Loop Statements

Loop Statements

One of the most common programming tasks is to perform the same set of statements multiple times. Rather than repeat a set of statements again and again, a loop can be used to perform the same set of statements repeatedly.

Note

IDL's array capabilities can often be used in place of loops to write much more efficient programs. For example, if you want to perform the same calculation on each element of an array, you could write a loop to iterate over each array element:

   array = INDGEN(10)
   FOR i = 0,9 DO BEGIN
      array[i] = array[i] * 2
   ENDFOR

This is much less efficient than using IDL's built-in array capabilities:

   array = INDGEN(10)
   array = array * 2

FOR...DO

The FOR statement is used to execute one or more statements repeatedly, while incrementing or decrementing a variable with each repetition, until a condition is met. It is analogous to the DO statement in FORTRAN.

In IDL, there are two types of FOR statements: one with an implicit increment of 1 and the other with an explicit increment. If the condition is not met the first time the FOR statement is executed, the subject statement is not executed.

FOR Statement with an Increment of One

The FOR statement with an implicit increment of one is written as follows:

FOR Variable = Expression, Expression DO Statement

The variable after the FOR is called the index variable and is set to the value of the first expression. The subject statement is executed, and the index variable is incremented by 1 until the index variable is larger than the second expression. This second expression is called the limit expression. Complex limit and increment expressions are converted to floating-point type.

Warning

The data type of the index variable is determined by the type of the initial value expression. Keep this fact in mind to avoid the following:

FOR I = 0, 50000 DO ... ...

This loop does not produce the intended result. Converting the longword constant 50,000 to a short integer yields -15,536 because of truncation. The loop is not executed. The index variable's initial value is larger than the limit variable. The loop should be written as follows:

FOR I = 0L, 50000 DO ... ...

Note also that changing the data type of an index variable within a loop is not allowed, and will cause an error.

Warning

Also be aware of FOR loops that are entered but are not terminated after the expected number of iterations, because of the truncation effect. For example, if the index value exceeds the maximum value for the initial data type (and so is truncated) when it is expected instead to exceed the specified index limit, then the loop will continue beyond the expected number of iterations.

The following FOR statement continues infinitely:

FOR i = 0B, 240, 16 DO PRINT, i

The problem occurs because the variable i is initialized to a byte type with 0B. After the index reaches the limit value 240B, i is incremented by 16, causing the value to go to 256B, which is interpreted by IDL as 0B, because of the truncation effect. As a result, the FOR loop "wraps around" and the index can never be exceeded.

Examples

A simple FOR statement:

FOR I = 1, 4 DO PRINT, I, I^2

This statement produces the following output:

The index variable I is first set to an integer variable with a value of one. The call to the PRINT procedure is executed, then the index is incremented by one. This is repeated until the value of I is greater than four at which point execution continues at the statement following the FOR statement.

The next example displays the use of a block structure (instead of a single statement) as the subject of the FOR statement. The example is a common process used for computing a count-density histogram. (Note that a HISTOGRAM function is provided by IDL.)

FOR K = 0, N - 1 DO BEGIN 
    C = A[K] 
    HIST(C) = HIST(C)+1 
ENDFOR

The next example displays a FOR statement with floating-point index and limit expressions, where X is set to a floating-point variable and steps through the values (1.5, 2.5, ..., 10.5):

FOR X = 1.5, 10.5 DO S = S + SQRT(X)

The indexing variables and expressions can be integer, longword, floating-point, or double-precision. The type of the index variable is determined by the type of the first expression after the "=" character.

Warning

Due to the inexact nature of IEEE floating-point numbers, using floating-point indexing can cause "infinite loops" and other problems. This problem is also manifested in both the C and FORTRAN programming languages. For example, the numbers 0.1, 0.01, 1.6, and 1.7 do not have exact representations under the IEEE standard. To see this phenomenon, enter the following IDL command:

PRINT, 0.1, 0.01, 1.6, 1.7, FORMAT='(f20.10)'

IDL prints the following approximations to the numbers we requested:

0.1000000015
0.0099999998
1.6000000238
1.7000000477

See "Accuracy & Floating-Point Operations" in the Using IDL manual for more information about floating-point numbers.

FOR Statement with Variable Increment

The format of the second type of FOR statement is as follows:

FOR Variable = Expression₁, Expression₂, Increment DO Statement

This form is used when an increment other than 1 is desired.

The first two expressions describe the range of numbers for the index variable. The Increment specifies the increment of the index variable. A negative increment allows the index variable to step downward.

Examples

The following examples demonstrate the second type of FOR statement.

;Decrement, K has the values 100., 99., ..., 1. 
FOR K = 100.0, 1.0, -1  DO ... 
 
;Increment by 2., loop has the values 0., 2., 4., ..., 1022. 
FOR loop = 0, 1023, 2       DO ... 
 
;Divide range from bottom to top by 4. 
FOR mid = bottom, top, (top - bottom)/4.0 DO ...

Warning

If the value of the increment expression is zero, an infinite loop occurs. A common mistake resulting in an infinite loop is a statement similar to the following:

FOR X = 0, 1, .1 DO ....

The variable X is first defined as an integer variable because the initial value expression is an integer zero constant. Then the limit and increment expressions are converted to the type of X, integer, yielding an increment value of zero because .1 converted to integer type is 0. The correct form of the statement is:

FOR X = 0., 1, .1 DO ....

which defines X as a floating-point variable.

Sequence of the FOR Statement

The FOR statement performs the following steps:

The value of the first expression is evaluated and stored in the specified variable, which is called the index variable. The index variable is set to the type of this expression.

The value of the second expression is evaluated, converted to the type of the index variable, and saved in a temporary location. This value is called the limit value.

The value of the third expression, called the step value, is evaluated, type-converted if necessary, and stored. If omitted, a value of 1 is assumed.

If the index variable is greater than the limit value (in the case of a positive step value) the FOR statement is finished and control resumes at the next statement. Similarly, in the case of a negative step value, if the index variable is less than the limit value, control resumes after the FOR statement.

The statement or block following the DO is executed.

The step value is added to the index variable.

Steps 4, 5, and 6 are repeated until the test of Step 4 fails.

REPEAT...UNTIL

REPEAT...UNIL loops are used to repetitively execute a subject statement until a condition is true. The condition is checked after the subject statement is executed. Therefore, the subject statement is always executed at least once. (See Definition of True and False for details on how the "truth" of an expression is determined.)

The syntax of the REPEAT statement is as follows:

REPEAT statement UNTIL expression

REPEAT BEGIN 
   statements 
ENDREP UNTIL expression

Examples

The following example finds the smallest power of 2 that is greater than B:

A = 1 
B = 10 
REPEAT A = A * 2 UNTIL A GT B

The subject statement can also be in the form of a block:

A = 1 
B = 10 
REPEAT BEGIN 
   A = A * 2  
ENDREP UNTIL A GT B

The next example sorts the elements of ARR using the inefficient bubble sort method. (A more efficient way to sort elements is to use IDL's SORT function.)

;Sort array. 
REPEAT BEGIN 
   ;Set flag to true. 
   NOSWAP = 1 
   FOR I = 0, N - 2 DO IF arr[I] GT arr[I + 1]THEN BEGIN 
     ;Swapped elements, clear flag. 
     NOSWAP = 0 
     T = arr[I] & arr[I] = arr[I + 1] & arr[I + 1] = T 
     ENDIF 
;Keep going until nothing is moved. 
ENDREP UNTIL NOSWAP

WHILE...DO

WHILE...DO loops are used to execute a statement repeatedly while a condition remains true. The WHILE...DO statement is similar to the REPEAT...UNTIL statement except that the condition is checked prior to the execution of the statement. (See Definition of True and False for details on how the "truth" of an expression is determined.)

The syntax of the WHILE...DO statement is as follows:

WHILE expression DO statement

WHILE expression DO BEGIN 
   statements 
ENDWHILE

When the WHILE statement is executed, the conditional expression is tested, and if it is true, the statement following the DO is executed. Control then returns to the beginning of the WHILE statement, where the condition is again tested. This process is repeated until the condition is no longer true, at which point the control of the program resumes at the next statement.

In the WHILE statement, the subject is never executed if the condition is initially false.

Examples

The following example reads data until the end-of-file is encountered:

WHILE ~ EOF(1) DO READF, 1, A, B, C

The subject statement can also be in the form of a block:

WHILE ~ EOF(1) DO BEGIN 
   READF, 1, A, B, C 
ENDWHILE

The next example demonstrates one way to find the first element of an array greater than or equal to a specified value assuming the array is sorted into ascending order:

array = [2, 3, 5, 6, 10] 
i = 0 ;Initialize index 
n = N_ELEMENTS(array) 
 
;Increment i until a point larger than 5 is found or the end of the  
;array is reached: 
 
WHILE (array[i] LT 5) AND (i LT n) DO i = i + 1 
 
PRINT, 'The first element >= 5 is element ', i

IDL Prints:

The first element >= 5 is element    2 
 


Tip 

   Another way to accomplish the same thing is with the WHERE command, which is used to find the subscripts of the points where ARR[I] is greater than or equal to X.
   P = WHERE(arr GE X)
   ;Save first subscript:
   I = P(0)