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convertIntToStr(42) → '42'
In Python, the values 42 and '42' are two different values: you can perform mathematics
on the integer 42, but the string '42' is the same as any other two-character text string. You
can perform mathematical addition on integer values, but not on strings. And
you can concatenate or replicate string values, but not integers. A common
programming task is to obtain the string equivalent of a number. You can use
the str() function to do this conversion but in this
exercise you’ll recreate this function yourself.
Exercise Description
Write a convertIntToStr() function
with an integerNum parameter. This function operates
similarly to the str() function in that it returns a
string form of the parameter. For example, convertIntToStr(42)
should return the string '42'. The function doesn’t
have to work for floating-point numbers with a decimal point, but it should
work for negative integer values.
Avoid using Python’s str() function in
your code, as that would do the conversion for you and defeat the purpose of
this exercise. However, we use str() with assert statements to check that your convertIntToStr()
function works the same as str() for all integers
from -10000 to 9999:
for i in range(-10000, 10000):
assert convertIntToStr(i) == str(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.
Prerequisite concepts: dictionaries, while
loops, string concatenation, integer division
Solution Design
To create the string equivalent of the integer value, we’ll convert
the integers to strings one digit at a time. Let’s create a variable named stringNum to hold the converted string. In a loop, the
expression integerNum % 10 evaluates to the digit in
the one’s place of integerNum. Store this result in
a variable named onesPlaceDigit.
Then we add the string equivalent of onesPlaceDigit
to stringNum with a series of if-elif statements such as the following:
if onesPlaceDigit == 0:
stringNum = '0' + stringNum
elif onesPlaceDigit == 1:
stringNum = '1' + stringNum
elif onesPlaceDigit == 2:
stringNum = '2' + stringNum
elif onesPlaceDigit == 3:
stringNum = '3' + stringNum
elif onesPlaceDigit == 4:
stringNum = '4' + stringNum
elif onesPlaceDigit == 5:
stringNum = '5' + stringNum
elif onesPlaceDigit == 6:
stringNum = '6' + stringNum
elif onesPlaceDigit == 7:
stringNum = '7' + stringNum
elif onesPlaceDigit == 8:
stringNum = '8' + stringNum
elif onesPlaceDigit == 9:
stringNum = '9' + stringNum
However, this code is pretty long and tedious. A more concise and common approach is to create a dictionary that maps individual integer digits to their string equivalents, and then look up the value for the onesPlaceDigit key:
DIGITS_INT_TO_STR = {0: '0', 1: '1', 2: '2', 3: '3', 4: '4', 5: '5', 6: '6', 7: '7', 8: '8', 9: '9'}
stringNum = DIGITS_INT_TO_STR[onesPlaceDigit] + stringNum
After this, we can integer divide it by 10
to “remove” the digit in the one’s place of integerNum.
For example, 41096 // 10 evaluates to 4109, effectively removing the 6
from the number and making the 9 the new digit in
the one’s place to convert. Our loop can continue looping and converting digits
until integerNum is 0. For
example, doing this to the integer 41096 would carry out the following
operations:
· 41096 // 10 = 4109
· 4109 // 10 = 410
· 410 // 10 = 41
· 41 // 10 = 4
· 4 // 10 = 0
At this point the algorithm is finished and stringNum
contains the string form.
Special Cases and Gotchas
At the start of the function, check if the integerNum
parameter is 0 and, if so, immediately return the
string '0'. Our algorithm is such that otherwise it
will return the blank string '' for 0, which is incorrect.
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/convertinttostr-template.py and paste it into your code editor. Replace the underscores with code to make a working program:
def convertIntToStr(integerNum):
# Special case: Check if integerNum is 0, and return '0' if so:
if integerNum == ____:
return ____
# This dictionary maps single integer digits to string digits:
DIGITS_INT_TO_STR = {0: '0', 1: '1', 2: '2', 3: '3', 4: '4',
5: '5', 6: '6', 7: '7', 8: '8', 9: '9'}
# Make a note whether the number is negative or not, and make
# integerNum positive for the rest of the function's code:
if integerNum < ____:
isNegative = ____
integerNum = ____
else:
isNegative = ____
# stringNum holds the converted string, and starts off blank:
stringNum = ____
# Keeping looping while integerNum is greater than zero:
while integerNum ____ 0:
# Mod the integerNum by 10 to get the digit in the ones place:
onesPlaceDigit = integerNum % ____
# Put the corresponding string digit at the front of stringNum:
stringNum = DIGITS_INT_TO_STR[onesPlaceDigit] + ____
# Divide integerNum by ten to remove one entire digit place:
integerNum //= ____
# If the number was originally negative, add a minus sign:
if isNegative:
return ____ + stringNum
else:
return ____
The complete solution for this exercise is given in Appendix A and https://invpy.com/convertinttostr.py. You can view each step of this program as it runs under a debugger at https://invpy.com/convertinttostr-debug/.
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