Iowa State University · Computer Science

Expressions, input, and type conversion

20-minute lesson · module-02-simple-data

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Expressions, input, and type conversion

20-minute lesson · module-02-simple-data

Build arithmetic expressions using +, -, *, /, //, %, and **.
Read user input with input() and convert strings to numbers.
Predict the difference between integer division (//) and true division (/).
Recognize TypeError and ValueError when conversions fail.

Expressions are recipes for a single value

An expression is anything Python can compute down to a single value. These are all expressions:

3 + 4         # arithmetic
"hi" + "!"    # string concatenation
10 / 4        # division -> 2.5
10 // 4       # integer (floor) division -> 2
10 % 4        # remainder (modulo) -> 2
2 ** 8        # exponent (2 to the 8th) -> 256

Python evaluates expressions following standard precedence: parentheses, exponents, then multiplication/division/modulo, then addition/subtraction, left to right within the same level.

input() asks the user for a line of text

name = input("What is your name? ")
print("Hello,", name)

The thing input() returns is always a string — even if the user types digits. To do math on it, convert with int(...) or float(...):

raw = input("How old are you? ")
age = int(raw)
next_age = age + 1
print("Next year you will be", next_age)

Type conversion can fail

int("12") → 12 (works)
int("twelve") → raises ValueError
int("12.5") → raises ValueError (use float first if there's a decimal)
"hi" + 5 → raises TypeError (can't add a string and an int)

Putting numbers back into a line of text

Going the other direction is just as common. print() takes several values separated by commas, but it puts a space between each one:

hours = 2
mins = 25
print(hours, "h", mins, "m")     # 2 h 25 m   <- spaces you did not ask for

When the pieces need to sit together with no gaps, you have two ways:

str(hours) + "h" gives 2h. str(...) turns a number into text so + can join it to other text. That is the answer to the "hi" + 5 problem above: convert first, then join.
f"{hours}h {mins}m" gives 2h 25m. This is an f-string: put an f before the opening quote, and anything you write inside { } is worked out and dropped into the line.

The f-string is the one you will see most often, and it is the shortest way to build a line out of several values.

Predict-then-run. Before clicking Run on the editor below, write down what the program would print on a piece of paper. Then run it and compare.

Examples

Tip calculator

bill = 50
tip_pct = 18
tip = bill * tip_pct / 100
total = bill + tip
print("Tip:", tip)
print("Total:", total)

`bill * tip_pct / 100` evaluates left to right: first the multiplication, then the division. The total adds the original bill back in.

Checkpoint questions

What does this program print?

print(7 // 2, 7 % 2)

a)3 1
b)3.5 0
c)3 0.5
d)An error

What will Python print on the last line?

raw = "42"
n = int(raw)
print(n + 1)

Your prediction

Which line raises a ValueError?

a = int("12")
b = int("twelve")
c = float("3.14")

a)Line 1
b)Line 2
c)Line 3
d)None of the above

Hands-on coding

Sum two integers from variables

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ActiveCode

# The variables a and b are already given as strings.
# Convert each to an int, add them, and print the result.
a = "15"
b = "27"
# TODO: convert and print the sum

Output

(no output yet)

Convert minutes to hours and minutes

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ActiveCode

# The variable total_minutes holds an integer.
# Print the equivalent in the form: "<h>h <m>m"
# For example, 145 -> "2h 25m"
total_minutes = 145
# TODO: compute hours and minutes and print

Output

(no output yet)

CodeLens — step through execution

Trace integer vs. true division

CodeLens — step through frames, objects, and references

CodeLens

Code · Python 3

  1a = 7
  2b = 2
  3true_div = a / b
  4floor_div = a // b
  5remainder = a % b
  6print(true_div, floor_div, remainder)
  7

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Print output

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Frames

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Objects

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Edit the code

a = 7
b = 2
true_div = a / b
floor_div = a // b
remainder = a % b
print(true_div, floor_div, remainder)

Debugging interlude

Predict the error first (+50 XP), then fix the code (+100 XP). Hints cost 30 XP each.

🐞

Why does this raise TypeError?

The student wanted to print "Total: 25" but Python raises TypeError. The variable `count` was read from a CSV (so it's a string). Identify the type mismatch and fix it.

Predict · 50 XPFix · 100 XP

count = "20"
bonus = 5
print("Total: " + count + bonus)

Step 1 — Predict the error

Read the program. Which kind of error (or non-error) will Python raise?

End-of-lesson quiz

What does input() return?
- a)Always a string.
- b)An integer when the user types digits, otherwise a string.
- c)Whatever Python guesses the user meant.

Reflection

What is one situation where you would prefer // (integer division) over / (true division)?

Source attribution

Status: adapted

How to Think Like a Computer Scientist: Interactive Edition — source · Simple Python Data / Variables / Values / Operators / Order of Operations / Input

Adaptation notes. Reading rewritten in our own words. Examples and hidden tests authored specifically for PyTutor AI. The order-of-operations table is summarized in our own phrasing.

License notes. Verify chapter-level license metadata on Runestone before copying any text verbatim. This lesson contains no verbatim quotations.