reveal.js

# Pause for Breath

---

CS 130 // 2021-09-22

## Administrivia
- Quiz 1 returned on Gradescope
    + Let me know if you have any questions!
- Assignment 2 is due tomorrow night

# Exam 1
- Monday, September 27th

## Exam 1
- **Format of the exam**:
    + Written, in-class, 75-minute exam
    + Closed textbook/notes/internet
    + Can bring a 8.5 x 11, single-sided "cheat sheet"
- 
 **Topics**:
    + Basic CLI usage
    + C programming fundamentals
        * Arrays / pointers / strings / structs / IO
        * Stack memory / heap memory / malloc / free

# Questions
## ...about anything?

# Pointer Arithmetic

## Pointer Arithmetic
- You can do arithmetic on pointers!
    ```c
    int arr[5] = {100, 200, 300, 400, 500};
    int *p1 = arr;
    int *p2 = &arr[2];
    int *p3 = p2 + 1;
    p2--;
    printf("%d %d %d\n", *p1, *p2, *p3);
    ```
- 
 What do you guess the output will be?

## Exercise 1: Whatzitdo?
- Draw the **state of the stack/heap** of the program execution at the specified position

```c
void foo(int *p) {
    int x = 11, y = 13;
    // <========= THIS IS THE SPOT
    *p = x + y;
}

int main() {
    int x = 5, y = 7, sum = 0;
    int *p = &x;
    foo(&sum);
    *p = x + y;
    printf("%d %d %d %d\n", x, y, sum, *p);
    return 0;
}
```

## Exercise 1: Whatzitdo?

- After the code finishes, what is printed?

```c
void foo(int *p) {
    int x = 11, y = 13;
    // <========= THIS IS THE SPOT
    *p = x + y;
}

int main() {
    int x = 5, y = 7, sum = 0;
    int *p = &x;
    foo(&sum);
    *p = x + y;
    printf("%d %d %d %d\n", x, y, sum, *p);
    return 0;
}
```

## Exercise 2: String Trimming
- In Python there is a string method called `trim` that removes spaces around a string:
    ```py
    s1 = "   abc   "
    s2 = s1.trim()  # "abc"
    ```
- 
 Write a C function that trims on the right:
    ```c
    // assume that s contains at least one non-space char
    void trim_right(char *s)
    ```
- 
 It should do the trimming "in place"
    ```c
    char s[10] = "abc   ";
    trim_right(s);          // becomes "abc"
    ```

## Exercise 3: Spot the Errors
- Consider the following C code
    ```c
    // Copies the string s, returning an entirely new string
    // with the same contents
    char *copy(char *s) {
        int n = strlen(s);
        char arr[n];
        for (int i = 0; i < n; i++) {
            arr[i] = s[i];
        }
        return arr;
    }
    ```
- 
 Anyone who uses the code above will almost certainly encounter a **segmentation fault**. Why?

## Exercise 3: Spot the Errors
- I've fixed the dangling pointer error now:
    ```c
    // Copies the string s, returning an entirely new string
    // with the same contents
    char *copy(char *s) {
        int n = strlen(s);
        char *arr = malloc(n * sizeof(char));
        for (int i = 0; i < n; i++) {
            arr[i] = s[i];
        }
        return arr;
    }
    ```
- 
 Anyone who uses this may encounter some unexpected behavior. Why?

## Exercise 3: Spot the Errors
- I've fixed the termination character error:
    ```c
    char *copy(char *s) {
        int n = strlen(s);
        char *arr = malloc((n + 1) * sizeof(char));
        for (int i = 0; i < n; i++) {
            arr[i] = s[i];
        }
        arr[i] = '\0';
        return arr;
    }
    ```
- 
 Now suppose I use it:
    ```c
    char s1[10] = "abc";
    char *s2 = copy(s1);
    ```
- 
 What do I need to do with `s2` when I am done?

## Exercise 4: Structs
- Suppose we have the following struct:
    ```c
    typedef struct {
        char name[20];
        int age;
    } PERSON;
    ```
- 
 Write a function that takes in an array of people and returns the youngest person in the array

## Exercise 5: 
- Write a **recursive** function that computes the sum of an array of integers
- 
 What would the prototype of the function be?

```c
    int sum(int *arr, int n)
    ```

- 
 Remember to think about the **base case** and **recursive case**