CS1010 AY20/21 S1 Lecture 5

# Lecture 5

### 6 September 2021

**Admin Matters** 
**Unit 10:** Assertion 
**Unit 11:** Loops 
**Unit 12:** Loop Invariants

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### Reminders

- Assignment 1 due tomorrow night (23:59)
- Please read:
    - [How Not to Go About a Programming Assignment](http://people.irisa.fr/Martin.Quinson/Teaching/how-not-to-code.pdf)
    - [About Assignments in CS1010](assignments.md)
    - [General CS1010 Policies](policy.md)

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### Grading

- Assignment 1 will be graded solely on correctness (including syntax, practices, approach, and logic) and whether the requirements given are followed.
- One random question will be graded by tutors -- the rest by bot.

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### Grading

- Make sure that your submission compiles cleanly without errors or warnings.  (0 if cannot compile; -1 per warning)
- Marks will be deducted if you do not demonstrate a full understanding of what we taught so far.  E.g., wrong type, redundant conditional checks.

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### Upcoming releases:

- Exercise 2 today
- Exercise 3 on Thursday
- Assignment 2 on Thursday
    - Additional grading criteria: [style](style.md) and efficiency.
- Anonymous feedback exercise on lab tutors and tutorial instructors on Friday
- Last year's midterm question next week

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### Midterm

- **Date**: 27 September, 2021 (Monday)
- **Time**: 12noon to 2pm
- **Duration**: 60 minutes
- **Online** via Luminus Quiz
- **10%** of your grade

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### Midterm
- **Scope:**
    - Units 1-12
	- Problem Set 1-12
	- Assignments 1-2
- **Format:** MCQs, MRQs, True/False, Fill-in-the-Blanks, and Short Structured Questions
- Open book (printed/written notes ONLY)

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### Read for more details:

- [SoC E-Exam procedure](https://mysoc.nus.edu.sg/academic/e-exam-sop-for-students/)
- [CS1010 midterm instructions](midterm.md)

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### Taking Online Exam on Campus

If you need to take the online midterm or online PE on campus, please fill up the LumiNUS Survey on "Taking Midterm/PE1 on Campus" before this Friday 23:59.
 
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### Practical Exam I

- Date: 2 October 2020 (Saturday)
- Time: 9 am to 12noon
- Duration: 2 hours 30 minutes
- Online via `ssh` into restricted PE hosts
- **10%** of your grade

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### Practical Exam I

- **Scope:** same as midterm
- **Format:** Five programming problems 
    - 3 on average, 4 to ace, 5 for glory
- Open Book (printed/written notes only)
- Calculator allowed
- Criteria: correctness, style, and efficiency

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### Read for more details:

- [SoC E-Exam procedure](https://mysoc.nus.edu.sg/academic/e-exam-sop-for-students/)
- [CS1010 PE1 instructions](pe1.md)

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# Warning

### Pay attention to exam regulations.  Disciplinary actions will be taken against violators.

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### Tutorial 3

- Problem Sets 9 - 10 (maybe 11)

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### Lab 3

- Assignment 1
- Exercise 4
- Preparing for Assignment 2

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## Assertion

```
// { ... }
```

A logical expression that must always be true 
at a given point in a program.

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```C
x = 1;
// { x == 1 }
```

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```C
if (x > y) {
  max = x;
} else {
  max = y;
}
```

how can we be sure that `max` is now the larger of the two?

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```C
if (x > y) {

max = x;

} else {

max = y;

}
```
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.tiny[
```C
  if (score >= 8) {
    cs1010_println_string("A");
  } else {
    if (score >= 5) {
      cs1010_println_string("B");
    } else {
      if (score >= 3) {
        cs1010_println_string("C");
      } else {
        cs1010_println_string("D");
      }
    }
  }
```
]

When will `C` gets printed?

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if (score >= 5) {
      cs1010_println_string("B");
    } else {

if (score >= 3) {

cs1010_println_string("C");
      } else {
        cs1010_println_string("D");
      }
    }
  }
```
]

When will `C` gets printed?

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# Loops

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background-image: url(figures/lec5/lec5.001.png)

max(L,k)

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background-image: url(figures/lec5/lec5.002.png)

Loop body: what we do repeatedly

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background-image: url(figures/lec5/lec5.003.png)

Loop initialization: what we do to set up the loop

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background-image: url(figures/lec5/lec5.004.png)

Loop update: what changes from one iteration to the next?

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background-image: url(figures/lec5/lec5.005.png)

Terminating condition: when to stop repeating?

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background-image: url(figures/lec5/lec5.012.png)

Finding factorial recursively

---

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background-image: url(figures/lec5/lec5.006.png)

Finding factorial with a loop

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background-image: url(figures/lec5/lec5.007.png)

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background-image: url(figures/lec5/lec5.008.png)

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background-image: url(figures/lec5/lec5.008.png)

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background-image: url(figures/lec5/lec5.009.png)

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background-image: url(figures/lec5/lec5.010.png)

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## Assertion

```
// { ... }
```

A logical expression that must be true at a given point in a program.

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```C
x = 1;
// { x == 1 }
```
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.tiny[
```C
if (x > y) {
 max = x;
 // { (x > y) && (max == x) && (max > y) }
} else {
 max = y;
 // { (x <= y) && (max == y) && (max >= x) }
}
// { ((max == x) || (max == y)) &&
// (max >= x) && (max >= y) }
```
]

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What does `y` count?
.tiny[
```C
long count(long n) {
  long y = 0;
  long x = n;

while (x > 0) {

x -= 1;

if (x % 5 == 0) {

y += 1;

}
  }

return y;
}
```
]
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## Loop Invariant

An assertion that holds (i) before the loop; (ii) after the loop; and (iii) after each iteration of the loop.

(but may be temporarily false during the loop)

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---

.tiny[
```C
long factorial(long n)
{
 if (n == 0) {
 return 1;
 }
 long product = 1;
 long i = 1;
 while (i < n) {

i += 1;

product *= i;

}
  return product;
}

```
]
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### Homework

- Assignment 1 and 2
- Quiz for Lecture 5
- Surveys (On-Campus Exam and Anon Feedback)
- Exercise 3 and 4
- Problem Sets

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Last Updated: Mon Sep  6 11:28:13 +08 2021
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