Jan 30, 2019 -- Makeup Day Notes

IT 211 -- Jan 30, 2019 -- Makeup Lecture Notes

Review Exercises

Write a statement that opens the file input.txt for reading; name the file object fin. Ans:
```
fin = open("input.txt", "r")
```
Write a statement that opens the file out.txt for writing; name the file object fout. Ans:
```
fin = open("out.txt", "w")
```
Write a statement that reads line from the input file accessed by the file object fin. Ans:
```
line = fin.readline( )
```
Write a statement that writes a string s to the output file accessed by the file object fout. Ans:
```
fout.write(s)
```
Write a statement that closes the file object fout. Ans:
```
fout.close( )
```

Examples Zipfile

Here is a zipfile of all the examples for these notes.

Here are also direct links to the examples files:

Example Group	Input File	Example Scripts
Grades	grades.txt	grades1.py grades2.py grades3.py
Kids	kids.txt	kids1.py kids2.py kids3.py
Employees	employees.txt	employees1.py employees2.py employees3.py

Sequential Processing

Sequential processing means to process the input values one at a time in the order of the inputs. The input values are not saved.
In many cases, sequential processing is used to obtain summary values of the dataset, such as a count, sum, product, average, minimum, or maximum.
Before beginning to process the inputs, the summary value must be set to an appropriate initial value. Here are some common initial values for summary values:

Summary Value Initial Value

count 0

sum 0.0

product 1.0

minimum First input value

maximum First input value
Note: a Python script computes the sum of input values by keeping track of the partial total. For example, to add the list 2, 4, 7, 5, 3, the script would add like this:
Therefore, the sum is 21.

Summary Value	Initial Value
count	0
sum	0.0
product	1.0
minimum	First input value
maximum	First input value

The general form of a sequential processing loop:

summary_value = initial_value
get input line
while input_value != ""
    extract fields from input line
    update summary_value(s) using input fields
    get input line  
end
output summary_value

Look at these Grades sequential processing examples:
Input file: grades.txt
1. Compute the sum of all grades. Ans: grades1.txt
2. Compute the average of all grades. Ans: grades2.txt
3. Compute the maximum of all grades. Ans: grades3.txt
Zipfile of all examples for these notes: sequential-processing.zip

Extracting Fields from a Line

The kids.txt file contains comma separated lines. Here is the first line of this file:
```
Samantha,F,5
```
To extract the fields (name, gender, age) from this line, we use the split method, which is called from an str object:
```
line = "Samantha,F,5"
fields = line.split( )
print(fields)

# Output:
["Samantha", "F", "5"]
```
line.split( ) returns a Python list, which is shown in the output. Here is a table showing the items and corresponding indices in the list:

Items: "Samantha" "F" "5"

Indices: 0 1 2
Items in the list are extracted by index. Notice that the indices are zero-based; they start at 0, not 1. (I have previously mentioned that computer scientists start counting with zero.)
```
fields = ["Samantha", "F", "5"]
name = fields[0]
gender = fields[1]
age = int(fields[2])
print(name, gender, age)

# Output:
Samantha F 5
```
A safer way to extract the fields from a line is to use the the str strip method, which removes leading and training white space from the field. In particular, the end of a line read from a file is the \n character:
```
line = "Samantha , F , 5\n"
fields = line.strip( )
name = fields[0].strip( )
gender = fields[1].strip( )
age = int(fields[2].strip( ))
print(name, gender, age)

# Output: 
Samantha F 5
```
Here are the Kids Examples:
Input file: kids.txt
1. What is the average age of all kids? Ans: kids1.txt
2. What is the average age of the girls? Ans: kids2.txt
3. What is the maximum age of the boys? Ans: kids3.txt
Zipfile of all examples for these notes: sequential-processing.zip

Employee Examples

Write Python scripts that print the requested information about the employees in the employees.txt Input file: employees.txt
1. What is the average of all the salaries? Ans: employees1.py
2. What is the average of all the Chicago employees? Ans: employees2.py
3. What is the maximum of the women's employees? Ans: employees3.py
Zipfile of all examples for these notes: sequential-processing.zip

Items:	"Samantha"	"F"	"5"
Indices:	0	1	2