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### Variables and Arithmetic Expressions

The program in Listing 1.1 shows the use of variables and expressions by performing a simple compound-interest calculation.

##### Listing 1.1. Simple Compound-Interest Calculation
 ```principal = 1000 # Initial amount rate = 0.05 # Interest rate numyears = 5 # Number of years year = 1 while year <= numyears: principal = principal*(1+rate) print year, principal year += 1```

The output of this program is the following table:

```1 1050.0
2 1102.5
3 1157.625
4 1215.50625
5 1276.2815625```

Python is a dynamically typed language in which names can represent values of different types during the execution of a program. In fact, the names used in a program are really just labels for various quantities and objects. The assignment operator ...

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