External function problem.

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I am trying to alter this code which I have included and is functioning to use an external function.
After a few attempts but always returning errors I am at an end.
I have tried replacing line 8 and 9 with a function look up and recreating the function in the external file but it throws errors.
I believe it is a simple fix but am a bit stuck.
clear; close all; clc
balance(1) = 1000;
contribution = 1200;
M = 1; % month ticker
interest = 0.0025;
while true %this makes the program run until the break condition is met
M = M + 1; % you need to increment months + 1 every cycle
balance(M) = balance(M-1) + interest*balance(M-1) + contribution;
if balance(end) > 500000;
break;
end
end
years = M/12; %year counter
fprintf('%g Months until $500000 reached\n', M);
fprintf('%g Years until $500000 reached\n', years);
format bank
plot(balance);
grid on
ytickformat('usd')
title('Saving 3% compound')
xlabel('Months')
ylabel('Saving balance')
Label = max(balance) %this labels the plot with max balance
txt = {'Max Balance:',Label,'\leftarrow'};
text(M,Label,txt,'HorizontalAlignment','right')
  4 Comments
Chett Manly
Chett Manly on 18 Sep 2021
This is the code I am trying to make work the same as the original post but this one has to use the function externally.
clear; close all; clc
balance(1) = 1000;
contribution = 1200;
M = 1; % month ticker
interest = 0.0025;
while true %this makes the program run until the break condition is met
M = M + 1; % you need to increment months + 1 every cycle
account(balance, contribution, interest, M)
if balance(end) > 500000;
break;
end
end
years = M/12; %year counter
fprintf('%g Months until $500000 reached\n', M);
fprintf('%g Years until $500000 reached\n', years);
format bank
plot(balance);
grid on
ytickformat('usd')
title('Saving 3% compound')
xlabel('Months')
ylabel('Saving balance')
Label = max(balance) %this labels the plot with max balance
txt = {'Max Balance:',Label,'\leftarrow'};
text(M,Label,txt,'HorizontalAlignment','right')
This is my external function file.
function account(balance, contribution, interest, M);
balance(M) = balance(M-1) + interest*balance(M-1) + contribution;

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Accepted Answer

Walter Roberson
Walter Roberson on 18 Sep 2021
in MATLAB, assigning to one of the input parameters only causes local changes, not change to variable passed in. To change the variable you need to return the new value and assign it to the appropriate location in the calling function.
  3 Comments
Chett Manly
Chett Manly on 21 Sep 2021
I was so close... Sorry for the late reply, this is exactly it. Thanks!

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