CS1010 AY20/21 S1 Lecture 12

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# Lecture 12

### 8 November 2021

.smaller[
Admin Matters<br>
Unit 28: struct<br>
Unit 29: stdio<br>
Recap of CS1010<br>
]
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.fit[![meme](figures/meme/from-students/Man-of-Culture-from-Hao-Yi.jpg)]

.tiny[Submitted by Hao Yi]

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background-image: url(figures/meme/from-students/making-meme.jpg)

.tiny[Submitted by Benjamin Poh]

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![meme](figures/meme/from-students/memes-appear-from-Benjamin-Poh.png)

.tiny[Submitted by Benjamin Poh]

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### Last Catch-Up Session

- This Saturday, 10 am - 12 noon
- Q and A format, as usual
- Watch out Piazza for Pigeonhole link

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### Last Tutorial

- Final questions (from last AY)
- Mounted as a ungraded Quiz

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### Last Lab

- PE2 questions (last Saturday)

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background-image: url(figures/meme/from-students/wrong-test-case-Muthya.jpg)

.tiny[Submitted by Muthya]

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background-image: url(figures/meme/from-students/pe2-two-from-Muthya.jpg)

.tiny[Submitted by Muthya]

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![two](figures/meme/from-students/pe2-empty-main-from-Hao-Yi.jpg)

.tiny[Submitted by Hao Yi]

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![5 mins](figures/meme/from-students/5-mins-before-pe2-from-Fuxi.png)

.tiny[Submitted by Fuxi (Tutor)]

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### Last Quiz

- New self-diagnostic quiz for this week
- All four self-diagnostic quizzes since midterm break re-opened
- Due on Wednesday night as usual

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### Last Assignment

- Due Friday midnight (but you can submit before next Tuesday, 23:59 without penalty)

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![two](figures/meme/from-students/assignment-weekend-wilson-ng.jpg)

.tiny[Submitted by Wilson Ng]

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### Final Exam

- **THURSDAY**, 25 Nov, 2021 (5pm - 7pm)
- Coverage: Everything (except stdio)
- Open Book

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

### `struct`

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### 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

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## Defining a `struct`

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

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```C
struct list {
  char *array;
  long length;
};
```

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```C
struct module {
  char *code;
  char *title;
  long mc;
};
```
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### Declaring a struct variable

```C
struct module cs1010;
```

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### Initializing a struct variable

.tiny[
```C
struct module cs1010;
cs1010.code = "CS1010";
cs1010.title = "Programming Methodology";
cs1010.mc = 4;
```
]

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### Declaring AND Initializing a struct variable

.tiny[
```C
struct module cs1010 = {
  .code = "CS1010",
  .title = "Programming Methodology",
  .mc = 4
};
```
]
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### Reading and writing members of a `struct`

.tiny[
```C
cs1010.mc = hours_spent_per_week/2.5;
cs1010_println_long(cs1010.mc);
```
]

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### `struct` as pass-by-value parameters to a function

.tiny[
```C
void update_mc(struct module cs1010, long weekly_hours_spent) {
  cs1010.mc = weekly_hours_spent/2.5;
}
```
]

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### `struct` as pass-by-reference parameters to a function

.tiny[
```C
void update_mc(struct module *cs1010, long weekly_hours_spent) {
  (*cs1010).mc = weekly_hours_spent/2.5;
}
```
]

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### `struct` as pass-by-reference parameters to a function

.tiny[
```C
void update_mc(struct module *cs1010, long weekly_hours_spent) {
  cs1010->mc = weekly_hours_spent/2.5;
}
```
]

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### How to pass multiple values back to caller (Method 1)

.tiny[
```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;
    }
  }
}
```
]

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### How to pass multiple values back to caller (Method 2)

```C
struct answer {
  long max_n;
  long max_num_steps;
};
```

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.tiny[
```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;
}
```
]

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## Defining Your Own Type

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

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```C
typedef struct module {
  char *code;
  char *title;
  long mc;
} module;
```

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```C
typedef struct {
  char *code;
  char *title;
  long mc;
} module;
```

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.tiny[
```C
void update_mc(module cs1010, long weekly_hours_spent) {
  cs1010.mc = weekly_hours_spent/2.5;
}
```
]

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## Reading and Printing without `libcs1010`

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## `printf` and `scanf`

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```C
printf("%s is a %ld-MC module\n",
   module.code, module.mc);
```

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.tiny[
```
%[flags][field_width][.precision][length_modifier]specifier
```
]

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

- It can take in different number of arguments of different type.

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

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.tiny[
```
%[*][field_width][length_modifier]specifier
```
]

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- 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

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## Recap of CS1010

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## What Have You Learned?

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### 1. How to write C

- types, functions, arithmetic ops, if-else, logical expressions, loops, arrays, pointers, memory management, preprocessor, struct, I/O

- Others we skipped: break/continue, switch, common C libraries API, bit operations, union, enum, argc/argv, separate compilation etc.

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background-image: url(figures/meme/from-students/pointers-zhen-wei.jpg)

.tiny[Submitted by Zhen Wei (Tutor)]

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![pointers](figures/meme/from-students/mem-leak-from-Jialun.png)

.tiny[Submitted by Jia Lun (Tutor)]

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### 2. How a C program behaves

- machine code, data in memory, types, call stack, memory addr, call by value/ref, heap

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### 3. Tools and Practices

- `clang`, `vim`, `bash`, style, assertion, documentation, (some) testing, (some) secure programming

- Others we skipped: debugger, valgrind, git, make

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background-image: url(figures/meme/from-students/clang-tidy-from-Hao-Yi.jpg)

.tiny[Submitted by Hao Yi]

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![pointers](figures/meme/from-students/wsl-from-Jialun.png)

.tiny[Submitted by Jialun (Tutor)]

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### 4. Problem Solving

- decomposition, recursion, flowchart, conditionals, assertions, loops, invariants, running time analysis, sorting, searching, efficiency

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background-image: url(figures/meme/from-students/ash-Omn-from-Joo-Hoe.png)

.tiny[Submitted by Joo Hoe]

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![pointers](figures/meme/from-students/non-trivial-solution-from-Pereira.jpg)

.tiny[Submitted by Pereira]

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### Computational Thinking

The mental process associated with computational problem solving

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### Four Pillars of Computational Thinking

- Decomposition
- Pattern Recognition
- Abstract
- Algorithms

(We have been teaching these without being explicit about it)

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## Decomposition

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### Find the Std Dev (Lecture 2)

- Give an algorithm to find the standard deviation of a list $L$ of $k$ integers.

$$\sqrt{\frac{\sum\_{i=0}^{k-1}(l\_i - \mu)^2}{k}}$$

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### Decomposition makes a problem easier to solve

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### Taxi

.tiny[
```C
double surcharge(long day, long hour, long minute) {
	if (is_weekday(day) && is_morning_peak(hour, minute)) {
		return MORNING_SURCHARGE;  
	}
	if (is_evening_peak(hour)) {
		return EVENING_SURCHARGE;  
	}
	if (is_midnight_peak(hour)) {
		return MIDNIGHT_SURCHARGE;  
	}
	return 1.0; 
}
```
]

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.tiny[
```C
bool is_weekday(long day) { 
  return (day >= 1 && day <= 5);
}
bool is_morning_peak(long hour, long minute) {
  return (hour >= 6 && hour < 9) || 
		 (hour == 9 && minute <= 29);
}
bool is_evening_peak(long hour) { 
  return (hour >= 18 && hour <= 23); 
} 
bool is_midnight_peak(long hour) { 
  return (hour >= 0 && hour < 6);
}
```
]

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### Social

.tiny[
```C
// is i and j connected through some m at h+1 hops?
char is_connected(long n, char **degree_1, 
		          char **degree_h, long i, long j) {
	for (long m = 0; m < n; m += 1) { 
		if (is_friend(degree_1, i, m) && 
		    is_friend(degree_h, m, j)) {
			return FRIEND;    
		}  
	} 
	return STRANGER;
}
```
]

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### Social

.tiny[
```C
bool is_friend(char **network, int i, int j) {
   if (i >= j) {
      return network[i][j] == FRIEND;  
   } 
   return network[j][i] == FRIEND;
}
```
]

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### Decomposition

Break complex problems down into "bite-size" sub-problems that you can solve

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### Decomposition

George Polya said: "If you can't solve a problem, there is an easier problem that you can solve: find it"

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### Decomposition

- Solve the easier problem first, then generalized.
- E.g., `social`: find two-hops friends, then generalized to k hops
- E.g., `pattern`: draw the left most cell, then generalized

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### Recursion

- Assume the easier problem is solved, then generalized.
- E.g., `reverse`: assume we can reverse the rest of the numbers, then reverse the number
- E.g., `tower of hanoi`: assume we know how to move k-1 discs, then solve for k disc.

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### Four Pillars of Computational Thinking

- Decomposition
- Pattern Recognition
- Abstract
- Algorithms

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## Pattern Recognition

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### Pattern Recognition

Observe trends and patterns, then generalized

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### Taxi

- The first 1 km or less (Flag Down) 	$3.20
- Every 400 m thereafter or less, up to 10 km 	$0.22
- Every 350 m thereafter or less, after 10 km 	$0.22

(See the pattern?)

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### Taxi

| unit dist | max dist  | fare |
|-----------|-----------|------|
| Every 1000 m | next 1 km | $3.20 |
| Every 400 m  | next 9 km | $0.22 |
| Every 350 m  | next $\infty$ km | $0.22 |

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.tiny[
```C
double fare = 0;
for (int i = 0; i < NUM_TIERS; i += 1) {
    if (distance < 0) {
		  return fare;
    }
	long min_dist = min(distance, tiers[i].max_distance);
	double units = ceil(min_dist*1.0/ tiers[i].unit_distance);
	fare += tiers[i].fare * units;
	distance -= tiers[i].max_distance;   
}
```
]

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### Inversion

![inversion](../figures/scripts/scripts.002.png)

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### Identifying loop invariants is partly about pattern recognition!

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## Abstraction

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### Abstraction

- Identifying and abstracting relevant information
- Hide details
- Adapt to change
- Generalize to other domain

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### Data Abstraction

Model the problem with only the necessary information

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.fit[
![queens](figures/recursion/recursion.007.png)
]
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### Peak

- Only an array of numbers is required to model the problem.

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### Functional Abstraction

Hide details and focus on higher-level logic

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What if the peak hours are extended?  surchange increases?

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Does it matter (to this code) if jagged or rectangular array is used?

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from Lecture 1..

### Work hard
### Work very very hard

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background-image: url(figures/meme/from-students/cs1010s-from-Leonardo.png)

.tiny[Submitted by Leonardo]

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background-image: url(figures/meme/from-students/hard-to-swallow-Hao-Yi.jpg)

.tiny[Submitted by Hao Yi]

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background-image: url(figures/meme/from-students/GPA-wilson.jpg)

.tiny[Submitted by Wilson Ng]

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## What doesn't kill you only makes you stronger

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Special Thanks to

- All the students
- All the tutors
- The e-exam logistic team (Song Kai, Xinjian, Chengxin)
- Mr. Tan Hsiao Wei and his team of sys admin

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.fit[![TAs](figures/tutors/group1.png)]

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.fit[![TAs](figures/tutors/group2.png)]

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![meme](figures/meme/goodbye-cs1010.jpg)

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.tiny[
Version: 1.0

Last Updated: Sun Nov  1 11:59:04 +08 2020
]