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Exercise #32: Convert Strings To IntegersconvertStrToInt('42') → 42
To complement Exercise #31, “Convert Integers to Strings”, in this exercise we’ll convert strings of numeric digits into their integer equivalents. The most common use case for this is taking the string returned from, say, the input() function or a text file’s read() method and converting it to an integer to perform mathematical operations on it. You can use Python’s int() function to do this conversion, but in this exercise, you’ll recreate this function yourself.
Exercise Description
Write a convertStrToInt() function with a stringNum parameter. This function returns an integer form of the parameter just like the int() function. For example, convertStrToInt('42') should return the integer 42. The function doesn’t have to work for floating-point numbers with a decimal point, but it should work for negative number values.
Avoid using int()in your code, as that would do the conversion for you and defeat the purpose of this exercise. However, we do use int() with assert statements to check that your convertStrToInt() function works the same as int() for all integers from -10000 to 9999:
for i in range(-10000, 10000):
assert convertStrToInt(str(i)) == i
Try to write a solution based on the information in this description. If you still have trouble solving this exercise, read the Solution Design and Special Cases and Gotchas sections for additional hints.
Solution Design
The solution for this exercise is quite different than the int-to-string algorithm. Still, they are both similar in that they convert one digit and use a dictionary to map between string digits and integer digits:
DIGITS_STR_TO_INT = {'0': 0, '1': 1, '2': 2, '3': 3, '4': 4, '5': 5, '6': 6, '7': 7, '8': 8, '9': 9}
The function creates an integerNum variable to hold the integer form of stringNum as we build it. This variable starts with the value 0. Your code must also note if there is a minus sign at the start of the string, in which case
Our algorithm loops over the individual digits in the stringNum parameter, starting on the left and moving right. The code multiplies current integer in integerNum by 10 to “move” all of these digits to the left by one place, then adds the current digit.
For example, if we needed to convert the string '41096' to an integer, the code needs to carry out the following operations:
· integerNum = 0
· integerNum = (0 * 10) + 4 = 4
· integerNum = (4 * 10) + 1 = 41
· integerNum = (41 * 10) + 0 = 410
· integerNum = (410 * 10) + 9 = 4109
· integerNum = (4109 * 10) + 6 = 41096
Before returning, we convert this integer to a negative number if the original string began with a minus sign.
Special Cases and Gotchas
The convertStrToInt() function must be able to handle strings representing negative integers. To do this, check if stringNum[0] (the first character in the string) is the '-' dash character. If so, we can mark an isNegative variable to True (and False otherwise). Then we can remove this dash character by setting stringNum = stringNum[1:], replacing the string in stringNum with a string of all the characters in stringNum after the first.
At the end of the function, the function can return -integerNum if isNegative was set to True.
Now try to write a solution based on the information in the previous sections. If you still have trouble solving this exercise, read the Solution Template section for additional hints.
Solution Template
Try to first write a solution from scratch. But if you have difficulty, you can use the following partial program as a starting place. Copy the following code from https://invpy.com/convertstrtoint-template.py and paste it into your code editor. Replace the underscores with code to make a working program:
def convertStrToInt(stringNum):
DIGITS_STR_TO_INT = {'0': 0, '1': 1, '2': 2, '3': 3, '4': 4,
'5': 5, '6': 6, '7': 7, '8': 8, '9': 9}
if stringNum[0] == '-':
isNegative = True
stringNum = stringNum[1:]
else:
isNegative = False
integerNum = 0
for i in range(len(stringNum)):
digit = DIGITS_STR_TO_INT[stringNum[i]]
integerNum = (integerNum * 10) + digit
if isNegative:
return -integerNum
else:
return integerNum
The complete solution for this exercise is given in Appendix A and https://invpy.com/convertstrtoint.py. You can view each step of this program as it runs under a debugger at https://invpy.com/convertstrtoint-debug/.
Prev - #31 Convert Integers To Strings | Table of Contents | Next - #33 Comma-Formatted Numbers
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