# Matlab: While loop in function

I am new to Matlab (& also to stackexchange) and I need to learn it for a dynamic Macro class.

I have trouble using a while loop in a function. The function is supposed to calculate the amount of coins one needs to pay price p. My function, however, just provides a vector displaying which coins I need and not how much of which. I hope, someone jumps in and helps me out of my trouble. Here is my code:

function[c] = achange(p,coins)
s=size(coins);
c=zeros(1,s(2));

while p>0
for i=1:s(2)
if coins(i) <= p
p=p-coins(i);
c(i)=c(i)+1;
end
end
end
end

• Okay, well you should know that the economics stackexchange is for economics questions, not Matlab questions. Commented Nov 18, 2020 at 19:19
• @user253751 no actually our help center explicitly states: "Software questions: Questions related to software used in economics and econometrics, are on-topic here." See here. So this question is totally on topic because it is application of matlab to some econ question. Matlab is used in economics quite a lot for numerical modeling - I dont recognize the type of the problem above since it is not connected to anything I specialize in but it looks like an Econ problem
– 1muflon1
Commented Nov 18, 2020 at 19:25
• Your function seems to be okay. Suppose that the good costs 60 cents, and you have the following vector of coins $(1,2,5,10,20)$, then the function returns $(4,3,2,2,1)$ telling you that you need 4 one cent coins, 3 two cents coins etc., adding up to 60 cents, which is the price of the good. Commented Nov 18, 2020 at 20:48
• @BrianRomanchuk that is correct, it works only with integers. But this is a refinement that we should let to the author. Commented Nov 18, 2020 at 20:58
• The algorithm doesn’t diagnose an inability to get the requested amount, and can miss the correct solution. Let’s say we have p=50 and c=[25, 10]. It will return c=[1,2] to add up to 45, and not [2,0] which gives 50. Commented Nov 18, 2020 at 21:01

## 1 Answer

I thought a little bit about the problem, and I came up with the following quick hack, that seems to work.

Here is the piece of code that should do the job in principle with integers:

function  [exch]=achange(p,coins)
s=size(coins);
c=zeros(1,s(2));
coins=sort(coins,'descend');
it=1:s(2);
isaninteger = @(x)isfinite(x) & x==floor(x);
if p<coins(end)
return
end
if isaninteger(p)==0
exch.coins=coins;
exch.c=inf(1,s(2));
exch.rest=inf;
return
elseif any(isaninteger(coins)==0)
exch.coins=coins;
exch.c=inf(1,s(2));
exch.rest=inf;
return
end
rest=0;
while p>0
trQ=mod(p,coins)==0;
idx=it(trQ);
li=length(idx);
if isempty(idx)
rst=1;
rest=rest+rst;
p=p-rst;
elseif li>0 && li<s(2)
ps=it(trQ)==0;
idx2=idx;
idx2(ps)=[];
for k=1:li
if p>0
p=p-coins(idx2(k));
c(idx2(k))=c(idx2(k))+1;
end
end
elseif li==s(2)
for k=1:li
if p>0
p=p-coins(idx(k));
c(idx(k))=c(idx(k))+1;
end
end
end
end
exch.coins=coins;
exch.c=c;
exch.rest=rest;
end


At least for the following example it works:

>> exch=achange(57,[10 25])
exch =
struct with fields:
coins: [25 10]
c: [1 2]
rest: 12

>> exch.coins*exch.c' + exch.rest
ans =
57


or for:

>> exch=achange(51,[10 25])
exch =
struct with fields:
coins: [25 10]
c: [1 2]
rest: 6

>> exch.coins*exch.c' + exch.rest
ans =
51


and finally my example from the comments:

>> exch=achange(60,[1 2 5 10 20])
exch =
struct with fields:
coins: [20 10 5 2 1]
c: [1 1 3 4 7]
rest: 0

>> exch.coins*exch.c'
ans =
60


If any input is not an integer, we obtain

>> exch=achange(57,[5 .10 25])
exch =
struct with fields:
coins: [25 5 0.1000]
c: [Inf Inf Inf]
rest: Inf


or similar

>> exch=achange(57.8,[5 10 25])
exch =
struct with fields:
coins: [25 10 5]
c: [Inf Inf Inf]
rest: Inf


UPDATE 21.11.2020

Due to the fact that the allocation of the species is not optimal by the above code, which can be seen, for instance, by first example, where the residual amount can still be allocated to some coins, I provide a small amendment that should produce better results.

The residual amount of the mentioned example is $$12$$ indicating that the structure of problem is unchanged, which allows us to solve the problem recursively. That is, we should replace the lines of code after the While-loop by the following If-construct to invoke a recursive call whenever it is necessary.

if any(rest>coins)
exch2=achange(rest,coins);
exch.coins=coins;
exch.c=c+exch2.c;
exch.rest=exch2.rest;
else
exch.coins=coins;
exch.c=c;
exch.rest=rest;
end


Then we can reduce the residual amount of the first example from $$12$$ to $$2$$, as we can see through

>> exch=achange(57,[10 25])
exch =
struct with fields:
coins: [25 10]
c: [1 3]
rest: 2

• Thank you very much! This code is very nice. As a newbie I guess I wouldn't have figured it out. Probably, it will take me some time to even understand the code completely. So again, thanks a lot for your help! Commented Nov 19, 2020 at 15:37
• @Hokkaido21 I provided a code amendment to optimize the allocation procedure of the coins. I hope this helps. Commented Nov 21, 2020 at 11:31