jhu_prgwr_w2.Rmd

---
title: "JHU - Programming with R"
author: 郭耀仁
output: 
  revealjs::revealjs_presentation:
    theme: black
    highlight: zenburn
    center: true
---

## 作業來源

[Coursera](https://www.coursera.org/) 的 [Programming with R](https://www.coursera.org/learn/r-programming) 課程第二週作業

## 作業目的

寫出三個函數與提供的資料集互動：

`pollutantmean(directory, pollutant, id = 1:332)`

`complete(directory, id = 1:332)`

`corr(directory, threshold = 0)`

## 資料集

[specdata.zip](https://storage.googleapis.com/jhu_coursera_data/specdata.zip)

## 資料集簡介

- 332 個 .csv 檔案
- 每個 .csv 檔案都有三個變數：
    - Date
    - sulfate
    - nitrate

# 函數一

## 函數一結構

`pollutantmean(directory, pollutant, id = 1:332)`

計算不同觀測站特定污染物的平均數值

## 函數一提示

數列中若有遺漏值，記得在 `mean()` 函數中加入參數 `na.rm = TRUE`

```{r}
mean(c(1, 2, NA), na.rm = TRUE)
```

## 函數一範例

```{r echo=FALSE}
pollutantmean <- function(directory, pollutant, id = 1:332) {
  csv_filenames <- c()
  for (i in id) {
    if (nchar(i) == 1) {
      csv_filename <- paste0("00", i, ".csv")
      csv_filenames <- c(csv_filenames, csv_filename)
    } else if (nchar(i) == 2) {
      csv_filename <- paste0("0", i, ".csv")
      csv_filenames <- c(csv_filenames, csv_filename)
    } else {
      csv_filename <- paste0(i, ".csv")
      csv_filenames <- c(csv_filenames, csv_filename)
    }
  }
  csv_lst <- list()
  for (i in 1:length(csv_filenames)) {
    csv_files_dir <- paste0(directory, "/", csv_filenames[i])
    csv_lst[[i]] <- read.csv(csv_files_dir, stringsAsFactors = FALSE)
  }
  df <- csv_lst[[1]]
  if (length(csv_lst) != 1) {
    for (i in 2:length(csv_lst)) {
      df <- rbind(df, csv_lst[[i]])
    }
  }
  filtered_vector <- df[, pollutant]
  ans <- mean(filtered_vector, na.rm = TRUE)
  return(ans)
}
```

```{r}
my_dir <- "~/Downloads/specdata"
pollutantmean(my_dir, "sulfate", 1:10)
pollutantmean(my_dir, "nitrate", 70:72)
pollutantmean(my_dir, "nitrate", 23)
```

# 函數二

## 函數二結構

`complete(directory, id = 1:332)`

計算不同 csv 檔案中的完整觀測值列數

## 函數二提示

使用 `complete.cases()` 來計算完整的觀測值個數。

```{r}
logi_vec <- complete.cases(cars)
n_completes <- sum(logi_vec)
n_completes

test_df <- cars
test_df[1, "dist"] <- NA
logi_vec <- complete.cases(test_df)
n_completes <- sum(logi_vec)
n_completes
```

## 函數二範例

```{r echo = FALSE}
complete <- function(directory, id = 1:332) {
  csv_filenames <- c()
  for (i in id) {
    if (nchar(i) == 1) {
      csv_filename <- paste0("00", i, ".csv")
      csv_filenames <- c(csv_filenames, csv_filename)
    } else if (nchar(i) == 2) {
      csv_filename <- paste0("0", i, ".csv")
      csv_filenames <- c(csv_filenames, csv_filename)
    } else {
      csv_filename <- paste0(i, ".csv")
      csv_filenames <- c(csv_filenames, csv_filename)
    }
  }
  csv_lst <- list()
  for (i in 1:length(csv_filenames)) {
    csv_files_dir <- paste0(directory, "/", csv_filenames[i])
    csv_lst[[i]] <- read.csv(csv_files_dir, stringsAsFactors = FALSE)
  }
  df_id <- id
  nobs <- c()
  for (i in 1:length(csv_lst)) {
    n_complete_cases <- sum(complete.cases(csv_lst[[i]]))
    nobs <- c(nobs, n_complete_cases)
  }
  return_df <- data.frame(id = df_id, nobs = nobs)
  return(return_df)
}
```

```{r}
my_dir <- "~/Downloads/specdata"
complete(my_dir, 1)
complete(my_dir, c(2, 4, 8, 10, 12))
```

## 函數二範例二

```{r}
complete(my_dir, 30:25)
complete(my_dir, 3)
```

# 函數三

## 函數三結構

`corr(directory, threshold = 0)`

計算完整觀測值個數大於等於 `threshold` 的 csv 檔案中污染物的相關係數

## 函數三提示

- 使用 `cor(x, y, use = "pairwise.complete.obs")` 函數計算相關係數

## 函數三範例

```{r echo = FALSE}
corr <- function(directory, threshold = 0) {
  csv_filenames <- list.files(directory)
  csv_directories <- paste0(directory, "/", csv_filenames)
  csv_filelist <- list()
  for (i in 1:length(csv_filenames)) {
    csv_filelist[[i]] <- read.csv(csv_directories[i])
  }
  nobs <- c()
  for (i in 1:length(csv_filelist)) {
    n_complete_cases <- sum(complete.cases(csv_filelist[[i]]))
    nobs <- c(nobs, n_complete_cases)
  }
  filter_vector <- nobs >= threshold
  if (sum(filter_vector) == 0) {
    cor_vector <- c()
    return(cor_vector)
  } else {
    filtered_list <- csv_filelist[filter_vector]
    cor_vector <- c()
    for (i in 1:length(filtered_list)) {
      cor_vector[i] <- cor(filtered_list[[i]]$sulfate, filtered_list[[i]]$nitrate, use = "pairwise.complete.obs")
    }
    cor_vector <- cor_vector[!is.na(cor_vector)]
    return(cor_vector)
  }
}
```

## 函數三範例二

```{r}
my_dir <- "~/Downloads/specdata"
cr <- corr(my_dir, 150)
head(cr)
summary(cr)
```

## 函數三範例三

```{r}
cr <- corr(my_dir, 400)
head(cr)
summary(cr)
```

## 函數三範例四

```{r}
cr <- corr(my_dir, 5000)
summary(cr)
length(cr)
```

## 函數三範例五

```{r}
cr <- corr(my_dir)
summary(cr)
length(cr)
```