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LESSON 76 OF 414 · CONDITIONAL STATEMENTS

Student Passed in Rust: Averages and if-else

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We created three variables to store the grades obtained by a student at math, english and science.

You need to compute the average of the grades and print to the console if the user has passed or not.

Assignment
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    RUST · CONDITIONAL STATEMENTS

    Student Passed in Rust, explained

    The task

    A student has three grades, stored in math_grade, english_grade and science_grade. The program computes their average and prints whether the student passed. A grade of 5 or more is a pass.

    This exercise combines the previous lessons: variables, arithmetic, a comparison and an if-else. The one new difficulty is the type of the average.

    Why the average is tricky

    The three grades are integers: let math_grade = 7; and so on, so Rust makes them i32 values. Their sum is an integer, and dividing an integer by an integer in Rust gives an integer, with the fractional part discarded:

    let sum = 7 + 4 + 9;
println!("{}", sum / 3);

    6

    The true average of 7, 4 and 9 is 6.666..., so the integer result loses information. For the pass mark of 5 the verdict would happen to be the same, but the average itself would be wrong the moment you print it or compare it with a mark such as 5.5. To keep the fraction, the division has to be done on floating-point numbers.

    Rust never converts between number types on its own. The integer sum can't be divided by 3.0 directly, and the compiler reports "cannot divide {integer} by {float}". You convert explicitly with as.

    Step by step

    1. Compute grade as a float. Add the three grades in parentheses, cast the sum with as f64, and divide by 3.0. The cast applies to the parenthesised sum, so the addition itself still happens on integers, and the division happens on f64 values.
    2. Write the if. Check grade >= 5.0. A pass is 5 or more, so the comparison is "greater than or equal to". The literal must be 5.0, because grade is an f64, and Rust doesn't compare a float with the integer 5.
    3. Print the pass message inside the block: "Passed the test".
    4. Add the else and print "Failed the test". It runs whenever the average is below 5.
    let math_grade = 7;
let english_grade = 4;
let science_grade = 9;

let grade = (math_grade + english_grade + science_grade) as f64 / 3.0;

if grade >= 5.0 {
    println!("Passed the test");
} else {
    println!("Failed the test");
}

    The sum is 20, the average is 6.666..., which is at least 5.0, so the program prints Passed the test.

    The cast and the precedence

    The expression (a + b + c) as f64 / 3.0 reads left to right with as binding tighter than /. It means ((a + b + c) as f64) / 3.0. Casting only one grade, as in a + b + c as f64, would be a type error, since the integers and the float would be added together. Casting the grades individually works too, but the cast on the sum is shorter.

    The cast from an integer to f64 is lossless for any i32. Casts in the other direction, from float to integer, cut off the fractional part, and that is not what we want here.

    Common mistakes

    • Dividing by 3 instead of 3.0. After the cast to f64, dividing by the integer 3 is a type error. The divisor must be a float too.
    • Casting too late. (sum / 3) as f64 divides as integers first and only then converts, so the fraction is already lost.
    • Comparing with 5. grade >= 5 doesn't compile when grade is a float.
    • Using > instead of >=. An average of exactly 5.0 should pass. For example grades 5, 5 and 5, or 3, 5 and 7.
    • Floating-point surprises. Floats can't store every decimal exactly (0.1 + 0.2 == 0.3 is false), so comparisons with a boundary can be off in other computations. Here the sums are whole numbers divided by 3, and exact boundary cases like 15 / 3 = 5.0 work correctly.

    Key takeaways

    • Integer division discards the fraction, so use f64 for averages.
    • Rust converts numbers only when you ask, with as.
    • Compare floats with float literals such as 5.0.
    • The if-else chooses exactly one message.
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