Lecture 10 Nested Loops

Joseph Haugh

University of New Mexico

Loop Refresher

Recall, the syntax of the while loop:

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

As well as the syntax of the for loop:

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

What statements are allowed in <<bodyStatements>>?
Any valid statements!
This includes loops!

Nested Loops

Nested loops can be intimidating at first
However, they follow the same rules as any other loop

For example, let’s step through the following code:

void main() {
    for (int i = 0; i < 3; i++) {
        for (int j = 0; j < 3; j++) {
            IO.println("(" + i + ", " + j + ")");
        }
    }
}

Copy and paste this code into Intellij and set a breakpoint on the first loop
What does it print?

Nested Loops

Code

void main() {
    for (int i = 0; i < 3; i++) {
        for (int j = 0; j < 3; j++) {
            IO.println("(" + i + ", " + j + ")");
        }
    }
}

Output

(0, 0)
(0, 1)
(0, 2)
(1, 0)
(1, 1)
(1, 2)
(2, 0)
(2, 1)
(2, 2)

Nested Loops

Code

void main() {
    for (int i = 0; i < 3; i++) {
        for (int j = 0; j < 3; j++) {
            IO.println("(" + i + ", " + j + ")");
        }
    }
}

Pseudocode

For every i in range [0, 3)
- For every j in range [0, 3)
  - Print: (i, j)

Output

(0, 0)
(0, 1)
(0, 2)
(1, 0)
(1, 1)
(1, 2)
(2, 0)
(2, 1)
(2, 2)

Dependent Nested Loop

The nested loop can see changes to variables declared in the outer loop
For example, we could start the nested loops j variable at i’s value

What does this code print?

void main() {
    for (int i = 0; i < 3; i++) {
        for (int j = i; j < 3; j++) {
            IO.println("(" + i + ", " + j + ")");
        }
    }
}

Dependent Nested Loop

Code

void main() {
    for (int i = 0; i < 3; i++) {
        for (int j = i; j < 3; j++) {
            IO.println("(" + i + ", " + j + ")");
        }
    }
}

Pseudocode

For every i in range [0, 3)
- For every j in range [i, 3)
  - Print: (i, j)

Output

(0, 0)
(0, 1)
(0, 2)
(1, 1)
(1, 2)
(2, 2)

Example: PlusOut

Let’s write a function together called plusOut
plusOut is given a String, s, and another String, word
It must then return a new String where all characters have been replaced with +
Except for appearances of the word parameter
For example:

plusOut("12xy34", "xy") -> "++xy++"
plusOut("12xy34", "1") -> "1+++++"
plusOut("12xy34xyabcxy", "xy") -> "++xy++xy+++xy"

Key phrases:
- New string
- All characters replaced with +
- Except for appearances of the word parameter

Example: PlusOut

Disclaimer: this function will get long and relatively complicated. I show it as an example of the thought process and trial and error which come with coding.
Note: I will put line numbers next to relevant bullet points which reference the code in parens (line number)

Example: PlusOut

To get started let’s write out our function with comments identifying the key aspects

String plusOut(String s, String word) {
    // Create a new string to return
    // All characters in s replaced with +
    // Except for appearances of word
}

Example: PlusOut

Now let’s add code for the easy lines which in this case is creating a result String (3)
As well as returning it (6)

String plusOut(String s, String word) {
    // Create a new string to return
    String result = "";
    // All characters in s replaced with +
    // Except for appearances of word
    return result;
}

Example: PlusOut

We still have low hanging fruit
Since the problem specifies all characters in s are affected we know we need to loop over s (5-7)

String plusOut(String s, String word) {
    // Create a new string to return
    String result = "";
    // All characters in s replaced with +
    for (int i = 0; i < s.length(); i++) {
        // Except for appearances of word 
    }
    return result;
}

Example: PlusOut

Do we only want to access the single character at index i?
No, instead we want the next n characters where n is the length of word
We also need a place to store these n characters as we get them, so we need another String (6)
How do we retrieve these n characters?
Another loop! (7-9)

String plusOut(String s, String word) {
    // Create a new string to return
    String result = "";
    // All characters in s replaced with +
    for (int i = 0; i < s.length(); i++) {
        String nextN = "";
        for (int j = i; j < word.length(); j++) {
            nextN += s.charAt(j);
        }
        // Except for appearances of word
    }
    return result;
}

Example: PlusOut

Reading the code
Store the answer in result (3)
For every index in s (5-11)
- Store the next n characters in nextN(6)
- For n iterations, where n is the length of word(7-9)
  - Append the character to nextN(8)

String plusOut(String s, String word) {
    // Create a new string to return
    String result = "";
    // All characters in s replaced with +
    for (int i = 0; i < s.length(); i++) {
        String nextN = "";
        for (int j = i; j < word.length(); j++) {
            nextN += s.charAt(j);
        }
        // Except for appearances of word
    }
    return result;
}

Example: PlusOut

Now we need to determine if the nextN is equal to word (11)
If it is we need to append word to the result (12)
Otherwise, we need to append a + (14)
Does this work?
Try this:
```
plusOut("1xy2", "xy");
```

String plusOut(String s, String word) {
    // Create a new string to return
    String result = "";
    // All characters in s replaced with +
    for (int i = 0; i < s.length(); i++) {
        String nextN = "";
        for (int j = i; j < word.length(); j++) {
            nextN += s.charAt(j);
        }
        // Except for appearances of word
        if (nextN.equals(word)) {
            result += nextN;
        } else {
            result += '+';
        }
    }
    return result;
}

Example: PlusOut

plusOut("1xy2", "xy");

Results in all +!
What went wrong?
Our nested loop is incorrect!
It needs to loop up to word.length() + i (7)

String plusOut(String s, String word) {
    // Create a new string to return
    String result = "";
    // All characters in s replaced with +
    for (int i = 0; i < s.length(); i++) {
        String nextN = "";
        for (int j = i; j < word.length() + i; j++) {
            nextN += s.charAt(j);
        }
        // Except for appearances of word
        if (nextN.equals(word)) {
            result += nextN;
        } else {
            result += '+';
        }
    }
    return result;
}

Example: PlusOut

plusOut("1xy2", "xy");

It right now?
No, now we get an index out of bounds error, why?
We perform the nested loop for every index in s including the last
We need to guard against this case (7)

String plusOut(String s, String word) {
    // Create a new string to return
    String result = "";
    // All characters in s replaced with +
    for (int i = 0; i < s.length(); i++) {
        String nextN = "";
        if (word.length() + i <= s.length()) {
            for (int j = i; j < word.length() + i; j++) {
                nextN += s.charAt(j);
            }
        }
        // Except for appearances of word
        if (nextN.equals(word)) {
            result += nextN;
        } else {
            result += '+';
        }
    }
    return result;
}

Example: PlusOut

plusOut("1xy2", "xy");

Please let it be right!
Not quite
We include an extra character
Can you see why?
We need to increment i by the number of iterations we took in nested loop when we find word

String plusOut(String s, String word) {
    // Create a new string to return
    String result = "";
    // All characters in s replaced with +
    for (int i = 0; i < s.length(); i++) {
        String nextN = "";
        if (word.length() + i <= s.length()) {
            for (int j = i; j < word.length() + i; j++) {
                nextN += s.charAt(j);
            }
        }
        // Except for appearances of word
        if (nextN.equals(word)) {
            result += nextN;
        } else {
            result += '+';
        }
    }
    return result;
}

Example: PlusOut

plusOut("1xy2", "xy");

To do this we need a while loop otherwise it get confusing (6-21)
Phew that was a lot of work!
Looking at it now it is hard to understand all at once!
But step-by-step is approachable

String plusOut(String s, String word) {
    // Create a new string to return
    String result = "";
    // All characters in s replaced with +
    int i = 0;
    while (i < s.length()) {
        String nextN = "";
        if (word.length() + i <= s.length()) {
            for (int j = i; j < word.length() + i; j++) {
                nextN += s.charAt(j);
            }
        }
        // Except for appearances of word
        if (nextN.equals(word)) {
            result += nextN;
            i += word.length();
        } else {
            result += '+';
            i++;
        }
    }
    return result;
}