Storage Types Display Format String Numeric (Dis)connect Cha

Transcription

1 Storage Types Display Format String Numeric (Dis)connect Characters Jeehoon Han Fall 2017

2 Storage Types Storage types Numbers (digits of accuracy) Integers: byte(2), int(4), long(9) Floating points: float(7), double(16) Strings: str1, str2,..., str# where str# can hold words with # characters or less The default storage type is float Storing a variable containing numbers > 7 digits 8-9 digit integer: gen long varname Otherwise: gen double varname

3 Storage Types Storage types Numbers (digits of accuracy) Integers: byte(2), int(4), long(9) Floating points: float(7), double(16) Strings: str1, str2,..., str# where str# can hold words with # characters or less The default storage type is float Storing a variable containing numbers > 7 digits 8-9 digit integer: gen long varname Otherwise: gen double varname Changing the storage type of an existing variable: recast type varname Use compress to save memory by storing variables in the smallest types without losing precision

4 Storage Types: Example set obs 1 gen var = 0.2 tab var if var == 0.2

5 Storage Types: Example set obs 1 gen var = 0.2 tab var if var == 0.2 no observation

6 Storage Types: Example set obs 1 gen var = 0.2 tab var if var == 0.2 no observation Problems Numbers are stored in binary form and most decimals have no exact representations in binary ( ) 0.2 is stored as in float in double When you create the variable var, 0.2 is stored in float but Stata does all calculations in double precision Two ways to deal with this issue Store data as double tab var if var==float(0.2)

7 Display Format Specify the display format format varlist %fmt Numeric formats Fixed format: %w.df General format: %w.dg where w: the total width of the display d: the number of decimals (fixed format) For general format, Stata decides the number of decimals to display (if d > 0, d indicates the maximum number of decimal places) String format: %ws where w: the width of characters

8 Display Format: example Default format byte, int: %8.0g long: %12.0g float: %9.0g double: %10.0g Examples clear set obs 1 gen double pi = list pi format pi %8.0 g format pi %8.5 f

9 Inspecting Data sysuse uslifeexp, clear browse list

10 Inspecting Data describe codebook region

11 Strings (pure text) Numerics String variable numeric variable encode country, gen(country code) Numeric variable string variable decode country code, gen(county str)

12 Strings (numeric text) Numerics String variable numeric variable destring varlist, {gen(varname) replace} [option] [option] ignore( chars ): remove the nonnumeric characters specified force: treat any values containing nonnumeric characters as missing values

13 Strings (numeric text) Numerics Example: use destring price, gen(pricea) ignore( $, ) destring price, gen(priceb) force

14 Strings (numeric text) Numerics Numeric variable string variable tostring varlist, {gen(varname) replace} [option] [option] format(%fmt): convert using specified format force: convert to string even if it entails information loss tostring pricea, gen(price stra) tostring pricea, gen(price strb) format(%8.1f) force

15 Disconnect the Characters of Variables substr(str,(-)n,m): extract a substring from str starting at position n (from the end of a string) for a length of m

16 Disconnect the Characters of Variables substr(str,(-)n,m): extract a substring from str starting at position n (from the end of a string) for a length of m gen year =

17 Disconnect the Characters of Variables substr(str,(-)n,m): extract a substring from str starting at position n (from the end of a string) for a length of m gen year = substr(date,1,4) gen month =

18 Disconnect the Characters of Variables substr(str,(-)n,m): extract a substring from str starting at position n (from the end of a string) for a length of m gen year = substr(date,1,4) gen month = substr(date,6,2) gen day =

19 Disconnect the Characters of Variables substr(str,(-)n,m): extract a substring from str starting at position n (from the end of a string) for a length of m gen year = substr(date,1,4) gen month = substr(date,6,2) gen day = substr(date,-2,.)

20 Disconnect the Characters of Variables substr(str,(-)n,m): extract a substring from str starting at position n (from the end of a string) for a length of m gen year = substr(date,1,4) gen month = substr(date,6,2) gen day = substr(date,-2,.)

21 Disconnect the Characters of Variables substr(str,(-)n,m): extract a substring from str starting at position n (from the end of a string) for a length of m gen year = substr(date,1,4) gen month = substr(date,6,2) gen day = substr(date,-2,.) gen length = strlen(pricea) gen decimal = substr(date,-2,.) gen integer =

22 Disconnect the Characters of Variables substr(str,(-)n,m): extract a substring from str starting at position n (from the end of a string) for a length of m gen year = substr(date,1,4) gen month = substr(date,6,2) gen day = substr(date,-2,.) gen length = strlen(pricea) gen decimal = substr(date,-2,.) gen integer = substr(date,1,length-3)

23 Connect the Characters of Variables gen date1 = year+ +month+ +day egen date2 = concat(year month day), punct( )

24 Sort Arrange the data in ascending order sysuse uslifeexp, clear sort le sort year le gsort -year

25 Sort: Applications Create a lagged variable sort year gen le lag = le[ n-1] Finding duplicates sort year list if year == year[ n-1]