Assignment question based on excel file I/O

Write a function called get_distance that accepts two character vector inputs representing the names of two cities. The function returns the distance between them as an output argument called distance. For example, the call get_distance('Alexandria, VA' , 'Allentown, PA') should return 195

. If one or both of the specified cities are not in the file, the function returns -1.

The code I tried is the following:

I get error in the " if text{i,1} == a" and "if text{1,j} == b" lines stating matrix dimensions must agree. Will be really helpful if someone can help me with it.

function distance = get_distance(a,b)
[~,text,raw] = xlsread('Distances.xlsx');
for i = 2 : size(text,1)
    if text{i,1} == a
       for j = 1 : size(text,2)
           if text{2,j} == b
               distance = raw{i,j};
           else
               distance = -1;
           end
       end
    else
        distance = -1;
    end
end

[Siddharth Vidyarthi]

have u got the correct answer for this?

if u got please post the code here

[Sanket khullr]

function distance = get_distance(a,b)
[~,text,raw] = xlsread('Distances.xlsx');


    for i=2:size(raw,1)
      if strcmp(text{i,1}, a)
          break
      end
    end
    if i>=size(raw,1)
        distance=-1;
    else
       for j=2:size(raw,2)
      if strcmp(text{1,j}, b)
          distance=raw{i,j};
          break
      end
       end
       if j>=size(raw,2)
       distance = -1;
       end
    end
       
     

[Guillaume]

@sanket,

Loops are not needed at all for this. strcmp is perfectly capable of the searching a whole cell array at once.

[Walter Roberson]

This code is wrong for the case where the match is the last row or column. You break out of the loop but the value of the index happens to exactly equal to the size so you think there was no match.

Answer 1

[Geoff Hayes]

Ajai - if you try to compare strings using the == operator, then you will see this error if the two strings being compared are of different lengths. Instead, you should be using strcmp or strcmpi to check for equality between two strings. Your code would then change from

text{i,1} == a

to

strcmp(text{i,1}, a)

Also, rather than using a loop, you could use find to see if the first column of text contains the cities that you are looking for (see the examples in the provided link for details).

[Ajai Kannan]

Thank you so much Geoff! That helped a lot!

[Tashu Bar]

I am facing some issue in writing code using find for simillar problem.

Geoff can you help to write the code,

[Priyamvada Shankar]

Ajai Kannan if you got the correct code.. please post here

[Walter Roberson]

This is obviously an assignment. We seldom post the code to answer assignments. However, if you post your attempt and indicate the error messages you are encountering, then we will assist you in debugging what you have.

Answer 2

[ABINAND PANDIYAN]

% This problem is right I think.
function distance = get_distance(A,B)
[~,~,dist] = xlsread('Distances.xlsx');


    for i=2:(size(dist,1)+1)
        if i>size(dist,1)
            distance=-1;
            return
        end
        if strcmp(dist{i,1}, A)
          break
      end
    end
       for j=2:(size(dist,2)+1)
      if j> size(dist,2)
          distance=-1;
          return
      end
           if strcmp(dist{1,j}, B)
          distance=dist{i,j};
          break
      end
       end
       
end
              

[Walter Roberson]

Suppose that A happens to exactly match the city name in the last row. Then i = size(dist,1) and success would be desired. The for loop would execute and on the last iteration, strcmp(dist{i,1},A) would succeed, so you would break; with i = size(dist,1). Then you do if i>=size(dist,1) and find that i == size(dist,1) so the if succeeds, and you declare the distance to be -1 and return, even though you had a match.

In the case where there was no match at all, the for i loop would complete, and afterwards i would be size(dist,1), same as if there had been a match on the last item. You test i>=size(dist,1) and that is true so you return -1, same as the case where you had a city match on the last row.

In MATLAB, after a for loop, if the body of the for did not change the loop variable, then the loop control variable is left at the last value it had, not one more than the last value it had. You will need to fix that termination condition.

[ABINAND PANDIYAN]

@Walter Roberson

Yeah I checked that, But now after hours of brainstroming I think I got the answer right with the edited content. You can check it now.

Answer 3

[Olel Arem]

function distance=get_distance(city_1,city_2)
n=0;m=0;
[value,name]=xlsread('Distances.xlsx');
for ii=1:size(name,2)
    if (strcmp(city_1,name(ii,1)))
        n=ii;
    end
end
for jj=1:size(name,1)
    if (strcmp(city_2,name(1,jj)))
        m=jj;
    end
end
if(n==0 && m==0 && n==1&&m==1)
    distance =-1;
elseif (n>0&&m>0)
    distance=value(n-1,m-1);
else
    distance=-1;
end

Answer 4

[Fazlul Haque]

done without using loops

[Walter Roberson]

You can omit the find() step: your a and b will be logical indexes that should work fine to pull out the needed data.

[Suraj Gawade]

It will not show value for non-existant cities.

[Walter Roberson]

The posted code appears to return -1 for non-existent cities.

[Krashank Kulshrestha]

so what should we do in this code I applied 0 in place of -1 its showing error in non existing cities

[Krashank Kulshrestha]

also please tell the reason behind applying -1 rather than 0

[Walter Roberson]

It is a requirement of the assignment, ". If one or both of the specified cities are not in the file, the function returns -1."

In terms of why, the answer is that it is valid for someone to ask about the distance between a city and the same city, and the distance that should be returned for that would be 0. If you returned 0 for unknown cities as well, then the 0 that was returned for city-to-itself could not be distinguished from the 0 that would be returned for the unknown-city case.

The place I live now, historically was a small village. There was a slightly larger village what is now two blocks away, half a kilometre. But historically miles were used, and to the nearest mile, the distance rounds down to 0. It is valid to ask about the distance between the two places, and the answer would be validly 0 ("to the nearest mile"), and that is a different situation than asking about the distance and being given a response that does not allow me to tell whether the two places are very close or if it does not have any information about one of the places.

[Farhaan Zaidi Bhat]

Can you please explain the a and b steps? The data part?

[Rik]

Did you read the documentation for the strcmpi function?

Answer 5

[Muhammad Qaisar Ali]

function distance = get_distance(city_1,city_2)
    persistent distt city_names;
    if isempty(distt) && isempty(city_names)                % Ceck wheater first time load or not.
        [distt,city_names,~]=xlsread('Distances.xlsx');     % Reading ditances and names of cities
    end
    rowIndex = find(ismember(city_names(:,1),city_1));   % Searching city_1 in col_1 of excell seet.finding row index.
    colIndex = find(ismember(city_names(1,:),city_2));   % Searching city_2 in row_1 of excell seet.finding col index.
    if isempty(rowIndex) || isempty(colIndex)           % If cities nit found return -1.
       distance=-1;
    else                                                % Find and retun distance.
        distance=distt(rowIndex-1,colIndex-1);
    end 
end

[Walter Roberson]

Caching the data is often a good performance improvement, but remember to document the method that would need to be used to signal that the Distances file had changed.

[Muhammad Qaisar Ali]

Thanks for your vaulable feedbak..indeed it helps me alot..

Answer 6

[Priyamvada Shankar]

Can you please write the code...it will be a great help... please I have tried too many times but still getting some problems

[Siddharth Vidyarthi]

function distance = get_distance(a,b)

[~,text,raw] = xlsread('Distances.xlsx');

for i = 2 : size(text,1)

if strcmp(text{i,1}, a)

for j = 1 : size(text,2)

if strcmp(text{2,j}, b)

distance = raw{i,j};

else

distance = -1;

end

end

else

distance = -1;

end

end

please tell whats wrong in this code its showing error

[Walter Roberson]

You do not stop looping when you find a match.

[Siddharth Vidyarthi]

can u please eloborate in which line i have to stop?

should i use end command?

[Walter Roberson]

The empty line after you assign raw to distance.

end is not a command. You want to find some way to break out of the loop early.

[Siddharth Vidyarthi]

so how should i end the loop after that please help me i am getting error again and again

[Walter Roberson]

You want to find some way to break out of the loop early.

[sadek kouz]

use break to break out of the looo

Answer 7

[XINYI CAI]

why my answer does not work for non-exsitent city?

[Rik]

Did you use the debugger to determine the shape and type of your variables? What value would row have for a non-existent city?

[XINYI CAI]

@Rikoh i see, variables row and column are arrays with a number of 0 instead of empty arrays, I should change column=value1==1 to coulmn=find(value1==1) and the same for variable row, then it makes sense. tq

[Rik]

No, you should use ~any(row) instead.