4
1.3 Mathematical Computation Using Operators
Python provides a variety of common mathematical operators, typical of most programming languages,
and many more through modules you import – we’ll look at these later. See one of the books
recommended above, or the help system (under “numeric types”), to learn more about these. Common
operators are +, -, *, /, ** (raise to a power), % (used for modulo, remainder from division).
Integer vs. Floating Point Results. One rule to remember is that if all of the inputs are integer, the
result will be integer – even for division. Any input that is floating point makes the output floating
point.
Try each of the following expressions on the interactive window to see how this works. Enter
expressions that Python will display the value for, and fill in the result here.
expression to enter result Notes
2 + 3
integer result
2. + 3
2. is floating point, so gives floating point result
2 – 3
2 * 3
integer result
2. * 3
floating point
5 / 2
Integer
5. / 2
5 % 2
not percent, but modulo – remainder from division
az = 270
newaz = az+180
print newaz % 360
one use for the modulo -- think azimuth
(add 180 to azimuth reverses direction)
5 ** 2
25 ** 0.5
there is no sqrt( ) function, unless you add the math module
See the other sources for more on these.
1.4 String manipulation
A string is a sequential set of characters of text, like ‘San Francisco’ or ‘d:/prog/hmbarea/landuse.shp’
or ‘94132’. It helps to think of them as a sequence of characters, a string of characters, with each
character having a position starting with zero. There are many ways to work with text strings in
Python, and many of these come in handy in GIS work, especially when working with datasets, fields,
and text field values. Learn more about strings in a Python manual, or the help system.
String methods can be used with string objects. A string
object might be a string literal (like ‘a string’ or
‘d:/prog/hmbarea/landuse.shp’) or a variable that has
been assigned a string. Methods are applied by writing
them in after a dot following the string object. Methods
always end with parentheses, even if no parameters are
needed.
The best way to understand strings is to try them in Python, which we’ll do next.
Using Help for String Methods in Python
There are more than 30 built-in methods that
can be used to process strings. They are each
described in the help system. To find them
from PythonWin, use the Help menu to go
Python Manuals, and in the Library Reference
find the section 2.1.5.1 String Methods, under
2.1.5 Se
uence T
es.
