# Editing Cookbook

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An Octave cookbook. Each entry should go in a separate section and have the following subsection: problem, solution, discussion and maybe a see also. | An Octave cookbook. Each entry should go in a separate section and have the following subsection: problem, solution, discussion and maybe a see also. | ||

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== Structures == | == Structures == | ||

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=== Retrieve a field value from all entries in a struct array === | === Retrieve a field value from all entries in a struct array === | ||

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==== Problem ==== | ==== Problem ==== | ||

− | You have a struct array with multiple fields, and you want to | + | You have a struct array with multiple fields, and you want to acess the value from a specific field from all elements. For example, you want to return the age from all patients in the following case: |

− | + | samples = struct ("patient", {"Bob", "Kevin", "Bob" , "Andrew"}, | |

− | samples = struct ("patient", {"Bob", "Kevin", "Bob" , "Andrew"}, | + | "age", [ 45 , 52 , 45 , 23 ], |

− | + | "protein", {"H2B", "CDK2" , "CDK2", "Tip60" }, | |

− | + | "tube" , [ 3 , 5 , 2 , 18 ] | |

− | + | ); | |

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==== Solution ==== | ==== Solution ==== | ||

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Indexing the struct returns a comma separated list so use them to create a matrix. | Indexing the struct returns a comma separated list so use them to create a matrix. | ||

− | + | [samples(:).age] | |

− | [samples(:).age] | ||

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This however does not keep the original structure of the data, instead returning all values in a single column. To fix this, use {{Codeline|reshape()}}. | This however does not keep the original structure of the data, instead returning all values in a single column. To fix this, use {{Codeline|reshape()}}. | ||

− | + | reshape ([samples(:).age], size (samples)) | |

− | reshape ([samples(:).age], size (samples)) | ||

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==== Discussion ==== | ==== Discussion ==== | ||

+ | Returning all values in a comma separated lists allows you to make anything out of them. If numbers are expected, create a matrix by enclosing them in square brackets. But if strings are to be expected, a cell array can also be easily generated with curly brackets | ||

− | + | {samples(:).name} | |

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− | {samples(:). | ||

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You are also not limited to return all elements, you may use logical indexing from other fields to get values from the others: | You are also not limited to return all elements, you may use logical indexing from other fields to get values from the others: | ||

− | + | [samples([samples(:).age] > 34).tube] ## return tube numbers from all samples from patients older than 34 | |

− | [samples([samples(:).age] > 34).tube] ## return tube numbers from all samples from patients older than 34 | + | [samples(strcmp({samples(:).protein}, "CDK2").tube] ## return all tube numbers for protein CDK2 |

− | [samples(strcmp({samples(:).protein}, "CDK2" | ||

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== Input/output == | == Input/output == | ||

− | + | == Mathematics == | |

− | === | + | === Find if a number is even/odd === |

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==== Problem ==== | ==== Problem ==== | ||

− | + | You have a number, or an array or matrix of them, and want to know if any of them is an odd or even number, i.e., their parity. | |

− | You have | ||

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==== Solution ==== | ==== Solution ==== | ||

+ | Check the remainder of a division by two. If the remainder is zero, the number is odd. | ||

− | + | mod (value, 2) ## 1 if odd, zero if even | |

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− | + | Since {{Codeline|mod()}} acceps a matrix, the following can be done: | |

− | + | any (mod (values, 2)) ## true if at least one number in values is even | |

− | + | all (mod (values, 2)) ## true if all numbers in values are odd | |

− | + | ||

+ | any (!logical (mod (values, 2))) ## true if at least one number in values is even | ||

+ | all (!logical (mod (values, 2))) ## true if all numbers in values are even | ||

==== Discussion ==== | ==== Discussion ==== | ||

+ | Since we are checking for the remainder of a division, the first choice would be to use {{Codeline|rem()}}. However, in the case of negative numbers {{Codeline|mod()}} will still return a positive number making it easier for comparisons. Another alternative is to use {{Codeline|bitand (X, 1)}} or {{Codeline|bitget (X, 1)}} but those are a bit slower. | ||

− | + | Note that this solution applies to integers only. Non-integers such as 1/2 or 4.201 are neither even nor odd. If the source of the numbers are unknown, such as user input, some sort of checking should be applied for NaN, Inf, or non-integer values. | |

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− | + | ==== See also ==== | |

− | + | Find if a number is an integer. | |

− | + | == Parametrized Functions == | |

− | + | ==== Problem ==== | |

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− | === | ||

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− | + | One sometimes needs to define a family of functions depending on a set of parameters, e.g., | |

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− | + | <math> f (x, y, z; a, b, c) </math> | |

− | + | where | |

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− | + | <math> x, y, z </math> | |

+ | denote a the variables on which the function operates and | ||

− | + | <math> a, b, c </math> | |

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− | + | are the parameters used to chose one specific element of the family of functions. | |

− | + | For example, let's say we need to compute the time evolution of the elongation of | |

+ | a spring for different values of the spring constant | ||

− | < | + | <math> k </math> |

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==== Solution ==== | ==== Solution ==== | ||

− | + | We could solve the problem with the following code | |

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− | + | {{Code|Solve spring equation for different values of the spring constant|<syntaxhighlight lang="octave" style="font-size:13px"> | |

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− | <syntaxhighlight lang=" | ||

t = linspace (0, 10, 100); | t = linspace (0, 10, 100); | ||

function sprime = spring (s, t, k) | function sprime = spring (s, t, k) | ||

Line 476: | Line 96: | ||

plot (t, x1, t, x2) | plot (t, x1, t, x2) | ||

legend ('x1', 'x2') | legend ('x1', 'x2') | ||

− | </syntaxhighlight> | + | </syntaxhighlight>}} |

− | [[File:solparfun.png | + | [[File:solparfun.png]] |

==== Discussion ==== | ==== Discussion ==== | ||

− | In the above example, the function "sprime" represents a family of functions of the variables | + | In the above example, the function "sprime" represents a family of functions of |

− | + | the variables | |

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− | + | <math>x,t</math> | |

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− | + | parametrized by the parameter | |

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− | + | <math>k</math>. | |

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− | + | The [http://www.gnu.org/software/octave/doc/interpreter/Anonymous-Functions.html#Anonymous-Functions anonympus function] | |

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− | + | @(x, t) sprime (x, t, k) | |

+ | is a function of only <math>x,t</math> where the parameter <math>k</math> is 'frozen' to the value it has at the moment in the current scope. | ||

− | + | == Plotting == | |

+ | == User input == |