-
Create X = INDGEN(100).
[NOTE on IDL syntax: INDGEN is a function. In IDL the arguments of
a function are enclosed in parentheses. In IDL versions 1 through 4,
the indices of vector or arra variables were also enclosed in parentheses,
as in X(10). Obviously, this introduced possible ambiguities between
function and array notation. Therefore, in IDL version 5, array
indices are expected to be enclosed by brackets, as in X[10]. You
should use this syntax when referring to array elements. This notation
for array elements is not compatible with IDL V.4 and earlier.
However,
IDL V.5 does accept the parenthesis syntax for array elements, so it is
compatible with earlier IDL code in this regard.
-
Use
built-in IDL functions N_ELEMENTS, MIN, MAX, and TOTAL to answer the
following:
-
How many elements does X contain?
What is the minimum value in X?
What is the maximum value in X?
What is the sum of all the elements in X?
-
Use the IDL
HyperHelp facility to obtain information on any of the built-in routines you
just used, e.g. ?TOTAL.
-
Print X to your
terminal window
-
How are values in
X related to the corresponding subscripts?
-
Is X a
floating-point array?
-
Print the
fifteenth entry in X to your terminal
-
Then print the
entry containing the number 15
-
Is X a row-vector
or a column-vector?
-
The default
configuration of the printed data on your screen will tell you. To
confirm this, try the following: PRINT,TRANSPOSE(X)
-
Try the following
one-line command and inspect its output:
FOR I=0,99 DO PRINT,I,X[I]
-
Now try: FOR
I=1,100 DO PRINT,I,X[I]
-
Using the WHERE
function:
-
Define Q=2*X,
then type FIND = WHERE(Q LE 40, COUNT)
-
Examine the
contents of FIND and COUNT so that you understand how the WHERE function
operates.
-
Predict and
confirm the response if you type PRINT,Q[FIND]
-
Print to your
terminal the values of the vector X/10
-
Then print the
values of X/10.0 and compare the results.
-
Do the same for
FLOAT(X)/10
-
Do the same for
FIX(X/10.0)
-
What is the
difference between Z = X*0.0 and Z = 0?
-
Print the 11
elements centered on X = 10
-
First do this
using a FOR loop (on a single line)
-
Then do this
using the standard IDL subscript range notation, e.g. X[2:6]. No FOR
loop is needed.
-
Compare to the
following: K=5 & PRINT,X[10-K:10+K]
-
Define Y to be
the subarray consisting of those 11 elements, using subscript range
notation; print Y to your window as a check.
-
Using information
utilities:
-
Verify lengths of
X,Y using the N_ELEMENTS utility
-
Use the SIZE
utility to find the sizes of X and Y; what other info does it supply?
-
What information
does the command HELP,X,Y provide?
-
Define Q = Y+3
-- Note the values that Q contains
-
Define Q = Y*3
-- Note the values that Q contains
-
Define Q = Y^3
-- Note the values that Q contains
-
Define Q = Y^4
-- Note the values that Q contains; why are they not monotonic?
-
Define Q = FLOAT(Y)^4
-- Note the change.
-
Print the vector
which results from Q = X*Y
-
What did IDL
actually do to arrive at this result?
-
Using the
built-in functions TOTAL and N_ELEMENTS:
-
Find and print
the mean values of X and Y;
-
Find and print
the variances of the two arrays
-
Do the same using
the built-in functions MEAN and VARIANCE.
-
Do the same using
the built-in function MOMENT
-
Determine the
nature of the vector which results when you write Q = X & Q[2] =
Y.
-
What will happen
if you write Q[98] = Y? Try it.
-
Predict, then
verify, the outcome of the following operations:
Q = [Y,Y]
Q = [Q,Y]
-
Create a
16-element vector [1.01, 1.02, 1.03...] using a simple one-line command
employing FINDGEN. Verify.
-
Create a
16-element vector: [1,2,4,8,16....] using a simple one-line command
employing FINDGEN. Verify.
-
Create the
100-element vector Z = 10*X - 0.1*X^2, where X contains the integers between
0 and 99.
-
Use MAX to
determine the maximum value of Z.
-
Use WHERE to
locate the X value for which this maximum occurs.
-
X was an integer
vector but Z is not. Why?
-
Using the SHIFT
function, shift the elements of the array you just created three entries to
the left. Verify that the maximum is now in the expected
location.
-
Create, using a
simple one-line command employing FINDGEN, a 1001-element vector containing
the base-10 logarithms of the integers between 0 and 1000. Name this
"ILOG".
Verify its contents.
-
Now create a five
element vector, Y, containing the numbers 2, 100, 500, 20, and
999.
-
Explain the
vector resulting when you type NEW = ILOG[Y]. Why did we include an
entry for 0 in defining ILOG?
-
Optional problem:
ILOG in the previous exercise is called a "lookup table." It
can be used to accelerate computations in problems where a large number of
time-consuming transformations such as logarithms are needed. You can
estimate the time savings for this example as follows:
-
Write a simple
one-line IDL command script using "&" as a link between
individual commands. Use the SYSTIME(1) function to determine the
start time. Then, use a FOR loop to compute the logarithm of an
integer 40000 times using the standard ALOG10 function. Then use
SYSTIME(1) again to determine the end time. Print the elapsed time.
-
Repeat, now using
the "ILOG" lookup table instead of ALOG10.
