head

Get top rows of table, timetable, or tall array

Syntax

B = head(A)

B = head(A,k)

head(___)

Description

B = head(A) returns the first eight rows of table, timetable, or tall array A.

B = head(A,k) returns the first k rows of A.

head(___) displays the first eight, or the first k, rows of A without returning a value.

Examples

Return First Eight Rows of Table

Create a table by reading data from a spreadsheet. Display the size of the table, showing that it has 1468 rows.

T = readtable("outages.csv","TextType","string");
size(T)

ans = 1×2

        1468           6

Return another table that has the first eight rows of T.

T2 = head(T)

T2=8×6 table
      Region          OutageTime        Loss     Customers     RestorationTime           Cause      
    ___________    ________________    ______    __________    ________________    _________________

    "SouthWest"    2002-02-01 12:18    458.98    1.8202e+06    2002-02-07 16:50    "winter storm"   
    "SouthEast"    2003-01-23 00:49    530.14    2.1204e+05                 NaT    "winter storm"   
    "SouthEast"    2003-02-07 21:15     289.4    1.4294e+05    2003-02-17 08:14    "winter storm"   
    "West"         2004-04-06 05:44    434.81    3.4037e+05    2004-04-06 06:10    "equipment fault"
    "MidWest"      2002-03-16 06:18    186.44    2.1275e+05    2002-03-18 23:23    "severe storm"   
    "West"         2003-06-18 02:49         0             0    2003-06-18 10:54    "attack"         
    "West"         2004-06-20 14:39    231.29           NaN    2004-06-20 19:16    "equipment fault"
    "West"         2002-06-06 19:28    311.86           NaN    2002-06-07 00:51    "equipment fault"

Display First Three Rows of Table

Create a table from a file with 1468 rows.

T = readtable("outages.csv","TextType","string");
size(T)

ans = 1×2

        1468           6

Display the first three rows. If you do not specify an output argument, head does not return a value. It only displays the top of the table.

head(T,3)

      Region          OutageTime        Loss     Customers     RestorationTime         Cause     
    ___________    ________________    ______    __________    ________________    ______________

    "SouthWest"    2002-02-01 12:18    458.98    1.8202e+06    2002-02-07 16:50    "winter storm"
    "SouthEast"    2003-01-23 00:49    530.14    2.1204e+05                 NaT    "winter storm"
    "SouthEast"    2003-02-07 21:15     289.4    1.4294e+05    2003-02-17 08:14    "winter storm"

Extract and View Top of Tall Table

Create a tall table. Then extract and view the first eight rows of data.

Create a tall table for the airlinesmall.csv data set. Select a subset of the table variables to work with.

varnames = ["Year","Month","ArrDelay","DepDelay","UniqueCarrier"];
ds = tabularTextDatastore("airlinesmall.csv","TreatAsMissing","NA",...
     "SelectedVariableNames",varnames,"TextType","string");
T = tall(ds)

T =

  Mx5 tall table

    Year    Month    ArrDelay    DepDelay    UniqueCarrier
    ____    _____    ________    ________    _____________

    1987     10          8          12           "PS"     
    1987     10          8           1           "PS"     
    1987     10         21          20           "PS"     
    1987     10         13          12           "PS"     
    1987     10          4          -1           "PS"     
    1987     10         59          63           "PS"     
    1987     10          3          -2           "PS"     
    1987     10         11          -1           "PS"     
     :        :         :           :              :
     :        :         :           :              :

To extract the first eight rows of data, use head. The result is a tall table with eight rows.

tt = head(T)

tt =

  8x5 tall table

    Year    Month    ArrDelay    DepDelay    UniqueCarrier
    ____    _____    ________    ________    _____________

    1987     10          8          12           "PS"     
    1987     10          8           1           "PS"     
    1987     10         21          20           "PS"     
    1987     10         13          12           "PS"     
    1987     10          4          -1           "PS"     
    1987     10         59          63           "PS"     
    1987     10          3          -2           "PS"     
    1987     10         11          -1           "PS"     

Collect the data into memory using the gather function. The result is an ordinary table with eight rows.

t8 = gather(tt)

t8=8×5 table
    Year    Month    ArrDelay    DepDelay    UniqueCarrier
    ____    _____    ________    ________    _____________

    1987     10          8          12           "PS"     
    1987     10          8           1           "PS"     
    1987     10         21          20           "PS"     
    1987     10         13          12           "PS"     
    1987     10          4          -1           "PS"     
    1987     10         59          63           "PS"     
    1987     10          3          -2           "PS"     
    1987     10         11          -1           "PS"     

Retrieve Specified Number of Rows in Tall Array

Preview the first 20 rows of data in a tall table.

Create a tall table for the airlinesmall.csv data set. Select a subset of the variables to work with, and treat "NA" values as missing data so that datastore replaces them with NaN values. Use head to view the first 20 rows of data.

varnames = ["Year","Month","ArrDelay","DepDelay","UniqueCarrier"];
ds = tabularTextDatastore("airlinesmall.csv","TreatAsMissing","NA",...
     "SelectedVariableNames",varnames,"TextType","string");
T = tall(ds)

T =

  Mx5 tall table

    Year    Month    ArrDelay    DepDelay    UniqueCarrier
    ____    _____    ________    ________    _____________

    1987     10          8          12           "PS"     
    1987     10          8           1           "PS"     
    1987     10         21          20           "PS"     
    1987     10         13          12           "PS"     
    1987     10          4          -1           "PS"     
    1987     10         59          63           "PS"     
    1987     10          3          -2           "PS"     
    1987     10         11          -1           "PS"     
     :        :         :           :              :
     :        :         :           :              :

tt = head(T,20)

tt =

  20x5 tall table

    Year    Month    ArrDelay    DepDelay    UniqueCarrier
    ____    _____    ________    ________    _____________

    1987     10          8          12           "PS"     
    1987     10          8           1           "PS"     
    1987     10         21          20           "PS"     
    1987     10         13          12           "PS"     
    1987     10          4          -1           "PS"     
    1987     10         59          63           "PS"     
    1987     10          3          -2           "PS"     
    1987     10         11          -1           "PS"     
     :        :         :           :              :
     :        :         :           :              :

Collect the results into memory to view the data.

t20 = gather(tt)

t20=20×5 table
    Year    Month    ArrDelay    DepDelay    UniqueCarrier
    ____    _____    ________    ________    _____________

    1987     10          8          12           "PS"     
    1987     10          8           1           "PS"     
    1987     10         21          20           "PS"     
    1987     10         13          12           "PS"     
    1987     10          4          -1           "PS"     
    1987     10         59          63           "PS"     
    1987     10          3          -2           "PS"     
    1987     10         11          -1           "PS"     
    1987     10          3           3           "PS"     
    1987     10          2           1           "PS"     
    1987     10         16          15           "PS"     
    1987     10          3           9           "PS"     
    1987     10         39          15           "PS"     
    1987     10         57          32           "TW"     
    1987     10          0          -3           "TW"     
    1987     10        -14           0           "TW"     
      ⋮

Input Arguments

A — Input array

table | timetable | tall array

Input array, specified as a table, timetable, or tall array.

k — Number of rows to extract

scalar

Number of rows to extract, specified as a positive scalar integer. If A has fewer than k rows, then head returns all of A.

Output Arguments

B — Requested rows

table | timetable | tall array

Requested rows, returned as a table, timetable, or tall array. The data type of B is the same as A.

Extended Capabilities

Tall Arrays

Calculate with arrays that have more rows than fit in memory.

This function fully supports tall arrays. For more information, see Tall Arrays.

You can use head and tail with tall arrays of any valid underlying data type (single, double, int8, datetime, table, and so on).

If you are unsure whether the result returned by gather(A) will fit in memory, then use gather(head(A)) or gather(tail(A)). These commands still fully evaluate the tall array A, but only return a small subset of the result in memory.

GPU Arrays

Accelerate code by running on a graphics processing unit (GPU) using Parallel Computing Toolbox™.

This function fully supports GPU arrays. For more information, see Run MATLAB Functions on a GPU (Parallel Computing Toolbox).

Distributed Arrays

Partition large arrays across the combined memory of your cluster using Parallel Computing Toolbox™.

This function fully supports distributed arrays. For more information, see Run MATLAB Functions with Distributed Arrays (Parallel Computing Toolbox).

Version History

Introduced in R2016b

R2022b: Calling head without specified output argument does not store output in ans

When you call head without a specified output argument, head displays the selected rows of the table but it does not store the output in the ans variable. In previous releases, calling head without a specified output argument causes the output to be stored in ans.

Calling head in a live script is usually not recommended. Instead, display the table or timetable by typing the variable name with no semicolon. The live editor provides a widget that enables you to examine the entire table or timetable. However, if you do call head in a live script, it is recommended that you assign the output to a variable so that the live script creates a widget for the output.