Input and Operators

1 Explain why age = input("Age: ") followed by if age > 18: causes an error. 3

One mark for each of the following, to a maximum of 3.

• input returns a string
• A string cannot be compared with a number
• Convert with int() first

Model answer. The input function always returns a string, so age holds the text "20" rather than the number 20 even if the user types digits. Python cannot compare a string with an integer using a greater than operator, because there is no meaningful way to order text against a number, so it raises a TypeError. The fix is to convert the input as it is read, writing age = int(input("Age: ")), after which the comparison works as intended.

2 Write a program that asks for a number and states whether it is even or odd. 4

One mark for each of the following, to a maximum of 4.

• Asks for input and converts it to an integer
• Uses the modulus operator to test divisibility by 2
• Prints Even when the remainder is 0
• Uses else to print Odd otherwise

Model answer. number = int(input("Enter a number: ")) if number % 2 == 0: print("Even") else: print("Odd") The input is converted to an integer so arithmetic can be performed. The modulus operator gives the remainder after dividing by 2, and a remainder of exactly 0 means the number divides evenly and is therefore even. Any other remainder means it is odd, which the else branch handles.

3 Explain the difference between the / operator and the // operator in Python. 2

One mark for each of the following, to a maximum of 2.

• / performs normal division and always gives a float
• // performs integer division, giving only the whole number part

Model answer. The single slash performs ordinary division and always produces a float, so 7 / 2 gives 3.5 and even 6 / 2 gives 3.0 rather than 3. The double slash performs integer division, discarding anything after the decimal point and returning only the whole number part, so 7 // 2 gives 3. Integer division is useful when a fractional result makes no sense, for example working out how many whole minutes are in a number of seconds.

4 A program asks the user for a number of pencils and a total cost, then works out the cost of one pencil. Explain which operators and conversions are needed and why. 4

One mark for each of the following, to a maximum of 4.

• input returns a string so both values must be converted
• The count is converted with int because it is a whole number
• The cost is converted with float because it has a decimal part
• Division with a single slash is used, which correctly gives a float

Model answer. count = int(input("How many pencils? ")) cost = float(input("Total cost: ")) print(f"Each pencil costs {cost / count:.2f}") The input function always returns a string, so neither value can be used in arithmetic until it is converted. The number of pencils is converted with int because a count is always a whole number, while the total cost is converted with float because prices include pence and an int would discard them. Ordinary division with a single slash is correct here because the answer genuinely may have a decimal part, and it returns a float even when the division happens to be exact. Integer division would be wrong, because it would throw away the pence.

5 Explain what the modulus operator does and give two situations where it is useful. 4

One mark for each of the following, to a maximum of 4.

• Modulus returns the remainder after division
• Gives a correct example such as 17 % 5 being 2
• First use such as testing whether a number is even
• Second use such as extracting time units or cycling a value

Model answer. The modulus operator performs a division and returns the remainder rather than the quotient, so 17 % 5 gives 2 because 5 goes into 17 three times with 2 left over. One common use is testing divisibility: if a number modulus 2 equals 0 then it divides exactly by two and is therefore even, and the same technique checks whether something should happen every third or every tenth time round a loop. A second use is extracting units from a total, for example converting a total number of seconds into minutes and seconds, where total // 60 gives the whole minutes and total % 60 gives the seconds remaining. Modulus is also used to wrap a value around a fixed range, such as moving a player back to the start of a board after the last square.