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Defining Functions

• Functions are everywhere in programming!
• You often use builtin ones such as IO.println
• However, you can also define your own

Example: Sum 1 to N

• Let’s say to write some code which sums the numbers from 1 to n and returns that sum
• We could partially do this inside of main like this:

void main() {
    int n = Integer.parseInt(IO.readln());
    int result = 0;
    int i = 1;
    while (i <= n) {
        result += i;
        i++;
    }
    IO.println(result);
}

Example: 1 to N

void main() {
    int n = Integer.parseInt(IO.readln());
    int result = 0;
    int i = 1;
    while (i <= n) {
        result += i;
        i++;
    }
    IO.println(result);
}

• Here you get the input from the outside, from the user of the program
• You then take that input and do the calculation
• Finally, you print the result since this is how you return the result to the user
• Defining functions give you a way to take input from within the program, and return the result to another part of the code

Example: 1 to N as a Function

int sum1ToN(int n) {
    int result = 0;
    int i = 1;
    while (i <= n) {
        result += i;
        i++;
    }
    return result;
}

• Here we take the input, n, from another piece of code
• We use this input to perform the calculation
• Then we return the result back
• Let’s directly compare the two

Comparison

void main() {
    int n = Integer.parseInt(IO.readln());
    int result = 0;
    int i = 1;
    while (i <= n) {
        result += i;
        i++;
    }
    IO.println(result);
}

int sum1ToN(int n) {
    int result = 0;
    int i = 1;
    while (i <= n) {
        result += i;
        i++;
    }
    return result;
}

Using the function

int sum1ToN(int n) {
    int result = 0;
    int i = 1;
    while (i <= n) {
        result += i;
        i++;
    }
    return result;
}

void main() {
    // Call function with argument 10
    IO.println(sum1ToN(10));
    // We can also save the result into a variable
    int result = sum1ToN(30);
    // Then print it or do other thing with it
    IO.println(result);
}

Why Functions?

int sum1ToN(int n) {
    int result = 0;
    int i = 1;
    while (i <= n) {
        result += i;
        i++;
    }
    return result;
}

void main() {
    int n = Integer.parseInt(IO.readln());
    IO.println(sum1ToN(n));
}

Two Parts of Functions

• Declaring the function does nothing other than allow us to use it later
  • This is the function definition itself
• In order to execute the code inside we must call it
  • This is what we did inside main
• Once we call it, it performs its code and then it might return a value back
• We can save this value into a variable, print it, pass it to another function, etc.

Function Syntax

int sum1ToN(int n) {
    int result = 0;
    int i = 1;
    while (i <= n) {
        result += i;
        i++;
    }
    return result;
}

Common Mistakes: Forgetting Return

• It is easy to forget the return keyword
• What happens when you do this?
• Let’s try it and see:

int add1(int x) {
    x + 1; // Forgot return!
}

Compile time vs runtime

• Java favors compile time errors over runtime errors
• Python is the opposite
• What does this mean?

Compile Time

• The Java compiler is a piece of software which is used to translate your code into something which can be run
• Since Java has more type information available to the compiler it can catch more mistakes before you run the code
• This process of compilation is often called compile time
• This tradeoff makes Java code more cumbersome to get up and running but once it does run it is more likely to be correct
• Common compile time errors:
  • Type mismatch
  • Forgot semicolon
  • Forgot return
  • Forgot parenthesis

Runtime

• After your code has been compile and you run it is when you are in runtime
• Runtime errors still exist in Java but they are less common than Python
• Common runtime errors:
  • Divided by zero
  • Integer overflow
  • Stack overflow
  • Null pointer exception

Common Mistakes: Wrong Number of Arguments

• Another common mistake is a mismatch in the number of arguments given to a function
• For example:

int add(int x, int y) {
    return x + y;
}

void main() {
    // Giving 3 instead of 2 arguments
    IO.println(add(1, 2, 3));
    // Giving 0 instead of 2 arguments
    IO.println(add());
}

Common Mistakes: Typo

• The last and perhaps easiest mistake to make is simply misspelling the name of the function
• For example:

int add(int x, int y) {
    return x + y;
}

void main() {
    IO.println(ad(1, 2));
    IO.println(adde(3, 4));
}

Again you will get a compile time error saying it cannot resolve the method with that name