When you need to extract data from a string in Python, you can use the built-in String.find() method in Python and the String[n:m] notation to extract the sub-string.
string.find(str) will return the starting position of the first instance of str in string. For example. 'abcdecd'.find('cd') will return 2, because the first 'cd' instance starts at position 2 (not 3, since position starts with 0). If no str can be found in the string, the method will return -1.
str[n:m] notation will extract the sub-string starting at position n and ending at position m-1, not including the m-th character. For example, if the content of my_str is 'I don't know', my_str[2:] will return 'don't know'.
By using string.find() and [n:m] notation, we can write a script to automatically extract the data we need from a string.
For example, we have a file test_score.txt that records the scores of all the students enrolled in a class. We know that each line records the student's first name and his or her score of the final as follow.
Peter 100
Anna 99
Henry 98
Jerry 97
…
If we want to calculate the average of the final, we can use find() and [n:m] notation to achieve this.
string.find(str) will return the starting position of the first instance of str in string. For example. 'abcdecd'.find('cd') will return 2, because the first 'cd' instance starts at position 2 (not 3, since position starts with 0). If no str can be found in the string, the method will return -1.
str[n:m] notation will extract the sub-string starting at position n and ending at position m-1, not including the m-th character. For example, if the content of my_str is 'I don't know', my_str[2:] will return 'don't know'.
By using string.find() and [n:m] notation, we can write a script to automatically extract the data we need from a string.
For example, we have a file test_score.txt that records the scores of all the students enrolled in a class. We know that each line records the student's first name and his or her score of the final as follow.
Peter 100
Anna 99
Henry 98
Jerry 97
…
If we want to calculate the average of the final, we can use find() and [n:m] notation to achieve this.
#average_sample.py
# used to accumulate the scores
total_score = 0.0
# used to count how many scores (students) we have
number_of_score = 0
# open the file for 'read'
file_h = open('test_score.txt', 'r')
# read the file line by line
for line in file_h:
# find the marker
white_space_position = line.find(' ')
# calculate the position of the score
# in relation to position of the marker
score_position = white_space_position + 1
# extract the substring and
# convert the substring into a floating point number
score = float(line[score_position:])
# print the score
print 'Score: {}'.format(score)
# accumulate the scores
total_score = total_score + score
# accumulate how many scores we got
number_of_score = number_of_score + 1
# calculate and display the average
print 'Average: {}'.format(total_score/number_of_score)
Score: 100.0
Score: 99.0
Score: 98.0
Score: 97.0
Average: 98.5
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