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R w-1 assignment.mdown

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+#R assignment w-1
+
+## Assignment Week 1
+https://github.com/absolutoslo/practice_assignment/blob/master/practice_assignment.rmd
+## Unzip Specdata in R working directory
+## check files
+```list.files("specdata")
+[1] "001.csv" "002.csv" "003.csv" "004.csv" "005.csv" "006.csv" "007.csv" "008.csv"
+  [9] "009.csv" "010.csv" "011.csv" "012.csv" "013.csv" "014.csv" "015.csv" "016.csv"
+ [17] "017.csv" "018.csv" "019.csv" "020.csv" "021.csv" "022.csv" "023.csv" "024.csv"```
+## inspect one file
+ ```read.csv("specdata/001.csv")
+ 1458 2006-12-28      NA      NA  1
+1459 2006-12-29      NA      NA  1
+1460 2006-12-30      NA      NA  1
+1461 2006-12-31      NA      NA  1```
+## assing name to file
+```first=read.csv("specdata/001.csv")```
+## look for columns name
+```names(first)```
+## check length of the table (nrow)
+```length(first$Date)
+[1] 1461```
+## check dim (nrow,ncol)
+```dim(first)
+[1] 1461    4```
+```summary(first)
+         Date         sulfate          nitrate             ID   
+ 2003-01-01:   1   Min.   : 0.613   Min.   :0.1180   Min.   :1  
+ 2003-01-02:   1   1st Qu.: 2.210   1st Qu.:0.2835   1st Qu.:1  
+ 2003-01-03:   1   Median : 2.870   Median :0.4530   Median :1  
+ 2003-01-04:   1   Mean   : 3.881   Mean   :0.5499   Mean   :1  
+ 2003-01-05:   1   3rd Qu.: 4.730   3rd Qu.:0.6635   3rd Qu.:1  
+ 2003-01-06:   1   Max.   :19.100   Max.   :1.8300   Max.   :1  
+ (Other)   :1455   NA's   :1344     NA's   :1339 ```
+ ```str(first)
+'data.frame':	1461 obs. of  4 variables:
+ $ Date   : Factor w/ 1461 levels "2003-01-01","2003-01-02",..: 1 2 3 4 5 6 7 8 9 10 ...
+ $ sulfate: num  NA NA NA NA NA NA NA NA NA NA ...
+ $ nitrate: num  NA NA NA NA NA NA NA NA NA NA ...
+ $ ID     : int  1 1 1 1 1 1 1 1 1 1 ...```
+ ## check first value of a column in data.frame(first)
+ ```first[1, "Date"]
+[1] 2003-01-01
+1461 Levels: 2003-01-01 2003-01-02 2003-01-03 2003-01-04 2003-01-05 ... 2006-12-31```
+## check final date
+```first[1461, "Date"]
+[1] 2006-12-31
+1461 Levels: 2003-01-01 2003-01-02 2003-01-03 2003-01-04 2003-01-05 ... 2006-12-31```
+## create a subset of data
+```first[which(first$Date == 2006-12-31), "sulfate"]```
+## or
+```first[which(first[, "Date"] == 2006-12-31), "sulfate"]
+numeric(0)```
+## or
+subset() function
+```subset(...)```
+
+## back to all files - assign vector to list of files
+```> files = list.files("specdata")
+> files
+  [1] "001.csv" "002.csv" "003.csv" "004.csv" "005.csv" "006.csv" "007.csv" "008.csv"
+  [9] "009.csv" "010.csv" "011.csv" "012.csv" "013.csv" "014.csv" "015.csv" "016.csv"
+ [17] "017.csv" "018.csv" "019.csv" "020.csv" "021.csv" "022.csv" "023.csv" "024.csv"```
+ > files[1]
+[1] "001.csv"
+> files[1:5]
+[1] "001.csv" "002.csv" "003.csv" "004.csv" "005.csv"