How does it work ? For loop

I didn’t understand how does this loop work. I know for loop from c language.

Answer 1

[Walter Roberson]

In pseudo-code, since MATLAB does not have any GOTO:

for n=4:N

means

test if N < 4
if it is then
      assign the empty vector to n
      skip the body of the loop
otherwise
      copy 4 to _HIDDEN_INTERNAL_VARIABLE_n
      copy N to _HIDDEN_INTERNAL_VARIABLE_n_upperbound
    Label: _HIDDEN_INTERNAL_LOOP_n
      copy _HIDDEN_INTERNAL_VARIABLE_n to n
      process the for i at this point (not shown here)
      copy _HIDDEN_INTERNAL_VARIABLE_n + 1 to _HIDDEN_INTERNAL_VARIABLE_n_new
      test if _HIDDEN_INTERNAL_VARIABLE_n_new > _HIDDEN_INTERNAL_VARIABLE_n_upperbound
      if it is then
          GOTO _HIDDEN_INTERNAL_LOOP_end_n
      otherwise
          copy _HIDDEN_INTERNAL_VARIABLE_n_new to _HIDDEN_INTERNAL_VARIABLE_n
          GOTO _HIDDEN_INTERNAL_LOOP_n
%end of loop n
Label: _HIDDEN_INTERNAL_LOOP_end_n

Important points:

• if the endpoint is already greater than the beginning, then the body of the loop is not done at all and the loop control variable is left as []
• hidden copies of the boundary values are taken before the loop begins, and the hidden copies are what is used. If the variables that were used for the boundary calculations are changed inside the loop, then that does not affect the loop since there is no reliance on those variables once the loop has started
• the last new value of the variable, the one that would take it over the upper bound, is not written to the loop control variable
• if the body of the loop (the for i loop in this case) changed the loop control variable, then if there are any remaining iterations, at the beginning of the next iteration, the change will be overwritten with the information stored in the hidden variables
• if the body of the loop (the for i loop in this case) changed the loop control variable, then if there are no remaining iterations, then after the loop the loop control variable will be left changed. The hidden values do not overwrite the loop control variable until you start a new iteration

I left out details about using increments other than 1, and details about looping backwards (negative increment). I also left out details about what happens if the right hand side of the for statement is : expression, or if it is not a row vector (for example suppose it was 2D on the right hand side of the "for n = ")

Anyhow, that is how a MATLAB for loop works.

... You did not ask anything about the mathematics involved in your loop.