# Conditional and unconditional interpretations of mean inter-collision times

P

#### Peter Leopold

##### Guest
Peter Leopold Asks: Conditional and unconditional interpretations of mean inter-collision times
This question appeared in a study guide for my graduate level written exam in physics. (It may have been the one from the University of Chicago.) I see that a similar question was asked here What is the average value of time since last collision in Drude model? but the discussion did not focus on the aspect that I'm going to present.

A particle in a gas undergoes random collisions with other gas particles. The inter-collision times are described by an exponential random process. Consider a particle at time $t_0$. The distribution of times until the next collision is exponentially distributed $f(t | \tau) = (1/\tau)\exp(-(t-t_0)/\tau)$. Clearly, $\mathbb{E}[t-t_0] = \tau$. But by time reversal symmetry, this also describes the time since the last collision. So which is it? Is the mean time between collisions $\tau$ or $2\tau$?

The question always felt like a swindle $-$ in part because I was unable to track down a generally agreed upon answer. (This was long before stackoverflow existed.)

My take is that conditional on knowing that the particle just endured a collision, the mean time to the next collision is indeed $\tau.$ But unconditionally, the time since/until the last/next collision is properly $\tau$, so we compute the last-to-next time to be $2\tau.$

The problem is with the exact definition of the term "mean inter-collision time." As it can be defined to be 1) unconditionally "time to next collision", then it is $\tau$ (but it makes no reference to the last collision, so perhaps this definition is non-responsive to the name) or 2) conditional on a collision having just occurred, "time to next collision" = $\tau$, or 3) unconditionally, "time since last collision to next collision," for a randomly selected particle whose past is as unknown as its future, in which case it is in fact $2\tau.$

Is there a consensus on this point here?

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## Unreplied Posts

#### Images in NextJS not working when replacing a new image

John Albert Flores Asks: Images in NextJS not working when replacing a new image
I try to replace my current banner with any other images to see if they works but any image that I put leads to error when using it using Image from next/image and even in just simple <image> tag. It says The requested resource isn't a valid image. when I go to the url. All images are under of /public/images.

Here's the code for Image (the working 1)

Code:
    <Image
src={"/banner.png"}
alt="Image"
layout="fill"
objectFit="contain"
/>

But when I change the /banner.png into new name (ofcourse existing image too under the /public/images/), it just error with code 400 on request.

SolveForum.com may not be responsible for the answers or solutions given to any question asked by the users. All Answers or responses are user generated answers and we do not have proof of its validity or correctness. Please vote for the answer that helped you in order to help others find out which is the most helpful answer. Questions labeled as solved may be solved or may not be solved depending on the type of question and the date posted for some posts may be scheduled to be deleted periodically. Do not hesitate to share your thoughts here to help others.

#### python regex solution giving me 4 values in a tuple instead of one

coderman69 Asks: python regex solution giving me 4 values in a tuple instead of one
im trying to do this (Write a regular expression that can detect dates in the DD/MM/YYYY format. Assume that the days range from 01 to 31, the months range from 01 to 12, and the years range from 1000 to 2999. Note that if the day or month is a single digit, it’ll have a leading zero. The regular expression doesn’t have to detect correct days for each month or for leap years; it will accept nonexistent dates like 31/02/2020 or 31/04/2021.)

the solution is giving me 4 values in a tuple (date, day, month, year) instead of just (date) how do i fix this? thanks in advance.

Code:
import re

dates_regex = re.compile(r'''(

#day
(0[1-9]|[12][0-9]|3[01])

/

(0[1-9]|[10][12])

/

([1-2][0-9][0-9][0-9])

)''', re.VERBOSE)

string = """ 05/06/400
06/05/2031
29/04/5000
08/07/2164
01/12/1832
27/09/6000
08/01/1810
16/05/1503
35/12/2000
21/09/1915"""

regex_search = dates_regex.findall(string)

print(regex_search)

running the code results in:

[('06/05/2031', '06', '05', '2031'), ('08/07/2164', '08', '07', '2164'), ('01/12/1832', '01', '12', '1832'), ('08/01/1810', '08', '01', '1810'), ('16/05/1503', '16', '05', '1503'), ('21/09/1915', '21', '09', '1915')]

SolveForum.com may not be responsible for the answers or solutions given to any question asked by the users. All Answers or responses are user generated answers and we do not have proof of its validity or correctness. Please vote for the answer that helped you in order to help others find out which is the most helpful answer. Questions labeled as solved may be solved or may not be solved depending on the type of question and the date posted for some posts may be scheduled to be deleted periodically. Do not hesitate to share your thoughts here to help others.

#### Why does this method for reading in a value from a file in C causes a segfault?

Anton Nørgaard Asks: Why does this method for reading in a value from a file in C causes a segfault?
The background for my problem

I am presently writing a video game in C as a personal project. Due to the increasing number of hard-coded values, it was becoming impractical to store them directly inside the source code. Instead, it was decided that the values should be stored in files and then be dynamically read in, when relevant. Several options were available for this, but all of them were too tedious to set up and overkill for the desired usage. Therefore, I decided to write my own implementation of a file reader that returns values stored in files.

Initial approach to the solution

To make my code as generalizeable as possible, it was decided that the strategy i.e function that reads the file and returns the value, should first of all, return a void pointer, because it can store arbitrary values. The way that you then retrieved a value from a specific file, was that you would provide an argument specifying the desired type of the return value e.g by using an enum. The function would then based on the enum, store the value as the corresponding data type in the void pointer and it would be your responsibility to cast the returned void pointer to the correct type.

Expected file format

The files that the code should read, would be simple text files. The files in question, would consist of lines of variable names with a value assigned to them. Here is an example of how a file for my project would look like.

Code:
name=main_character
money=1000
damage_multiplier=1.2

So for example, the variable name would have the char* value of "main_character", the variable money would have the int value of 1000 and damage_multiplier, the float value of 1.2

Current code

Obviously including the entire code repository of my game won't be useful, so instead I have attempted to the best of my abilities to provide a minimum reproducible example.

We define the enum Return_Type that we pass as an argument to the function in order to retrieve a variable as a specific type

Code:
typedef enum{integer, string, floating,struct_limb,struct_attributes}Return_Type;

So if we want to return a variable as an integer, we pass the value integer to the method

The file i am presently trying to read from is a file that represents a creature struct. Its contents looks like this.

Code:
species=0
name=Short-faced bear
representation=a
id=0
limb_count=7
limbs=[[0,0,100,100],[1,0,100,100],[2,0,100,100],[2,0,100,100],[3,0,100,100],[3,0,100,100],[4,0,100,100]]
weight=900
height=3.4
attributes=[12,12,12,12,12,12,12,12]
max_health=100
max_carry=100
preferred_attack_type=1

The method used to read the file is this function

Code:
void *ir_readin_data(char *file_path, char *variable, Return_Type expected_type){
void *value;
FILE *fp = fopen(file_path, "r");
char * line = NULL;
size_t len = 0;
while((getline(&line, &len, fp)) != -1){
char *variable_pointer = strstr(line, variable);
if(variable_pointer != NULL){
char *value_as_str = strtok(strchr(line, '=')+1, "\n");
if(expected_type == integer){
/*Deprecated, but still used for sake of ease, strtol doesn't
work either */
int value_as_integer = atoi(value_as_str);
value = &value_as_integer;
fclose(fp);
return value;
}
}
}
}

For sake of readability, I have reduced the function to only consider the case when we want to retrieve the variable as an integer.

For the minimum reproducible example, i am trying to read the variable species from the file and i want it to be retrieved as an integer

Code:
*(int *)ir_readin_data(creature_file_path,"species", integer);

I would have gone on to demonstrate all the different approaches I have tried to remedy the problem, but I feel that this question is already dragging on quite a bit. I have tried approaches like passing a void pointer as an argument and store the value in that instead, I have tried to return the value as a char pointer, return the value in a union and so on. None of the approaches work. I feel like there is some subtlety in reading files that I haven't caught up on yet. Can you pinpoint me as to what the error is?

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When I try to run the command npx prisma migrate dev --name init I get this error message:

Code:
Error: P1010: User johndoe was denied access on the database mydb.public

I don't remember setting up a user "jhondoe", I just installed postgres desktop on my mac and that's it. I'm new to databases and backend dev so I'm not sure if I did setup a user.

SolveForum.com may not be responsible for the answers or solutions given to any question asked by the users. All Answers or responses are user generated answers and we do not have proof of its validity or correctness. Please vote for the answer that helped you in order to help others find out which is the most helpful answer. Questions labeled as solved may be solved or may not be solved depending on the type of question and the date posted for some posts may be scheduled to be deleted periodically. Do not hesitate to share your thoughts here to help others.

#### git config --list has two entries for user.name

user1936752 Asks: git config --list has two entries for user.name
I don't know how I've ended up at this stage but on my Mac, using the terminal, if I enter git config --list, I obtain

Code:
credential.helper=osxkeychain
init.defaultbranch=main
user.name=username2