is uninitialized, so accessing its value is undefined behavior. Since you accidentally assigned to it instead of comparing to it, if (i=0) doesn't invoke undefined behavior, but it would if you fixed the first bug.
i++;
printf("%d", number[i]);
Incrementing i before the print means you always print the cell right after the one you were working on.
You don't actually need i; t does everything you want i to do. We'll remove i and rename t to i:
for (int i = 0; i < 5; i++) {
if (i == 0) {
number[i] = rand() % 8 + 1;
} else {
number[i] = rand() % 10;
}
printf("%d", number[i]);
}
Several things are wrong in your code ...
1)
You didn't initialize the variable i
2)
if(i=0)
should be
if( i == 0 )
3)
You can use variable t instead of i -- means variable i is unnecessary
4)
You should have a function call for randomize() function before rand() function call, so that you can get the real random numbers
5)
If you just want to show the random numbers, you don't even need to use array
You will get the same sequence of 'pseudo-random' values by calling rand() in your program. To change this sequence the Random Number generator has to be 'seeded'. This is done by calling srand() function once with a seed value at the beginning of your program.
Ideally the value used for seed should change with each run of the program. Things that you could use as the seed are the process id, time, date or any other system provided value that is guaranteed to be different for each run of the program.
void srand(unsigned int seed);
Apart from that there are logical flaws in your program as others have highlighted, of which the if conditional assignment error is the most serious.