Tracing/Conversion
Name the two functions convert cell arrays to arrays or double arrays to cell arrays, respectively.
cell2mat & num2cell
What happens when you have an NxM array, arr, and do the following operation?
arr(:)
It linearizes the array, turning it into a NxM column vector
When is it better to use if statements and when is it better to use switch statements?
if: continuous values (integers)
switch: discrete values (strings, limited integers)
Explain when you should use a while loop and when you should use a for loop
While: when you DON'T know how many times you are iterating beforehand
For: when you DO know how many times you are iterating beforehand
What is the difference between {} and () when indexing with Cell Arrays?
{}: obtain the actual value WITHIN the cell
(): obtain the entire cell(s) with all the data still contained within their individual cells
Given the following direct for loop, turn it into an indirect for loop.
out = [];
for x = vec
out = [out, mod(x,2)];
end
out = [];
for x = 1:length(vec)
out(x) = mod(vec(x),2);
end
Given a NxM array, arr, output a (N/2)x(M/2) array of the bottom left quadrant. N and M are even numbers.
out = arr(end/2+1:end, 1:end/2);
What are the 7 saved words we use for if and switch statements (i.e. key words we use to create these statements)
if, elseif, else, switch, case, otherwise, end
What is the code to delete every other element in a vector (i.e. delete every even INDEX) USING ITERATION?
for i = length(vec):-2:1
vec(i) = [ ];
end
Given a Nx1 cell array, col, append that to the end of (N+1)xM cell array, ca, with the header, 'People', to create a new cell array that is (N+1)x(M+1)
col = [{'People'}; col];
ca = [ca, col];
Given the following for loop, convert it to a while loop.
vec = [ ];
for i = 1:5
vec(i) = i^2;
end
vec = [ ];
i = 1;
while i < 6
vec(i) = i^2;
i = i + 1
end
Given a doubles NxM array, find the sum of all the numbers in the array that are less than 10.
mask = arr < 10;
summ = sum(arr(mask));
What is the value of out when the following code is run:
num = 8;
if mod(num,2) == 1 & num == 8
out = 1;
elseif mod(num,2) == 0 & num < 12
out = 2;
else
out = 3;
end
2
If we wanted to iterate through each element of a NxM array, arr, what would be the "template" we would want to use?
[r,c] = size(arr);
for i = 1:r
for j = 1:c
%Code using arr(i,j)
end
end
Given a NxM cell array, ca, with a header row, how do you isolate JUST THE DATA of the column with the header 'Age'?
headers = ca(1,:);
data = ca(2:end,:);
mask = strcmp(headers, 'Age');
ageCol = data(:, mask);
Given the following switch statement, convert it to an if statement.
switch var
case {1, 3}
str = 'It is a great number!';
case 'Joshua'
str = 'It is a cool person!';
otherwise
str = 'I don''t care!';
end
if var == 1 | var == 3
str = 'It is a great number!';
elseif strcmp(var,'Joshua')
str = 'It is a cool person!';
else
str = 'I don''t care!';
end
Given a NxM array of doubles, delete the columns with a below average sum.
sums = sum(arr);
avg = mean(sums);
mask = sums >= avg;
arr = arr(:,mask);
Given a 1xN vector, vec, of a given datatype, do the following:
If double, sort it in ascending order.
If char, sort it in descending order.
If logical, reverse the vector.
if isnumeric(vec(1))
vec = sort(vec,'a');
elseif ischar(vec(1))
vec = sort(vec,'d');
else
vec = vec(end:-1:1);
end
Using iteration, determine the largest number than is divisible by 3 in a given vector, vec. The max function is banned.
maxx = 0;
for x = vec
if mod(x,3) == 0 & x > maxx
maxx = x;
end
end
Given a NxM cell array, ca, where the first COLUMN are names and the remaining columns contain test scores, delete all the rows where the average test score of that row is below 80.
data = cell2mat(ca(:,2:end));
avg = mean(data,2);
mask = avg < 80;
ca(mask,:) = [];
What does the following code accomplish given an input string, str.
ca = {};
[word, rest] = strtok(str);
while ~isempty(word)
mask = 'a' >= lower(word) & lower(word) <= 'z';
word = upper(word(mask));
ca = [ca; {word}];
[word, rest] = strtok(rest);
end
It creates a vertical cell array where each cell contains a word (in order) from the string and each word is uppercase & only contains letters.
Given an odd number, N, create a NxN array that is 'o' everywhere except on the diagonal, where it will be 'x' (i.e. create a square array that makes a big X)
arr = repelem('o',N,N);
arr(1:N+1:end) = 'x';
arr(N:N-1:end) = 'x';
Given a variable, str, write a SWITCH statement that does the following:
When str is the string 'joshua' (any combination of capital or lowercase letters), output 'Cool'
When str is the string 'pete' or 'Savannah' (any combination of capital or lowercase letters), output 'Less Cool'
When str is none of the TA names, output 'irrevelant'
switch lower(str)
case 'joshua'
out = 'Cool';
case {'pete','savannah'}
out = 'Less Cool';
otherwise
out = 'irrevelant';
end
What is the code to reverse every individual word in a string, str (string only contains spaces and letters)?
newStr = [ ];
[word, rest] = strtok(str);
while ~isempty(word)
newStr = [newStr, ' ', word(end:-1:1)];
[word, rest] = strtok(rest);
end
newStr(1) = [ ];
Given a NxM cell array, ca, where the first row is a header column, sort the data in the cell array by the column with the header 'Name', which contain string values, in descending order. Your output should be the entire cell array.
headers = ca(1,:);
data = ca(2:end,:);
mask = strcmp(headers,'Name');
names = ca(:,mask);
[~,inds] = sort(names);
inds = inds(end:-1:1);
data = data(inds,:);
ca = [headers; data];