Lecture 07 More Control

Joseph Haugh

University of New Mexico

Free Recall

Get out a sheet of paper or open a text editor
For 2 minutes write down whatever comes to mind about the last class
- This could be topics you learned
- Questions you had
- Connections you made

Control so far

So far we have seen the following control structures:
- if: allows for branching
- while: allows for looping
You could theoretically do everything with just these two control structures
However, it is often nice to have specialized ones as well

Example: Validating Input

For example, let’s try to write a function which does the following:
- Reads in input from the user
- Converts it to a number
- Makes sure that number is positive
- Repeats this until the user gets it right
Our first attempt will be with a while loop

Example: Validating Input With While Loop

boolean isPositive(int n) {
    return n >= 0;
}

int validateUserInput() {
    String input = IO.readln("Enter a positive number: ");
    int n = Integer.parseInt(input);
    while (!isPositive(n)) {
        input = IO.readln("Enter a positive number: ");
        n = Integer.parseInt(input);
    }
    return n;
}

void main() {
    int n = validateUserInput();
    IO.println(n);
}

Example: Validating Input With While Loop

Works but we have to repeat ourselves
We have to repeat the lines to read and parse the input
Can it be improved?

boolean isPositive(int n) {
    return n >= 0;
}

int validateUserInput() {
    String input = IO.readln("Enter a positive number: ");
    int n = Integer.parseInt(input);
    while (!isPositive(n)) {
        input = IO.readln("Enter a positive number: ");
        n = Integer.parseInt(input);
    }
    return n;
}

void main() {
    int n = validateUserInput();
    IO.println(n);
}

While vs Do While

Of course!
We can use a do while loop instead
Recall, the semantics of a while loop
- Check the condition
- If it is true, do the body statements inside the loop
- If it is false, continue past the loop
A do while loop has the following semantics:
- Do the body statements inside the loop
- Check the condition
- If it is true, do the body statements inside the loop
- If it is false, continue past the loop

Do While

Do while has the following syntax:

do {
    <<bodyStatements>>
} while (<<booleanCondition>>);

Do while is useful when you need the body of the loop to run first then check the condition
This is exactly what we need for our example!

Example: Validating Input With Do While Loop

boolean isPositive(int n) {
    return n >= 0;
}

int validateUserInput() {
    String input; // Is this necessary?
    int n;        // Or this? Why?
    do {
        input = IO.readln("Enter a positive number: ");
        n = Integer.parseInt(input);
    } while (!isPositive(n));
    return n;
}

void main() {
    int n = validateUserInput();
    IO.println(n);
}

Scoping

We had to declare input and n outside of the loop two different reasons
For input it is best to declare it outside to prevent creating a new version of input each time around the loop
For n we must declare it outside of the loop due to scope
The scope of a variable can broadly be understood as where in the code a variable can be referred to or seen
In Java the scope of a variable is bounded by the curly braces it was defined within
Therefore, if we defined n inside the loop then hit the closing curly brace of the loop n is no longer in scope and cannot be used

Example: Validating Input Scoping Problems

int validateUserInput() {
    do {
        // We could put input inside the loop without
        // error, but it would be suboptimal
        String input = IO.readln("Enter a positive number: ");
        int n = Integer.parseInt(input);
    } // n is out of scope after this curly brace
    // Thus, it cannot be seen in this condition!
    while (!isPositive(n));
    // Can also not be seen here!
    return n;
}
// Or here for that matter!

Common Looping Pattern

Many loops take on the following form:

<<initStatements>>
while (<<booleanCondition>>) {
    <<bodyStatements>>
    <<updateStatements>>
}

For example, this loop we saw a few days ago has this form:

int result = 0;  // Init statement
int i = 1;       // Init statement
while (i <= n) {
    result += i; // Body statement
    i++;         // Update statement
}

For Loop

This pattern is so common there is a dedicated loop construct for it
Its called a for loop
Caution:
- Java’s for loop is different than Python’s for loop
- We will see later that Python’s is closer to Java’s foreach loop

For loop syntax:

for (<<initStatement>>; <<booleanCondition>>; <<updateStatement>>) {
    <<bodyStatements>>
}

For vs While

For example our previous while loop example:

int result = 0;  // Init statement
int i = 1;       // Init statement
while (i <= n) {
    result += i; // Body statement
    i++;         // Update statement
}

Could be rewritten with a for loop:

int result = 0;
for (int i = 1; i <= n; i++) {
    result += i;
}

For vs While

While loops can do everything a for loop can
The choice of which to use should be made on per problem basis
Each have their niche