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[Contents](../Contents.md) \| [Previous (1.1 Python)](01_Python.md) \| [Next (1.3 Numbers)](03_Numbers.md)
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# 1.2 A First Program
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This section discusses the creation of your first program, running the
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interpreter, and some basic debugging.
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### Running Python
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Python programs always run inside an interpreter.
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The interpreter is a "console-based" application that normally runs
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from a command shell.
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```bash
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python3
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Python 3.6.1 (v3.6.1:69c0db5050, Mar 21 2017, 01:21:04)
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[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
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Type "help", "copyright", "credits" or "license" for more information.
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>>>
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```
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Expert programmers usually have no problem using the interpreter in
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this way, but it's not so user-friendly for beginners. You may be using
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an environment that provides a different interface to Python. That's fine,
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but learning how to run Python terminal is still a useful skill to know.
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### Interactive Mode
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When you start Python, you get an *interactive* mode where you can experiment.
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If you start typing statements, they will run immediately. There is no
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edit/compile/run/debug cycle.
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```python
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>>> print('hello world')
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hello world
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>>> 37*42
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1554
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>>> for i in range(5):
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... print(i)
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...
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0
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1
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2
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3
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4
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>>>
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```
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This so-called *read-eval-print-loop* (or REPL) is very useful for debugging and exploration.
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**STOP**: If you can't figure out how to interact with Python, stop what you're doing
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and figure out how to do it. If you're using an IDE, it might be hidden behind a
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menu option or other window. Many parts of this course assume that you can
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interact with the interpreter.
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Let's take a closer look at the elements of the REPL:
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- `>>>` is the interpreter prompt for starting a new statement.
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- `...` is the interpreter prompt for continuing a statement. Enter a blank line to finish typing and run what you've entered.
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The `...` prompt may or may not be shown depending on your environment. For this course,
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it is shown as blanks to make it easier to cut/paste code samples.
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The underscore `_` holds the last result.
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```python
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>>> 37 * 42
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1554
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>>> _ * 2
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3108
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>>> _ + 50
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3158
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>>>
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```
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*This is only true in the interactive mode.* You never use `_` in a program.
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### Creating programs
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Programs are put in `.py` files.
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```python
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# hello.py
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print('hello world')
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```
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You can create these files with your favorite text editor.
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### Running Programs
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To execute a program, run it in the terminal with the `python` command.
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For example, in command-line Unix:
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```bash
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bash % python hello.py
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hello world
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bash %
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```
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Or from the Windows shell:
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```
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C:\SomeFolder>hello.py
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hello world
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C:\SomeFolder>c:\python36\python hello.py
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hello world
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```
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Note: On Windows, you may need to specify a full path to the Python interpreter such as `c:\python36\python`.
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However, if Python is installed in its usual way, you might be able to just type the name of the program
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such as `hello.py`.
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### A Sample Program
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Let's solve the following problem:
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> One morning, you go out and place a dollar bill on the sidewalk by the Sears tower in Chicago.
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> Each day thereafter, you go out double the number of bills.
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> How long does it take for the stack of bills to exceed the height of the tower?
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Here's a solution:
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```python
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# sears.py
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bill_thickness = 0.11 * 0.001 # Meters (0.11 mm)
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sears_height = 442 # Height (meters)
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num_bills = 1
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day = 1
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while num_bills * bill_thickness < sears_height:
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print(day, num_bills, num_bills * bill_thickness)
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day = day + 1
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num_bills = num_bills * 2
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print('Number of days', day)
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print('Number of bills', num_bills)
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print('Final height', num_bills * bill_thickness)
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```
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When you run it, you get the following output:
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```bash
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bash % python3 sears.py
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1 1 0.00011
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2 2 0.00022
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3 4 0.00044
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4 8 0.00088
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5 16 0.00176
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6 32 0.00352
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...
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21 1048576 115.34336
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22 2097152 230.68672
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Number of days 23
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Number of bills 4194304
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Final height 461.37344
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```
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Using this program as a guide, you can learn a number of important core concepts about Python.
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### Statements
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A python program is a sequence of statements:
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```python
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a = 3 + 4
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b = a * 2
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print(b)
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```
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Each statement is terminated by a newline. Statements are executed one after the other until control reaches the end of the file.
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### Comments
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Comments are text that will not be executed.
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```python
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a = 3 + 4
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# This is a comment
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b = a * 2
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print(b)
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```
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Comments are denoted by `#` and extend to the end of the line.
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### Variables
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A variable is a name for a value. You can use letters (lower and
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upper-case) from a to z. As well as the character underscore `_`.
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Numbers can also be part of the name of a variable, except as the
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first character.
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```python
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height = 442 # valid
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_height = 442 # valid
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height2 = 442 # valid
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2height = 442 # invalid
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```
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### Types
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Variables do not need to be declared with the type of the value. The type
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is associated with the value on the right hand side, not name of the variable.
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```python
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height = 442 # An integer
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height = 442.0 # Floating point
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height = 'Really tall' # A string
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```
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Python is dynamically typed. The perceived "type" of a variable might change
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as a program executes depending on the current value assigned to it.
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### Case Sensitivity
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Python is case sensitive. Upper and lower-case letters are considered different letters.
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These are all different variables:
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```python
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name = 'Jake'
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Name = 'Elwood'
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NAME = 'Guido'
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```
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Language statements are always lower-case.
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```python
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while x < 0: # OK
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WHILE x < 0: # ERROR
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```
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### Looping
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The `while` statement executes a loop.
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```python
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while num_bills * bill_thickness < sears_height:
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print(day, num_bills, num_bills * bill_thickness)
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day = day + 1
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num_bills = num_bills * 2
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print('Number of days', day)
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```
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The statements indented below the `while` will execute as long as the expression after the `while` is `true`.
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### Indentation
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Indentation is used to denote groups of statements that go together.
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Consider the previous example:
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```python
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while num_bills * bill_thickness < sears_height:
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print(day, num_bills, num_bills * bill_thickness)
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day = day + 1
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num_bills = num_bills * 2
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print('Number of days', day)
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```
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Indentation groups the following statements together as the operations that repeat:
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```python
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print(day, num_bills, num_bills * bill_thickness)
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day = day + 1
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num_bills = num_bills * 2
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```
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Because the `print()` statement at the end is not indented, it
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does not belong to the loop. The empty line is just for
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readability. It does not affect the execution.
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### Indentation best practices
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* Use spaces instead of tabs.
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* Use 4 spaces per level.
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* Use a Python-aware editor.
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Python's only requirement is that indentation within the same block
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be consistent. For example, this is an error:
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```python
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while num_bills * bill_thickness < sears_height:
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print(day, num_bills, num_bills * bill_thickness)
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day = day + 1 # ERROR
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num_bills = num_bills * 2
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```
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### Conditionals
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The `if` statement is used to execute a conditional:
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```python
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if a > b:
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print('Computer says no')
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else:
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print('Computer says yes')
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```
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You can check for multiple conditions by adding extra checks using `elif`.
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```python
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if a > b:
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print('Computer says no')
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elif a == b:
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print('Computer says yes')
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else:
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print('Computer says maybe')
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```
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### Printing
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The `print` function produces a single line of text with the values passed.
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```python
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print('Hello world!') # Prints the text 'Hello world!'
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```
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You can use variables. The text printed will be the value of the variable, not the name.
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```python
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x = 100
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print(x) # Prints the text '100'
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```
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If you pass more than one value to `print` they are separated by spaces.
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```python
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name = 'Jake'
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print('My name is', name) # Print the text 'My name is Jake'
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```
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`print()` always puts a newline at the end.
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```python
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print('Hello')
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print('My name is', 'Jake')
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```
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This prints:
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```code
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Hello
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My name is Jake
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```
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The extra newline can be suppressed:
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```python
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print('Hello', end=' ')
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print('My name is', 'Jake')
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```
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This code will now print:
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```code
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Hello My name is Jake
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```
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### User input
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To read a line of typed user input, use the `input()` function:
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```python
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name = input('Enter your name:')
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print('Your name is', name)
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```
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`input` prints a prompt to the user and returns their response.
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This is useful for small programs, learning exercises or simple debugging.
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It is not widely used for real programs.
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### pass statement
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Sometimes you need to specify an empty code block. The keyword `pass` is used for it.
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```python
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if a > b:
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pass
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else:
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print('Computer says false')
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```
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This is also called a "no-op" statement. It does nothing. It serves as a placeholder for statements, possibly to be added later.
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## Exercises
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This is the first set of exercises where you need to create Python
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files and run them. From this point forward, it is assumed that you
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are editing files in the `practical-python/Work/` directory. To help
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you locate the proper place, a number of empty starter files have
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been created with the appropriate filenames. Look for the file
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`Work/bounce.py` that's used in the first exercise.
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### Exercise 1.5: The Bouncing Ball
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A rubber ball is dropped from a height of 100 meters and each time it
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hits the ground, it bounces back up to 3/5 the height it fell. Write
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a program `bounce.py` that prints a table showing the height of the
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first 10 bounces.
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Your program should make a table that looks something like this:
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```code
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1 60.0
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2 36.0
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3 21.599999999999998
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4 12.959999999999999
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5 7.775999999999999
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6 4.6655999999999995
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7 2.7993599999999996
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8 1.6796159999999998
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9 1.0077695999999998
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10 0.6046617599999998
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```
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*Note: You can clean up the output a bit if you use the round() function. Try using it to round the output to 4 digits.*
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```code
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1 60.0
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2 36.0
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3 21.6
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4 12.96
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5 7.776
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6 4.6656
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7 2.7994
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8 1.6796
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9 1.0078
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10 0.6047
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```
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### Exercise 1.6: Debugging
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The following code fragment contains code from the Sears tower problem. It also has a bug in it.
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```python
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# sears.py
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bill_thickness = 0.11 * 0.001 # Meters (0.11 mm)
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sears_height = 442 # Height (meters)
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num_bills = 1
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day = 1
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while num_bills * bill_thickness < sears_height:
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print(day, num_bills, num_bills * bill_thickness)
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day = days + 1
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num_bills = num_bills * 2
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print('Number of days', day)
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print('Number of bills', num_bills)
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print('Final height', num_bills * bill_thickness)
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```
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Copy and paste the code that appears above in a new program called `sears.py`.
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When you run the code you will get an error message that causes the
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program to crash like this:
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```code
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Traceback (most recent call last):
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File "sears.py", line 10, in <module>
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day = days + 1
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NameError: name 'days' is not defined
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```
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Reading error messages is an important part of Python code. If your program
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crashes, the very last line of the traceback message is the actual reason why the
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the program crashed. Above that, you should see a fragment of source code and then
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an identifying filename and line number.
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* Which line is the error?
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* What is the error?
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* Fix the error
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* Run the program successfully
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[Contents](../Contents.md) \| [Previous (1.1 Python)](01_Python.md) \| [Next (1.3 Numbers)](03_Numbers.md)
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Reference in new issue
Block a user