vault backup: 2024-01-25 14:02:02

Semester 2/Computer Systems Internals & Linux/Week 2/delete.sh

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+#!/bin/bash
+if [ $# -ne 1 ]; then
+	echo You need to give just one argument
+elif [ -d "$1" ]; then
+	echo Deleting directory \"$1\"
+	mv "$1" ~/wastebasket/
+elif [ -f "$1" ]; then
+	echo Deleting file \"$1\"
+	mv "$1" ~/wastebasket/
+else
+	echo \"$1\" does not exist
+fi

Semester 2/Computer Systems Internals & Linux/Week 2/delete2.sh

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+#!/bin/bash
+if [ $# -ne 1 ]; then
+	echo You need to give just one argument
+elif [ -e ~/wastebasket/"$1" ]; then
+	echo \"$1\" already exists in wastebasket, so I will not delete
+elif [ -d "$1" ]; then
+	if [ $(ls -A "$1" | wc -l) -gt 0 ]; then
+		echo Directory \"$1\" is not empty, so I will not delete
+	else
+		echo Deleting directory \"$1\"
+		mv "$1" ~/wastebasket
+	fi
+elif [ -f "$1" ]; then
+	echo Deleting file \"$1\"
+	mv "$1" ~/wastebasket
+else
+	echo \"$1\" does not exist
+fi

Semester 2/Computer Systems Internals & Linux/Week 2/script4.sh

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+#!/bin/bash
+num=1;
+while [ $# -gt 0 ]; do
+	echo argument $num is \"$1\".
+	shift
+	let num++
+done 

Semester 2/Computer Systems Internals & Linux/Week 2/script5.sh

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+#!/bin/bash
+cat ~/.bash_history | while read line; do
+	if echo $line | grep -Eq "^(mkdir|cd)"; then
+		dir=$(echo $line | sed -r 's/(mkdir|cd) +//')
+		if [ -d "$dir" ]; then
+			echo "YES: $dir"
+		else
+			echo "NO: $dir"
+		fi
+	fi
+done

Semester 2/Computer Systems Internals & Linux/Week 2/sounds.sh

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+#!/bin/bash
+file=1;
+for audio in media/*.mp3; do
+  if [ $audio != "media/footsteps_forest_01.mp3" ]; then
+    echo Processing $audio
+    filename="00$file"
+    filename=${filename:(-3)}
+    sox $audio temp.wav silence 1 0 0
+    sox media/footsteps_forest_01.mp3 temp.wav -C 128 $filename.mp3
+    rm temp.wav
+    let file=file+1
+  fi
+done

Semester 2/Computer Systems Internals & Linux/Week 2/thumbs.sh

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+#!/bin/sh
+date=$(date "+%a %d-%m-%Y %H:%M")
+for image in media/*.jpg; do
+  filename=$(basename $image)
+  if [ -e thumbs/$filename ]; then
+    echo $filename has already been converted
+  else
+    echo Converting $filename
+    convert $image -resize "640x480>" -rotate 90 -background magenta -pointsize 15 -gravity Center label:"$date" -append -rotate -90 thumbs/$filename
+  fi
+done

Semester 2/Computer Systems Internals & Linux/Week 2/undelete.sh

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+#!/bin/bash
+dir=$(dirname "$1")
+file=$(basename "$1")
+if [ $# -ne 1 ]; then
+	echo You need to give just one argument
+elif [ ! -e ~/wastebasket/"$file" ]; then
+	echo \"$file\" does not exist in wastebasket
+elif [ -e "$1" ]; then
+	echo \"$1\" already exists, so I will not undelete
+else
+	echo Restoring \"$file\" to \"$dir\"
+	if [ ! -d "$dir" ]; then
+		mkdir "$dir"
+	fi
+	mv ~/wastebasket/"$file" "$dir"
+fi

Semester 2/Computer Systems Internals & Linux/Week 2/videos.sh

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+#!/bin/bash
+for video in media/*.mp4 media/*.mov; do
+  if [ ! -e $video ]; then
+    echo No videos to convert
+    exit 1
+  fi
+  echo Processing $video
+  dir=$(dirname "$video") 
+  file=$(basename "$video" .mp4)
+  file=$(basename "$file" .mov)
+  avconv -i $video -s 80x50 $dir/$file.mpg
+
+  mv $video $video.backup
+done

Semester 2/Database Systems/Week 2/Week 2 Database Systems.md

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+# Query Optimisation
+
+Dominant cost in query processing is secondary storage access. The fewer blocks accessed, the faster the database queries can be.
+Many tuples will fit into a single block, requiring the query to: find the block, find the tuple, edit the tuple, and place it back in the block. This is inefficient if we are doing many different queries accessing different blocks.
+
+# Types of Index
+
+## Primary Index
+
+Data file sequentially ordered by ordering key field, indexing field is built on the ordering key field. Guaranteed to have unique value for each tuple.
+
+## Clustering Index
+
+Data file sequentially ordered on non-key field, indexing field built on same non-key field. Can be more than one tuple corresponding to a value in the indexing field.
+
+## Primary / Clustered Indices
+
+Affect the order the data is stored in a file.
+
+## Secondary Indices
+
+Give a lookup table to the file.
+
+# Index Restrictions
+
+- Table can have 1 primary index OR 1 clustering index.
+	- Most frequently looked up value is often the best choice
+	- Some DBMS' assume PK is primary index, as it is usually used to refer to rows
+
+# Exercise
+
+1. What is a Query Tree?
+A query tree is a visual model used to represent a logical model of database queries. Each leaf node represents a relation. Each internal node is a different query function ex. selection, projection, product, etc.
+
+2. Write a relational algebra expression for the following query
+
+```sql
+SELECT lecName, schedule 
+FROM lecturer, module, enroll, student 
+WHERE lastName=“Burns” 
+AND firstName=“Edward” 
+AND module.moduleNumber=enrol.moduleNumber 
+AND lecturer.lecID=module.lectID 
+AND student.stuID=enrol.stuID;
+```
+![](Pasted%20image%2020231121134251.png)
+
+pi lecName, schedule 
+( sigma lastName = burns
+	( sigma firstName = edwards 
+		( sigma module.moduleNumber=enrol.moduleNumber 
+			( sigma (lecturer.ledID=module.lectID (lecturer x module ) x enrol ) x student
+		)
+	)
+)
+
+Draw a Query Tree for this SQL query:
+
+3. Why can we not use SQL for query optimisation?
+Using relational algebra is easier for us to visualise, and less abstracted than SQL, allowing us to optimise the query flow.
+
+4. List the heuristics that optimisers use to reduce optimisation cost.
+- Begin with initial query tree for SQL
+- Move SELECT operations down the tree
+- Apply more restrictive SELECT operations first ( eg. equalities before range queries )
+- Replace Cartesian products followed by selection with theta joins ( eg. *sigma(f) ( RxS )* -> *R theta(f) S* )
+- Move PROJECT operations down the query tree ( add project operations as inputs to theta joins ).
+
+5. Draw a near optimal query tree for the following SQL query, and write a relational algebra expression for this tree.
+```sql
+SELECT sailors.name 
+FROM sailors, reservations 
+WHERE reservations.sID=sailors.ID 
+AND reservations.bID=100 
+AND sailors.rating=7;
+```
+	
+![](Pasted%20image%2020240123164006.png)

Semester 2/HCI/Week 2/Week 2 Human Computer Interfaces.md

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+#