Thanks for your time!
So I am trying to learn how to sort algorithms and watched a couple tutorial videos. Right now I am trying to declare an int variable "num" for values let's say 10-300. I am trying to create an int array "nums" that has "num" elements. Using random () to generate numbers from 1-1000 and put them in "nums". I wanted to implement 3 functions bubbleSort(), selectionSort(), and inserstionSort() that do bubble sort, selection sort, and insertion sort. After that I want to declare another int array "numt", copy all elements of nums array to numt and perform sorting in numt. After sorting completes, print the results with each number separated by a blank space. Hopefully this makes sense!
This question is a bit too broad for Stack Overflow. It's hard to answer general "how do I do this" type questions. Stack Overflow is more designed for specific "I tried X, expected Y, but got Z instead" type questions.
That being said, I'll try to help you in a general sense:
I am trying to learn how to sort algorithms
Small nitpick: you aren't sorting algorithms, you're using sorting algorithms to sort an array.
Right now I am trying to declare an int variable "num" for values let's say 10-300.
I'm not exactly sure why you need this variable, but you could do it pretty easily:
int num = 100; //assigns 100 to num
int num = int(random(10, 300)); //assigns random number between 10 and 300 to num
I am trying to create an int array "nums" that has "num" elements.
Just use the standard array syntax, passing in num as the size:
int[] nums = new int[num];
Using random () to generate numbers from 1-1000 and put them in "nums".
For this you'd use a for loop to assign each index of the array a random value.
After that I want to declare another int array "numt", copy all elements of nums array to numt and perform sorting in numt.
You can use another for loop to copy values from one array into another.
I wanted to implement 3 functions bubbleSort(), selectionSort(), and inserstionSort()
Start with something much simpler first, like a function that simply prints out the values in the array. From there you can implement more complicated logic.
do bubble sort, selection sort, and insertion sort
You can look these algorithms up on the internet. Wikipedia has entries on all of them.
But you might want to take a step back and ask yourself: how would you do this by yourself, without reading about any of these algorithms? If somebody handed you a bunch of index cards and asked you to put them in order, how would you do it? Try implementing that before you implement any other algorithms.
Also, you might want to ask yourself why you're doing this. Is it for homework? Or do you just need to sort an array? If so you might just use a built-in sorting function to do the work for you.
Related
I'm sure there may be a matlab function to do this but I'm required to write my own. As the title says, I need to write a function which when given a single cell array of strings, returns a structure array, containing the same strings but in alphabetical order. Furthermore, the 'count' fields must contain the number of times that that particular string has occurred eg
z=myfunction({'bag','dig','bag'})
ans =
str: 'bag'
count = 2
Ideally, the method should have an expected number of comparisons for n strings of O(n log n)
Assuming you don't want to use standard functions like sort or unique this is not an easy question. Furthermore it is more about math then about programming.
If you just want to practice programming, try implementing something simple like bubble sort.
This will not achieve O(n log n) however, if you really want that look into merge sort for example.
Several options are explained roughly here, and with a bit of searching it should not be hard to find what you need.
I have been given an assignment to write a program that reads in a number of assignment marks from a text file into an array, and then counts how many marks there are within particular brackets, i.e. 40-49, 50-59 etc. A value of -1 in the text file means that the assignment was not handed in, and a value of 0 means that the assignment was so bad that it was ungraded.
I could do this easily using a couple of for loops, and then using if statements to check the values whilst incrementing appropriate integers to count the number of occurences, but in order to get higher marks I need to implement the program in a "better" way. What would be a better, more efficient way to do this? I'm not looking for code right now, just simply "This is what you should do". I've tried to think of different ways to do it, but none of them seem to be better, and I feel as if I'm just trying to make it complicated for the sake of it.
I tried using the 2D array that the values are stored in as a parameter of a function, and then using the function to print out the number of occurences of the particular values, but I couldn't get this to compile as my syntax was wrong for using a 2D array as a parameter, and I'm not too sure about how to do this.
Any help would be appreciated, thanks.
Why do you need a couple for loops? one is enough.
Create an array of size 10 where array[0] is marks between 0-9, array[1] is marks between 10-19, etc. When you see a number, put it in the appropriate array bucket using integer division, e.g. array[(int)mark/10]++. when you finish the array will contain the count of the number of marks in each bucket.
As food for thought, if this is a school assignment, you might want to apply other things you have learned in the course.
Did you learn sorting yet? Maybe you could sort the list first so that you are not iterating over the array several times. You can just go over it once, grab all the -1's, and spit out how many you have, then grab all the ones in the next bracket and so on.
edit: that is of course, assuming that you are using a 1d array.
I am currently reading Cormen's "Introduction to Algorithms" and I found something called a sentinel.
It's used in the mergesort algorithm as a tool to decide when one of the two merging lists is exhausted. Cormen uses the infinity symbol for the sentinels in his pseudocode and I would like to know how such an infinite value can be implemented in C.
A sentinel is just a dummy value. For strings, you might use a NULL pointer since that's not a sensible thing to have in a list. For integers, you might use a value unlikely to occur in your data set e.g. if you are dealing with a list ages, then you can use the age -1 to denote the list.
You can get an "infinite value" for floats, but it's not the best idea. For arrays, pass the size explicitly; for lists, use a null pointer sentinel.
in C, when sorting an array, you usually know the size so you could actually sort a range [begin, end) in which end is one past the end of the array. E.g. int a[n] could be sorted as sort(a, a + n).
This allow you to do two things:
call your sort recursively with the part of the array you haven't sorted yet (merge sort is a recursive algorithm)
use end as a sentinel.
If you know the elements in your list will range from the smallest to the highest possible values for the given data type the code you are looking at won't work. You'll have to come up with something else, which I am sure can be done. I have that book in front of me right now and I am looking at the code that is causing you trouble and I have a solution that will work for you if you know the values range from the smallest for the given data type to the largest minus one at most. Open that book back up to page 31 and take a look at the Merge function. The lines causing you problems are lines 8 and 9 where the sentinel value of infinity is being used. Now, we know the two arrays are each sorted already and that we just need to merge them to get the array that is twice as big and in sorted order. This means that the largest elements in each half is at the end of the sub-arrays, and that the larger of the two is the largest in the array that is twice as big and we will have sorted once the merge function has completed. All we need to do is determine the largest of those two values, increment that value by one, and use that as our sentinel. So, lines 8 and 9 of the code should be replaced by the following 6 lines of code:
if L[n1] < R[n2]
largest = R[n2]
else
largest = L[n1]
L[n1 + 1] = largest + 1
R[n2 + 1] = largest + 1
That should work for you. I have a test tomorrow in my algorithms course on this stuff and I came across your post here and thought I'd help you out. The authors' use of sentinels in this book is something that has always bugged me, and I absolutely can not stand how much they are in love with recursion. Iteration is faster and in my opinion usually easier to come up with and grasp.
The trick is that you don't have to check array bounds when incrementing the index in only one of the lists in the inner while loops. Hence you need sentinels that are larger than all other elements. In c++ I usually use std::numeric_limits<TYPE>::max().
The C-equivalent should be macros like INT_MAX, UINT_MAX, LONG_MAX etc. Those are good sentinels. If you need two different sentinels, use ..._MAX and ..._MAX - 1
This is all assuming you're merging two lists that are ordered ascending.
I am trying to divide arrays recursively... I think that is what this would be called haha....
For instance, lets say the initial array contains 50 values the highest being 97 and the lowest being 7... I want to split this array into two, dividing them based on whether they are greater or lower than the midrange of the entire set. The midrange being 52...( (97+7)/2 )
Then I want to divide these two arrays using the same method and so on, ideally having a program that repeat this process an arbitrary number of times....
Load Values into array1
Find Midrange
For every value in array1{
if value > midrange{
assign value to ArrayHigh1}
Else{ assign value to ArrayLow1}
}
Perform same thing on ArrayHigh1 and ArrayHigh2
Etc etc etc.
I'm having trouble figuring out how I would create the successive arrays (ArrayHigh2 3 4 etc)
Also, I feel like there must be an easier way to do this, but I cannot think of one at the moment...
Thanks for the help
You seem to be working your way towards a B-tree or an implementation of Merge- or Quicksort. Plenty of reference implementations are available online.
Though speaking generally, you might benefit greatly from reading a book many here are familiar with.
I totally understand the purpose of arrays, yet I do not feel I have "mastered" them. Does anyone have some really good problems or readings involving arrays. I program in PHP and C++ so if there are examples with those languages that would be preferable but is not necessary.
Draw everything out on graph paper.
Memory is just little boxes, it's a lot clearer to see on paper with a few arrows than in some complex markup language
Define 'mastered'.
Does anyone have some really good problems or readings involving arrays.
Try array based
Linked-list implementation.
Implement stacks, queues, etc and then use stack(s) to emulate a queue etc
Heaps
A lot of people seem to struggle with the concept of arrays at first, particularly arrays of >2 dimensions.
It's a little too abstract. However, getting past that initial block just requires a concrete exploration of the mechanics. So, here's an example I've used a lot that shows the basic mechanics in a nerd-friendly way:
Concrete Example (in psuedocode)
Let's say you're building a role playing game and you want to keep track of your character's stats. You could use an array of integers like this:
Stats(0) could be strength
Stats(1) could be dexterity
Stats(2) could be intelligence
...And so on.
Now let's add a level of complexity. Maybe we want to introduce a potion of strength that increases strength by 5 for 10 turns. We could represent the stat side of that by making this into a 2-dimensional array:
Stats(0, 0) - this is my current strength.
Stats(0, 1) - this is my normal strength.
Stats(0, 2) - this is the number of turns until the strength potion wears off.
Stats(1, 0) - this is my current dexterity.
...And so on. You get the idea.
So I've added a 2nd dimension to hold details about the 1st dimension. What if I wanted our Stats array to handle statistics for more than one character? I could represent that by making this into a 3-dimensional array:
Stats(0, 0, 0) - character 0's current strength.
Stats(1, 1, 0) - character 1's current dexterity.
It would be even better to create some constants or enums to eliminate magic numbers from the code:
Const Strength = 0
Const Dexterity = 1
Const Intelligence = 2
Const CurrentValue = 0
Const NormalValue = 1
Const PotionTurns = 2
Then I could do:
Stats(1, Dexterity, NormalValue) = 5 'For character 1, set the normalvalue of dex to 5.
A few more thoughts about arrays... At least in the .Net world where I live, most of us don't have to use them in our day-to-day lives too often because they're slowly being relegated to underpinnings for more elaborate data structures like collections.
In fact, if I were to implement character stats, realistically I would not use arrays.
However, it's still important to get your head around them because they are rocket-fast and there are definitely cases where they're invaluable.
Try manually (no built in methods) sorting with arrays (bubblesort is a good one to get yourself going)
An array is a contiguous block of memory devoted to N items of the same type, where N is a fixed number indicating the number of items. In order to expand an array (as they are fixed in size), you can use the C function realloc(..). In C++, you can manually re-allocate an array by creating a new, larger array and copying the contents of the old array into the new one (an expensive operation).
Modern languages have options that supplant arrays (so as to overcome the inherent limitations of arrays). In C++, you can use the Standard Template Library (STL) for this purpose; the STL "vector" data type is a typical replacement for standard arrays in C++. Platform-dependent APIs like MFC have built-in constructs like the CArrayList for a richer array usage experience. ;-)
I'm going to try to explain the best i can arrays in their fundamental form.
Ok, so an array is basically a way to store data. For instance if you want a list of shopping items, we would use a one dimensional array:
[0] - "Bread"
[1] - "Milk"
[2] - "Eggs"
[3] - "Butter"
.
.
.
[n] - "Candy"
Each index 0,1,2,3,...n holds a specific data. The data as you can see are represented as Strings. Now i can't use something like :
[n+1] = 1000
because this will put an integer as index n+1, the compiler will tell you that it's no good and you need to fix that issue.
Moving on to matrices or 2-dimensional arrays. Take a piece of squared paper like the ones you use for math and draw a Cartesian system and some dots. Put the coordinates on different piece of paper and next to them put a 1. For example:
[0,0] = 1
[0,1] = 1
[2,3] = 1
What this means is that at index [0,0],[0,1],[2,3] i have 1. A representation would be like so:
Cartesian System in form of a matrices :
1) 2) 3)
1) 1 1 0
2) 0 0 1
3) 0 0 0
I used just simple arrays to illustrate what are they, for example if you want to go 3D the data structure for that would be a array of matrices aka a list which holds each Cartesian location at a specific height.
If we had 10 as height and the same dots as above it would be something like:
[10,0,0] - 1
[10,0,1] - 1
[10,2,3] - 1
If you want a way to master: grab a simple list of problems and try to implement them using arrays of any kind. There is no quick way to do it, just have patience and practice.
Alright, so a one-dimensional array is just a grouping of variables. Useful if you've got a lot of something, and you like to save time and space. Functionally,
element1:=5;
element2:=6;
element3:=7;
...
is the same as saying
element[1]:=5;
element[2]:=6;
element[3]:=7;
...
except now the computer knows what you're talking about and you can write something like:
for i:=1 to n do
element[i]:=element[i]+1;
Moving on, a two dimensional array is somewhat more complicated, but can be thought of as an array of arrays. So we could have something like this:
type
arrayA=array[1..50] of integer;
arrayB=array[1..50] of arrayA;
arrayB=array[1..50,1..50] of integer; //an equivalent declaration in Pascal to the above two
More specifically, a two-dimensional array is a table. So for example, if arrayA contains the grades of a student in 50 classes, then arrayB represents the grades of a bunch of students. So, arrayB[3,5] would be the grade of the third student on class number 3.
It's easy to expand the same logic to add another dimension to the array:
arrayC=array[1..50] of arrayB;
We could say that C represents a school, so arrayC[2,4,6] is the grade of the second schools fourth student in class 6.
Now, what are arrays used for? Storing groups of similar information, or information which'll need to be processed in bulk. In practice, though, you'll mostly be using a one-, at most two-, dimensional array. If you can imagine your data as a table, you'll almost definitely want an two-dimensional array, for example. How else would you represent a chess board, if you had to?
Three-dimensional arrays tend to be used less, but what if you had to save some sort of state for each of your table elements? Something like:
Table=array[1..50, 1..50, 0..1] of integer;
Then the third value is 1 if some condition is true for a given element, and 0 otherwise. Of course, a trivial example, but it's another way of understand three-dimensional arrays more easily.