High Performance Scientific Computing   AMath 483/583 Class Notes   Spring Quarter, 2013

Python functions

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Numerics in Python

# Python strings¶

## String formatting¶

Often you want to construct a string that incorporates the values of some variables. This can be done using the form format % values where format is a string that describes the desired format and values is a single value or tuple of values that go into various slots in the format.

This is best learned from some examples:

>>> x = 45.6
>>> s = "The value of x is %s"  % x
>>> s
'The value of x is 45.6'


The %s in the format string means to convert x to a string and insert into the format. It will use as few spaces as possible.

>>> s = "The value of x is %21.14e"  % x
>>> s
'The value of x is  4.56000000000000e+01'


In the case above, exponential notation is used with 14 digits to the right of the decimal point, put into a field of 21 digits total. (You need at least 7 extra characters to leave room for a possible minus sign as well as the first digit, the decimal point, and the exponent such as e+01.

>>> y = -0.324876
>>> s = "Now x is %8.3f and y is %8.3f" % (x,y)
>>> s
'Now x is   45.600 and y is   -0.325'


In this example, fixed notation is used instead of scientific notation, with 3 digits to the right of the decimal point, in a field 8 characters wide. Note that y has been rounded.

In the last example, two variables are inserted into the format string.