calc.py could look like:
calculation = input("> ")
parts = calculation.split() # Split e.g. "4 * 6" into ["4", "*", "6"]
lhs = int(parts[0]) # Extract e.g. "4" and turn it into 4
operation = parts[1] # Extract e.g. "*"
rhs = int(parts[2]) # Extract e.g. "6" and turn it into 6
if operation == "+":
print(calculation, "is", lhs + rhs)
elif operation == "-":
print(calculation, "is", lhs - rhs)
elif operation == "*":
print(calculation, "is", lhs * rhs)
elif operation == "/":
print(calculation, "is", lhs / rhs)
python calc.py
python calc.py
python calc.py
python calc.py
The code above works fine, but there's always more than one way to approach a problem like this.
One thing that we could improve would be the repetition in the print lines. Each of them are almost the same as each other and if we wanted to change the output from
4 * 6 is 24to something like
4 * 6 = 24then we'd have to edit all four lines of code.
Remembering our three-part pattern from earlier in the course of input→calculation→output, it's a good idea to split out the calculation of data from the printing and display of data. In our case we could change it to look like:
calculation = input("> ")
# Prepare the parts
parts = calculation.split()
lhs = int(parts[0])
operation = parts[1]
rhs = int(parts[2])
# Calculate the answer
if operation == "+":
result = lhs + rhs
elif operation == "-":
result = lhs - rhs
elif operation == "*":
result = lhs * rhs
elif operation == "/":
result = lhs / rhs
# Output the result
print(calculation, "is", result)
python calc.py
python calc.py
python calc.py
python calc.py
operator module¶We haven't covered using extra modules in this course so far, but you might have come across these if you've done some Python before. We cover this in more detail in the Intermediate Python chapter Using Functions. I'm showing you this way to solve the problem, not because I expect you to able to do this already, but so that when you look back at these notes in the future you see how you might solve the problem differently.
import operator
# Make a dictioanry that maps from
# the operator string passed in and
# a function which does the operation
operators = {
"+": operator.add,
"-": operator.sub,
"*": operator.mul,
"/": operator.truediv,
}
calculation = input("> ")
# Prepare the parts
parts = calculation.split()
lhs = int(parts[0])
operation = parts[1]
rhs = int(parts[2])
# We extract the operator function from the operators dictionary
# allowing us to then call the function with the round brackets
result = operators[operation](lhs, rhs)
# Output the result
print(calculation, "is", result)
python calc.py
python calc.py
python calc.py
python calc.py