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And that brings us to the modulus operator (%).
Enter the code shown in Figure 9 . (You don't need to enter the comments. They are there to explain what is going on.)
| Figure 9 . Whole number quotient and remainder. |
|---|
>>>23//4 #get integer quotient
5>>>23%4 #get remainder
3>>>23.0//4 #get quotient
5.0>>>23.0%4 #get remainder
3.0>>> |
It is probably safe to say that the purpose of the // division operator is to produce a whole number quotient (no digits to the right of the decimal point even if the decimal point is showing as in the last operation in Figure 8 ) .
The purpose of the modulus operator is to produce the remainder resulting from a division.
As you can see from the example in Figure 9 , both division operations using the // division operator produced the whole number quotient of 5, and bothmodulus operations produced the whole number remainder of 3.
What is the result of evaluating the expression shown in Figure 10 on a hand calculator?
| Figure 10 . A simple expression. |
|---|
3+5*4 |
Try it on your hand calculator. (You will probably need to use an X instead of an * to indicate multiplication.) My hand calculator gives an answer of 32.
Now try it with Python and you should get the result shown in Figure 11 .
| Figure 11 . A simple expression in Python. |
|---|
>>>3+5*4
23>>> |
Oops! This answer doesn't match the answer produced by my hand calculator, and I'll bet that it doesn't match your calculator either unless you are using a fancyscientific calculator.
The answer depends on the order in which you perform the various arithmetic operations. Ordinary hand calculators usually do the arithmetic in the orderthat the terms are fed into the keyboard.
However, most computer programming systems, including Python, use a precedence system to decide which operations to perform first, which operationsto perform second, etc.
I'm not going to go into the Python precedence system in detail. (If you are interested in the order of precedence of all the operators, you can find aprecedence table in Python Language Reference -- 6.15. Operator precedence .)
Rather, I am going to show you how to group terms using parentheses so that you can control the order of operations without worrying about the precedencesystem.
The Python code fragment in Figure 12 shows how I can produce both results simply by grouping terms using parentheses.
| Figure 12 . Grouping terms with parentheses. |
|---|
>>>(3+5)*4 # do addition first
32>>>3+(5*4) # do multiplication first, default
23>>> |
The first expression produces 32 as produced by the hand calculator. The second expression produces 23 as produced by the earlier Python expression.
A Python expression is evaluated by first evaluating each of the sub-expressions inside the parentheses and then using those values to complete the evaluation.
In the first expression, the code in Figure 12 forced Python to perform the addition first by placing the addition inside the parentheses. This produced an intermediate valueof 8 when the sub-expression inside the parentheses was evaluated. The remaining part of the overall expression was then evaluated by multiplying theintermediate value by 4, producing a result of 32.
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