CS1010 AY20/21 S1 Lecture 4

class: middle, center

# Lecture 4

### 31 August 2020

Unit 8: **Conditionals**<br>
Unit 9: **Logical Expression**<br>
Unit 10: **Assertion**<br>

---
class: middle

## Tutorial/Lab 2

- Problem sets from Units 3, 5, and 8. 
- Prepare you for Assignment 1

---
class: middle

## Assignment 1

- Release this Thursday morning
- Due next Tuesday 23:59

---
class: middle

## Survey

- Are you coping with online lecture/lab in CS1010?
- We want to know: Luminus -> CS1010 -> Survey

---
class: middle

## Mock Exam Session

- Register by 5pm TODAY
- Read [SoC's Procedure for Mock Exam](https://mysoc.nus.edu.sg/wp-login.php?redirect_to=%2Facademic%2Fmock-e-exams-for-students%2F)

---
class: middle

## 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)

---
class: middle
<table width=100% height=100%>
	<tr style="height: 50%">
	<td style="border-right:solid 1px; border-bottom:solid 1px;">
		<h5 style="margin:0px";>
			Problem Solving
		</h5>
		<ul style="margin:1px;font-size:80%;">
		<li>decomposition
		<li>recursion
		<li>flowchart
		<li>
		<li>
		</ul>
	</td>
	<td style="border-bottom:solid 1px;">
		<h5 style="margin:0px";>
			C Language/Syntax
		</h5>
		<ul style="margin:1px;font-size:80%;">
		<li>types 
		<li>functions 
		<li>arithmetic ops 
		<li>
		<li>
		</ul>
	</td>
	</tr>
	<tr style="height: 50%">
	<td style="border-right:solid 1px; border-bottom:none;">
		<h5 style="margin:0px";>
			Behavioural/Mental Model
		</h5>
		<ul style="margin:1px;font-size:80%;">
		<li>machine code
		<li>data in memory
		<li>types
		</ul>
	</td>
	<td style="border-bottom:none;">
		<h5 style="margin:0px";>
			Tools/Good Practices
		</h5>
		<ul style="margin:1px;font-size:80%;">
		<li>clang
		<li>vim
		<li>bash
		</ul>
	</td>
	</tr>
</table>
---
class: middle

<table width=100% height=100%>
	<tr style="height: 50%">
	<td style="border-right:solid 1px; border-bottom:solid 1px;">
		<h5 style="margin:0px";>
			Problem Solving
		</h5>
		<ul style="margin:1px;font-size:80%;">
		<li>decomposition
		<li>recursion
		<li>flowchart
		<li>conditionals
		<li>assertion
		</ul>
	</td>
	<td style="border-bottom:solid 1px;">
		<h5 style="margin:0px";>
			C Language/Syntax
		</h5>
		<ul style="margin:1px;font-size:80%;">
		<li>types 
		<li>functions 
		<li>arithmetic ops 
		<li>if-else
		<li>logical expression
		</ul>
	</td>
	</tr>
	<tr style="height: 50%">
	<td style="border-right:solid 1px; border-bottom:none;">
		<h5 style="margin:0px";>
			Behavioural/Mental Model
		</h5>
		<ul style="margin:1px;font-size:80%;">
		<li>machine code
		<li>data in memory
		<li>types
		</ul>
	</td>
	<td style="border-bottom:none;">
		<h5 style="margin:0px";>
			Tools/Good Practices
		</h5>
		<ul style="margin:1px;font-size:80%;">
		<li>clang
		<li>vim
		<li>bash
		</ul>
	</td>
	</tr>
</table>
---
class: middle,wide

.fit[![max](../figures/max-flowchart/max-flowchart.001.png)]

---
class: middle,wide

.fit[![max](../figures/max-flowchart/max-flowchart.010.png)]

---
class: middle,wide

.small[
```C
long factorial(long n)
{
  long answer;
  if (n == 0) {
    answer = 1;
  } else {
    answer = n * factorial(n - 1);
  }
  return answer;
}
```
]

---
class: middle

| Score                       | Letter Grade |
| --------------------------- | ------------ |
| 8 or higher                 | A            |
| Less than 8 but 5 or higher | B            |
| Less than 5 but 3 or higher | C            |
| Less than 3                 | D            |

---
class: middle

| Score       | Letter Grade |
| ----------- | ------------ |
| 8 or higher | A            |
| Less than 8 | See Table 1  |

---
class: middle

Table 1 (less than 8)

| Score       | Letter Grade |
| ----------- | ------------ |
| 5 or higher | B            |
| Less than 5 | See Table 2  |

---
class: middle

Table 2 (less than 5)

| Score       | Letter Grade |
| ----------- | ------------ |
| 3 or higher | C            |
| Less than 3 | D            |

---
class: middle

.tiny[
```C
void print_score(double score)
{
  if (score >= 8) {
    cs1010_println_string("A");
  } else { // Table 1
    if (score >= 5) {
      cs1010_println_string("B");
    } else { // Table 2
      if (score >= 3) {
        cs1010_println_string("C");
      } else {
        cs1010_println_string("D");
      }
    }
  }
}
```
]

---
class: middle

Rewritten with `else if` 
.tiny[
```C
void print_score(double score)
{
  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");
  }
}
```
]
---
class: middle

This is accepted in C (but not CS1010)
.tiny[
```C
void print_score(double score)
{
  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");
}
```
]
---
class: middle

When will cause `good effort` to be printed?

.tiny[
```C
void print_score(double score)
{
  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");
    cs1010_println_string("good effort");
}
```
]

---
class: middle

What will get printed in `score` is 5?

.tiny[
```C
if (score >= 8)
  if (late_penalty != 0)
      cs1010_println_string("late submission");
else
  cs1010_println_string("you can do better!");
```
]

---
class: middle,center

.fit[![meme](figures/meme/bad-good-curly.png)]

---
class: middle

.smaller[
```C
if (max_so_far < x) {
  max_so_far = x;
} else {
}
```
can be simplified to
```C
if (max_so_far < x) {
  max_so_far = x;
}
```
]

---
class: middle

Set `max` to the larger of `x` and `y`

.smaller[
```C
if (x > y) {
  max = x;
}
if (x < y) {
  max = y;
}
```
]

What is wrong?

---
class: middle

.smaller[
```C
if (x > y) {
  max = x;
}
if (x < y) {
  max = y;
}
if (x == y) {
  max = y;
}
```
]

---
class: middle

.smaller[
```C
if (x > y) {
  max = x;
}
if (x <= y) {
  max = y;
}
```
]

---
class: middle

.smaller[
```C
if (x > y) {
  max = x;
} else {
  max = y;
}
```
]

---
class: middle

.smaller[
```C
if (x > y) {
  max = x;
} else {
  max = y;
}
```
]

---
class: middle
```C
long factorial(long n)
{
  long answer;
  if (n == 0) {
    answer = 1;
  } else {
    answer = n * factorial(n - 1);
  }
  return answer;
}
```
]

---
class: middle
```C
long factorial(long n)
{
  if (n == 0) {
    return 1;
  } else {
    return n * factorial(n - 1);
  }
}
```

---
class: middle

`else` can be skipped

```C
long factorial(long n)
{
  if (n == 0) {
    return 1;
  } 
  return n * factorial(n - 1);
}
```
---
class: middle

The `?:` operator.

.tiny[
```C
long factorial(long n)
{
  return n == 0 ? 1 : n * factorial(n - 1);
}
```
]

---
class: middle,center

.fit[![meme](figures/meme/bad-good-tertiary.png)]

---
class: middle

What could go wrong?

.tiny[
```C
double z;

if (z == 0.03) {
  :
}
```
]

---
class: middle

Comparing floating-point numbers:

.tiny[
```C
double z;
double e = 0.0000001; // epsilon
if (fabs(z - 0.03) < e) {
  :
}
```
]

---
class: middle,center

.fit[![meme](figures/meme/bad-good-fp-equal.png)]

---
class: middle

The `bool` type in C can take either the value `true` or `false`.

To use:
```C
#include <stdbool.h> 
```

---
class: middle

Old fashion C uses `int` as boolean.  `0` for false, non-zero for true.

```C
if (x - y) {

}
```

---
class: middle,center

.fit[![meme](figures/meme/bad-good-bool.png)]

---
class: middle

- Comparison operations : `>`, `<`, `>=`, `<=`, `==`, `!=`
- Logical operators : `&&`, `||`, `!`

---
class: middle,center

## De Morgan's Law

`!(e1 && e2)`

is the same as

`(!e1) || (!e2)`

---
class: middle,center

## De Morgan's Law

`!(e1 || e2)`

is the same as

`(!e1) && (!e2)`

---
class: middle,center

## Short Circuiting

### `a && b`
### `a || b`

---
class: middle

Which one is better?

.tiny[
```C
if (number < 100000 && is_prime(number)) {
 :
}
```
]

.tiny[
```C
if (is_prime(number) && number < 100000) {
 :
}
```
]

---
class: middle,center

## Assertion

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

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

---
class: middle

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

---
class: middle

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

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

---
class: middle

```C
if (x > y) {

max = x;

} else {

max = y;

}
```
---
class: middle

.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?

---
class: middle

.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?

---
class: bottom

.tiny[
Version: 1.1

Last Updated: Mon Aug 31 11:05:14 +08 2020
]