| Larch: Data Analysis Tools for X-ray Spectroscopy

# 5.4. Tutorial: Conditional Execution and Flow Control

Two important needs for a full-featured language are the ability to run different statements under different conditions, and to repeat certain calculations. These are generally called ‘flow control’, as these statements control how the program will flow through the text of the script. In the discussion here, we will also introduce a few new concepts and Larch statements.

So far in this tutorial, all the text written to the Larch command line has been a single line of text that is immediately run, either printing output to the terminal or assigning a value to a variable. These are both examples of statements, which are the basic pieces pf text you send to the program. So far we’ve seen three types of statements:

> 1. simple statements or expressions, such as:

>        larch> 1+sqrt(3)

>    or:

>        larch> atomic_weight.keys()

>    where we have an expression evaluated. At the command line, these values are printed – in a script they would not be printed.

> 2. print statements, such as:

>        larch> print sqrt(3)

>    where we explicitly command larch to print the evaluated expression – this would print if run from a script.

> 3. assignment statements, such as:

>        larch> x = sqrt(3)

>    where we assign the name ‘x’ to hold the evaluated expression.

In fact, though these are the most common types of statements, there are many more types of statements. We will introduce a few more statement types here, including compound statements that take up more than one line of test.

## 5.4.1. Conditional Evaluation with if and else

A fundamental need is to be able to execute some statement(s) when some condition is met. This is done with the if statement, an example of which is:

    larch> if x == 0: print 'x is 0!'

Which will print ‘x is 0!’ if the value of x is equal to 0. The x == 0 in this if statement is called the test. A test is a Boolean mathematical expression. While most usual mathematical expressions use operators such as ‘+’, ‘-‘, a Boolean expression uses the operators listed in Table of Boolean Operators to evaluate to a value of `True` or `False`.

A single-line if statement as above looks like this::
  if <test>: statement

The ‘if’ and ‘:’ are important, while ‘<test>’ can be any Boolean expression. If the test evaluates to `True`, the statement is executed.

If statements can execute multiple statements by putting the statements into a “block of code”:

    if x == 0:
        print 'x is equal to 0!'
        x = 1
    endif

Which is to say that the multiple-line form of the if statement looks like this:

    if <test>:
      <statements>
    endif

where ‘<statements>’ here means a list of statements, and the ‘endif’ is required (see 12.1.1). For the above, two statements will be run if x is equal to 0 – there is no restriction on how many statements can be run.

An ‘else’ statement can be added to execute code if the test is False:

    if x == 0:
        print 'x is equal to 0!'
        x = 1
    else:
        print 'x is not 0'
    endif

Multiple tests can be chained together with the ‘elif’ (a contraction of ‘else if’):

    if x == 0:
        print 'x is equal to 0!'
    elif x > 0:
        print 'x is positive'
    elif x > -10:
        print 'x is a small negative number'
    else:
        print 'x is negative'
    endif

Here the ‘x > 0’ test will be executed if the ‘x == 0’ test fails, and the ‘x > -10’ test will be tried if that fails.

Table of Boolean Operators The operators here all take the form right OP left where OP is one of the operators below. Note the distinction between ‘==’ and ‘is’. The former compares values while the latter compares the identity of two objects.

> boolean operator | meaning
> --- | ---
> == | has equal value
> != | has unequal value
> > | has greater value
> >= | has greater or equal value
> < | has smaller value
> <= | has smaller or equal value
> is | is identical to
> not | is not `True`
> and | both operands are `True`
> or | either operand is `True`

Note that in Larch, as in Python, any value can be used as a test, not just values that are `True` or `False`. As you might expect, for example, the value 0 is treated as `False`. An empty string is also treated as `False`, as is an empty list or dictionary. Most other values are interpreted as `True`.

## 5.4.2. For loops

It is often necessary to repeat a calculation multiple times. A common method of doing this is to use a loop, including using a loop counter to iterate over some set of values. In Larch, this is done with a for loop. For those familiar with other languages, a Larch for loop is a bit different from a C for loop or Fortran do loop. A for loop in Larch iterates over an ordered set of values as from a list, tuple, or array, or over the keys from a dictionary. Thus a loop like this:

    for x in ('a', 'b', 'c'):
        print x
    endfor

will go through values ‘a’, ‘b’, and ‘c’, assigning each value to x, then printing the value of x, which will result in printing out:

    a
    b
    c

Similar to the if statement above, the for loop has the form:

    for <varlist> in <sequence>:
        <statements>
    endfor

Compared to a C for loop or Fortran do loop, the Larch for loop is much more like a foreach loop. The common C / Fortran use case of interating over a set of integers can be emulated using the builtin `range()` function which generates a sequence of integers. Thus:

    for i in range(5):
       print i, i/2.0
    endfor

will result in:

    0, 0.0
    1, 0.5
    2, 1.0
    3, 1.5
    4, 2.0

Note that the builtin `range()` function generates a sequence of integers, and can take more than 1 argument to indicate a starting value and step. It is important to note that the sequence that is iterated order does not be generated from the `range()` function, but can be any list, array, or Python sequence. Importantly, this includes strings(!) so that:

    for char in 'hello':  print char

will print:

    h
    e
    l
    l
    o

This can cause a common sort of error, in that you might expect some variabe to hold a list of string values, but it actually holds a single string. Notice that:

    filelist = ('file1', 'file2')
    for fname in filelist:
        fh = open(fname)
        process_file(fh)
        fh.close()
    endfor

would act very differently if filelist was changed to ‘file1’!

Multiple values can be assigned in each iteration of the for loop. Thus, iterating over a sequence of equal-length tuples, as in:

    for a, b in (('a', 1), ('b', 2), ('c', 3)):
        print a, b
    endfor

will print:

    a 1
    b 2
    c 3

This may seem to be mostly of curious interest, but can be extremely useful especially when dealing with dictionaries or with arrays or lists of equal length. For a dictionary d, d.items() will return a list of two-element tuples as above of key, value. Thus:

    mydict = {'a':1, 'b':2, 'c':3, 'd':4}
    for key, val in mydict.items():
        print key, val
    endfor

will print (note that dictionaries do no preserve order, but the (key, val) pairs match):

    a 1
    c 3
    b 2
    d 4

The builtin `zip()` function is similarly useful, turning a sequence of lists or arrays into a sequence of tuples of the corresponding elements of the lists or arrays. Thus:

    larch> a = range(10)
    larch> b = sin(a)
    larch> c = cos(a)
    larch> print zip(a, b, c)
    [(0, 0.0, 1.0), (1, 0.8414709848078965, 0.54030230586813977),
     (2, 0.90929742682568171, -0.41614683654714241), ....]

(Note that for arrays or lists of unequal length, `zip()` will return tuples until any of its arguments runs out of elements). Thus a for loop can make use of the `zip()` function to iterate over multiple arrays:

    larch> a = arange(101)/10.0
    larch> print 'X   SIN(X)  SIN(Y)\n================\n'
    larch> for a, sval, cval in zip(a, sin(a), cos(a)):
    .....>     print '%.3f, %.5f, %.5f' % (a, sval, cval)
    .....> endfor

will print a table of sine and cosine values.

A final utility of note for loops is `enumerate()` which will return a tuple of (index, value) for a sequence. That is:

    larch> for i, a in enumerate('a', b', 'c'):
    .....>     print i, a
    .....> endfor

will print:

    0 a
    1 b
    2 c

It is sometimes useful to jump out of a for loop, or go onto the next value in the sequence. The break statement will exit a for loop immediately:

    for fname in filelist:
        status = get_status(fname)
        if status < 0:
           break
        endif
        more_processing(fname)
    endfor
    print 'processed up to i = ', i

may jump out of the loop before the sequence generated by ‘range(10)’ is complete. The variable ‘i’ will have the final used value.

To skipover an iteration of a loop but continue on, use the continue statement:

    for fname in filelist:
        status = get_status(fname)
        if status < 0:
           continue
        endif
        more_processing(fname)
    endfor

## 5.4.3. While loops

While a for loop generally walks through a pre-defined set of values, a while loop executes as long as some test is `True`. The basic form is:

    while <test>:
       <statements>
    endwhile

Here, the test works as for if – it is a Boolean expression, evaluated at each iteration of the loop. Generally, the expression will test something that has been changed inside the loop (even if implicitly). The classic while loop increases a counter at each iteration:

    counter = 0
    while counter < 10:
       do_something(counter)
       counter = counter + 1
    endwhile

A while loop is easily turned into an infinite loop, simply by not incrementing the counter. Then again, the above loop would easily be converted into a for loop, as the counter is incremented by a fixed amout at each iteration. A more realistic use would be:

    n = 1
    while n < 100:
       n = (n + 0.1) * n
       print n
    endwhile

An additional use for a while loop is to use an implicit or external condition, such as time:

    now = time.time() # return the time in seconds since Unix epoch
    while time.time() - now < 15:   # That is 'for 15 seconds'
       do_someting()
    endwhile

The break and continue statements also work for while loops, just as they do with for loops. These can be used as ways to exit an other-wise infinite while loop:

    while True:  # will never exit without break!
       answer = raw_input('guess my favorite color>')
       if answer == 'lime':
           break
       else:
           print 'Nope, try again'
       endif
    endwhile