CS1010 AY24/25 S1 Lecture 11

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# Lecture 11

### 4 November 2024

.smaller[
Admin Matters<br>
Unit 26: Struct<br>
Unit 27: Standard I/O<br>
]
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### PE1

- Watch out for emails from SoftMark tonight/tomorrow

- Check marking scheme and common mistakes

- File remarking request ticket by this Friday

---
### PE2

- Next Tuesday, 6 - 9 PM

- Scope: until Unit 25

---
### Exercise 8

- Released today

- Due next Friday

- If you need more time, you can submit by the end of Monday, Reading Week

---

### Past Year Papers

- Posted for both final exam and PE2

- Will go through some final questions during tutorials

---
class:

### Open call for CS1010 memes

- Email your best memes to us!

- Feel free to roast us or the course :)

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background-image: url(figures/ceg-shirt.jpg)

### `struct`

---
### Some things belong together

- 1D array and its length
- 2D array, its width, and its height
- x and y position in a maze
- r, g, b of a pixel

---
### Defining a `struct`

```C
struct matrix {
  double** cells;
  long num_of_rows;
  long num_of_columns;
};
```

---
```C
struct course {
  char *code;
  char *title;
  long units;
};
```

---
### Declaring a struct variable

```C
struct course cs1010;
```

---
### Possible to have a `struct` of `struct`

```C
struct point {
  long x;
  long y;
};

struct rectangle {
  struct point top_left;
  struct point bottom_right;
};
```

---
### Initializing a struct variable

.small[
```C
struct course cs1010;
cs1010.code = "CS1010";
cs1010.title = "Programming Methodology";
cs1010.units = 4;
```
]

---
### Declaring AND Initializing a struct variable

.small[
```C
struct course cs1010 = {
  .code = "CS1010",
  .title = "Programming Methodology",
  .units = 4
};
```
]
---
### Reading and writing members of a `struct`

.small[
```C
cs1010.units = hours_spent_per_week/2.5;
cs1010_println_long(cs1010.units);
```
]

---
### Assigning one `struct` to another

.small[
```C
struct color black = {
  .r = 0;
  .g = 0;
  .b = 0;
};

struct color background = black;
```
]

---
### `struct` are passed by-value

.smaller[
```C
void update_units(struct course c, long weekly_hours) {
  c.units = weekly_hours/2.5;
}
  :

struct course cs1010;
cs1010.units = 4;
update_units(cs1010, 80);
// { cs1010.units == 4 }
```
]

---
### Passing a `struct` by reference

Using `*` dereference operator

.smaller[
```C
void update_units(struct course *c, long weekly_hours) {
  (*c).units = weekly_hours/2.5;
}
  :

struct course cs1010;
update_units(&cs1010, 80);
// cs1010.units becomes 32
```
]

---
### Passing a `struct` by reference

Using `->` arrow operator

.smaller[
```C
void update_units(struct course *c, long weekly_hours) {
  c->units = weekly_hours/2.5;
}
  :

struct course cs1010;
update_units(&cs1010, 80);
// cs1010.units becomes 32
```
]

---
### Passing multiple values back to caller

Method 1

.smaller[
```C
void find_max_steps(long n, long *max_n, long *max_num_steps) {
  *max_num_steps = 0;
  *max_n = 1;
  for (long i = 1; i <= n; i += 1) {
    long num_of_steps = count_num_of_steps(i);
    if (num_of_steps >= *max_num_steps) {
      *max_n = i;
      *max_num_steps = num_of_steps;
    }
  }
}
```
]

---
### Passing multiple values back to caller

Method 2

.smaller[
```C
struct answer {
  long max_n;
  long max_num_steps;
};
```
]

---
### Passing multiple values back to caller

.smaller[
```C
struct answer find_max_steps(long n) {
  struct answer ans = {
    .max_n = 1,
    .max_num_steps = 0
  };
  for (long i = 1; i <= n; i += 1) {
    long num_of_steps = count_num_of_steps(i);
    if (num_of_steps >= ans.max_num_steps) {
      ans.max_n = i;
      ans.max_num_steps = num_of_steps;
    }
  }
  return ans;
}
```
]

---
### Defining Your Own Type

.smaller[
```C
typedef unsigned long person_t;
  :
void is_contact(char **network, person_t i, person_t j) {
  :
}
```
]

---
.small[
```C
typedef struct course {
  char *code;
  char *title;
  long units;
} course;

void update_units(course *c, long weekly_hours)
{
  c->units = weekly_hours/2.5;
}
```
]

---
### Where to declare type and struct

- Same scoping rules apply.

- Structs and types are usually used across multiple functions -- you can define them globally.

- Struct variables are still variables -- avoid global variables.

---
### Life After CS1010

- No `Makefile`, `compile_flags.txt`, etc.

- No CS1010 I/O library

- Input is not always valid

---
### Reading and Printing without `libcs1010`

- Most C introductory courses teach `printf` and `scanf`

- How to use them correctly

---
.smaller[
```C
printf("%s is a %ld-unit course\n", course.code, course.units);

long n = 42;
printf("%ld\n", n);
printf("%5ld\n", n);
printf("%05ld\n", n);

double pi = 3.1415926;
printf("%.9lf\n", pi);
printf("%010.3le\n", pi);
```
]

---
- `printf` behaves differently from functions that you have been writing.

- It can take in a _variable number_ of arguments of _variable types_.

---
### Wrong/Dangerous use of `printf`

```C
double pi = 3.1415;
long x = 0;
printf("%ld\n", pi);
printf("%ld %s\n", x, x);
printf("%s %s\n");
```

---
```C
long width;
long height;
scanf("%ld %ld", &width, &height);
```

---
- Space in between the format specifier matches zero or more white spaces (space, tab, newline)

- Scanning stops when an input character does not match such a format character or when an input conversion fails

---
Reading a string with `scanf`

```C
char word[10];
scanf("%s", word);
```

What could possible go wrong?

---
Reading a number with `scanf`

```C
long n;
scanf("%ld", &n);
```

What could possible go wrong?

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### Homework

- Quiz 11 (last quiz)

- Exercise 8 (last exercise)

- Look at past year questions

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