Controlling the format of printed variables

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This was first worked out in this original Matlab post.

Often you will want to control the way a variable is printed. You may want to only show a few decimal places, or print in scientific notation, or embed the result in a string. Here are some examples of printing with no control over the format.

a = 2./3
print a
print 1/3
print 1./3.
print 10.1
print "Avogadro's number is ", 6.022e23,'.'
0.666666666667
0
0.333333333333
10.1
Avogadro's number is  6.022e+23 .

There is no control over the number of decimals, or spaces around a printed number.

In python, we use the format function to control how variables are printed. With the format function you use codes like {n:format specifier} to indicate that a formatted string should be used. n is the n^{th} argument passed to format, and there are a variety of format specifiers. Here we examine how to format float numbers. The specifier has the general form “w.df” where w is the width of the field, and d is the number of decimals, and f indicates a float number. “1.3f” means to print a float number with 3 decimal places. Here is an example.

print 'The value of 1/3 to 3 decimal places is {0:1.3f}'.format(1./3.)
The value of 1/3 to 3 decimal places is 0.333

In that example, the 0 in {0:1.3f} refers to the first (and only) argument to the format function. If there is more than one argument, we can refer to them like this:

print 'Value 0 = {0:1.3f}, value 1 = {1:1.3f}, value 0 = {0:1.3f}'.format(1./3., 1./6.)
Value 0 = 0.333, value 1 = 0.167, value 0 = 0.333

Note you can refer to the same argument more than once, and in arbitrary order within the string.

Suppose you have a list of numbers you want to print out, like this:

for x in [1./3., 1./6., 1./9.]:
    print 'The answer is {0:1.2f}'.format(x)
The answer is 0.33
The answer is 0.17
The answer is 0.11

The “g” format specifier is a general format that can be used to indicate a precision, or to indicate significant digits. To print a number with a specific number of significant digits we do this:

print '{0:1.3g}'.format(1./3.)
print '{0:1.3g}'.format(4./3.)
0.333
1.33

We can also specify plus or minus signs. Compare the next two outputs.

for x in [-1., 1.]: 
    print '{0:1.2f}'.format(x)
-1.00
1.00

You can see the decimals do not align. That is because there is a minus sign in front of one number. We can specify to show the sign for positive and negative numbers, or to pad positive numbers to leave space for positive numbers.

for x in [-1., 1.]: 
    print '{0:+1.2f}'.format(x) # explicit sign

for x in [-1., 1.]: 
    print '{0: 1.2f}'.format(x) # pad positive numbers
-1.00
+1.00
-1.00
 1.00

We use the “e” or “E” format modifier to specify scientific notation.

import numpy as np
eps = np.finfo(np.double).eps
print eps
print '{0}'.format(eps)
print '{0:1.2f}'.format(eps)
print '{0:1.2e}'.format(eps)  #exponential notation
print '{0:1.2E}'.format(eps)  #exponential notation with capital E
2.22044604925e-16
2.22044604925e-16
0.00
2.22e-16
2.2E-16

As a float with 2 decimal places, that very small number is practically equal to 0.

We can even format percentages. Note you do not need to put the % in your string.

print 'the fraction {0} corresponds to {0:1.0%}'.format(0.78)
the fraction 0.78 corresponds to 78%

There are many other options for formatting strings. See http://docs.python.org/2/library/string.html#formatstrings for a full specification of the options.

Copyright (C) 2013 by John Kitchin. See the License for information about copying.

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