logical index plot specific columns

Hi,

I made excel file with date and US dollar spent from 2015 to 2018. I graphed the two columns in excel but I also want to apply same knowledge in MATLAB using logical index. I want to use logical index using string compare to make a new table with each year corresponds with the amount of money spent in that specific year.

here is my first try but not sure why logical index doesn't work.

%%
clc
clear
%%
% read table
data = readtable('SpentinUSDollar.xlsx')
Udate = strcmp(data(:,1),'2015');
tdate = data(Udate,:);

does this work to use logical index? I was thinking to use for loop and the size of the table. Then to make MATLAB to look for 2015 and create table between this year and US Dollar. See the file for more details.

Answer 1

[Voss]

%%
clc
clear
%%
% read table
data = readtable('SpentinUSDollar.xlsx','NumHeaderLines',1)

data = 51×4 table
       Var1        Var2     Var3      Var4 
    ___________    ____    ______    ______

    23-Aug-2015    5000    1332.5       NaN
    09-Sep-2015    1000    266.51       NaN
    16-Sep-2015    6000    1599.1    3.7522
    27-Sep-2015    3000    799.53       NaN
    01-Oct-2015    3000    799.53       NaN
    08-Oct-2015    1000    266.51       NaN
    18-Oct-2015    6000    1599.1       NaN
    25-Oct-2015    1000    266.51       NaN
    01-Nov-2015    3000    799.53       NaN
    24-Nov-2015    3000    799.53       NaN
    09-Dec-2015    3000    799.53       NaN
    10-Jan-2016    1500    399.76       NaN
    13-Jan-2016    2000    533.02       NaN
    18-Jan-2016    3500    932.78       NaN
    31-Jan-2016    1500    399.76       NaN
    21-Feb-2016    3500    932.78       NaN

% use curly braces { } to access data from a table;
% parentheses ( ) give you a sub-table
class(data{:,1})

ans = 'datetime'

% use year() function to get the year from a datetime array
Udate = year(data{:,1}) == 2015;
% logical indexing to get a sub-table containing data from year 2015
tdate = data(Udate,:)

tdate = 11×4 table
       Var1        Var2     Var3      Var4 
    ___________    ____    ______    ______

    23-Aug-2015    5000    1332.5       NaN
    09-Sep-2015    1000    266.51       NaN
    16-Sep-2015    6000    1599.1    3.7522
    27-Sep-2015    3000    799.53       NaN
    01-Oct-2015    3000    799.53       NaN
    08-Oct-2015    1000    266.51       NaN
    18-Oct-2015    6000    1599.1       NaN
    25-Oct-2015    1000    266.51       NaN
    01-Nov-2015    3000    799.53       NaN
    24-Nov-2015    3000    799.53       NaN
    09-Dec-2015    3000    799.53       NaN

[Wal MathEngineering]

Thanks

3 questions if I may ask

But what does 'NumHeaderLines' do ?

why we used class(data{:,1})?

Lastly why 'year' in (date{:,1})==2015?

Thanks a lot

[Voss]

NumHeaderLines tells readtable how many lines there are in the file above where the data starts. Without setting that to 1, readtable returns this:

data = readtable('SpentinUSDollar.xlsx')

data = 49×4 table
       Var1        Var2     Var3      Var4 
    ___________    ____    ______    ______

    16-Sep-2015    6000    1599.1    3.7522
    27-Sep-2015    3000    799.53       NaN
    01-Oct-2015    3000    799.53       NaN
    08-Oct-2015    1000    266.51       NaN
    18-Oct-2015    6000    1599.1       NaN
    25-Oct-2015    1000    266.51       NaN
    01-Nov-2015    3000    799.53       NaN
    24-Nov-2015    3000    799.53       NaN
    09-Dec-2015    3000    799.53       NaN
    10-Jan-2016    1500    399.76       NaN
    13-Jan-2016    2000    533.02       NaN
    18-Jan-2016    3500    932.78       NaN
    31-Jan-2016    1500    399.76       NaN
    21-Feb-2016    3500    932.78       NaN
    13-Mar-2016    3500    932.78       NaN
    01-May-2016    3500    932.78       NaN

Note that starts with 16-Sep-2015, skipping the first two lines of data from the file.

Using class is unnecessary. I put it there to check the class of the first column of data in the table, the dates. I left it there so you could see the first column is of class datetime.

The year function returns the year of each element of a datetime array:

year(data{:,1})

ans = 49×1

2015 2015 2015 2015 2015 2015 2015 2015 2015 2016

dt = datetime('2022-04-20')

dt = datetime
   20-Apr-2022

year(dt)

ans = 2022