How do I change the increment in a loop

Question

MATTHEW FOX 2017-5-9

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/339481-how-do-i-change-the-increment-in-a-loop

评论： Steven Lord 2023-3-12

I have been asked to write a function that calculates the sum of the series;

2^(5*i-1)

where i = 1,3,5... I currently have

function s = summation(N)
% Syntax
% s = summation (N)
% Input
% N = Number of terms in series
%Output
% sum of series
%Initilising loop to be zero
s = 0;
for i = 1:N
    % loop adds previous value of s to the next one
    s = s + 2^(5*i-1); 
end
% increasing increment i by 2 from 1 [1,3,5...etc]
   i = i+2;
end

which calculates the sum for i=1,2,3.... How do I change the increment of i?

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Answer 1

Stephen23 2017-5-9

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编辑：Stephen23 2017-5-9

 i = 1:2:N
       ^ define the step size here!

The colon operator is clearly explained in the documentation:

https://www.mathworks.com/help/matlab/ref/colon.html

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Jan 2017-5-9

@MATTHEW FOX: See also: doc for

Note: Whenever you have questions concerning a specific command, read the documentation at first. Matlab's docs are the best I've ever read. They are useful and clear, and the "See also:" lines are smart guesses of what the user might be interested also in, when the command does not perfectly solve the problem.

Noman Rathore 2018-12-5

Hello i am New researcher , and new to Matlab programming , but i understand the basius of programming , my querry is how i can use help and support for guidance for programming my own program. i most of the time do not find the useful help.

Nomi

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Answer 2

Phirime Monyeki 2020-2-10

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N = 100 ;   % should be multiple of the number of parts you want 
th = linspace(0,2*pi) ;
y = cos(th) ;
plot(th,y) 
% divide the section into 5 equal parts
th1 = reshape(th,5,[]) ;
y1 = reshape(y,5,[]) 

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Walter Roberson 2023-3-12

@HENY

help colon
 :  Colon.
    J:K  is the same as [J, J+1, ..., J+m], where m = fix(K-J). In the 
    case where both J and K are integers, this is simply [J, J+1, ..., K]. 
    This syntax returns an empty matrix if J > K.
 
    J:I:K  is the same as [J, J+I, ..., J+m*I], where m = fix((K-J)/I). 
    This syntax returns an empty matrix when I == 0, I > 0 and J > K, or 
    I < 0 and J < K.
 
    COLON(J,K) is the same as J:K and COLON(J,I,K) is the same as J:I:K.
 
    The colon notation can be used to pick out selected rows, columns
    and elements of vectors, matrices, and arrays.  A(:) is all the
    elements of A, regarded as a single column. On the left side of an
    assignment statement, A(:) fills A, preserving its shape from before.
    A(:,J) is the J-th column of A.  A(J:K) is [A(J),A(J+1),...,A(K)].
    A(:,J:K) is [A(:,J),A(:,J+1),...,A(:,K)] and so on.
 
    The colon notation can be used with a cell array to produce a comma-
    separated list.  C{:} is the same as C{1},C{2},...,C{end}.  The comma
    separated list syntax is valid inside () for function calls, [] for
    concatenation and function return arguments, and inside {} to produce
    a cell array.  Expressions such as S(:).name produce the comma separated
    list S(1).name,S(2).name,...,S(end).name for the structure S.
 
    For the use of the colon in the FOR statement, See FOR.
    For the use of the colon in a comma separated list, See VARARGIN.

    Documentation for colon
       doc colon

    Other uses of colon

       codistributed/colon                             duration/colon
       codistributor1d/colon                           embedded.fi/colon
       codistributor2dbc/colon                         gpuArray/colon
       datetime/colon                                  matlab/colon
       deep.internal.recording.RecordingArray/colon    sym/colon
       distributed/colon                               symbolic/colon
       dlarray/colon

Steven Lord 2023-3-12

For the original question looking at the documentation for the colon function (which is the function name for the : operator) is correct. That's the tool you use to create a vector if you know the two desired endpoints and the increment.

For the answer under which these comments lie, you'd want to look at the documentation for the linspace function. That's the tool you use to create a vector if you know the two endpoints and how many elements you want the vector to have.

If you know one of the endpoints, the increment, and the number of elements you want the vector to have you can do that with colon as well.

startPoint = 1;
increment = 1;
numPoints = 10;
x = startPoint + increment*(0:numPoints-1)
x = 1×10
     1     2     3     4     5     6     7     8     9    10