Use for-loop and if function to create a new vector?

Question

I want to do the following operation with the code: I want to get a sample of n = 30 out of a given normal distribution and calculate the mean of each sample. (until this step my function works without any problem). After that I want to create a new vector with yes or no , dependent on if the mean is in a certain range or not. Sadly the code does notconduct this step. I always get a vector with 13 elements,but there should be 500. What is the problem? Where is my mistake?

o = 13
u = 7
d = c()
for (i in 1:500){
  i = rnorm(30,mean = 10,sd = 6.04)
  i = mean(i)
  if (i <= o & i >=u) {
    d[i]=("Yes")
  } else {
    d[i]=("No")
  }
}

Benjamin · Accepted Answer · 2021-11-19 16:41:18Z

1

You should avoid changing the value of your iterator (i) within your loop. In your case, your i is becoming a non-integer value. When you try to index your d vector, it takes the integer portion of i.

Consider what happens when I have a vector

x <- 1:4

and I take the pi index of it.

x[pi]
# [1] 3

Your code should look more like this:

o = 13
u = 7

d = c()

for (i in 1:500){
  sample_i = rnorm(30, mean = 10, sd = 6.04)
  mean_i = mean(sample_i)
  if (mean_i <= o & mean_i >=u) {
    d[i]=("Yes")
  } else {
    d[i]=("No")
  }
}

If you would like to improve your code some, here are some suggestions:

First, avoid "growing" your results. This has performance implications. It is better to decide how long your result (d) should be and set it to that length to begin with.

Next, try not to hard code the number of iterations into your loop. Get familiar with seq_along and seq_len and use them to count iterations for you.

o = 13
u = 7

d = numeric(500)    #  I made a change here

for (i in seq_along(d)){  # And I made a change here
  sample_i = rnorm(30, mean = 10, sd = 6.04)
  mean_i = mean(sample_i)
  if (mean_i <= o & mean_i >=u) {
    d[i]=("Yes")
  } else {
    d[i]=("No")
  }
}

answered Nov 19, 2021 at 16:41

Benjamin

17.3k8 gold badges53 silver badges67 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Georgie Baul Over a year ago

Thank you very much for the detailed answer. This helps a lot !

jpenzer · Accepted Answer · 2021-11-19 16:47:01Z

1

Re-assigning i looks like a bad idea to me.

Are you sure you want to do this in a for loop? If not, a vectorised solution with crossing (tidyverse - nice explanations at varianceexplained.org ) should work pretty nicely, I think?

o = 13
u = 7

crossing(trial = 1:500,
         rounds = 1:30)%>%
  mutate(num = rnorm(n(), mean = 10,  sd = 6.04))%>%
  group_by(trial)%>%
  summarise(mean = mean(num))%>%
  mutate(d = case_when(mean <= o & mean >= u ~ "Yes",
                       TRUE ~ "No"))%>%
  count(d)

answered Nov 19, 2021 at 16:47

jpenzer

9295 silver badges9 bronze badges

1 Comment

Georgie Baul Over a year ago

Thanks for your answer. I will take a look at this possibility and try to understand it.

Collectives™ on Stack Overflow

Use for-loop and if function to create a new vector?

2 Answers 2

1 Comment

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related