Seite - 49 - in Programming for Computations – Python - A Gentle Introduction to Numerical Simulations with Python 3.6, Band Second Edition

2.3 NumericalPythonArrays 49 The Built-In Len Function It is appropriate here to also mention the built-in function len, which is useful when you need to find the length (i.e., the number ofelements)ofanarray.11 A quickdemonstrationofzerosandlenmaygo like this, In [1]: from numpy import zeros In [2]: x = zeros(3, int) # get array with integer zeros In [3]: x Out[3]: array([ 0, 0, 0]) In [4]: y = zeros(3) # get array with floating point zeros In [5]: y Out[5]: array([ 0., 0., 0.]) In [6]: y[0] = 0.0; y[1] = 1.0; y[2] = 2.0 # overwrite In [7]: y Out[7]: array([ 0., 1., 2.]) In [8]: len(y) Out[8]: 3 Note that the linex = zeros(3, int)could,alternatively,havebeenwrittenabit more informativeasx = zeros(3, dtype=int),wheredtypemeans data type. Just like with linspace, the line y = zeros(3) instructs Python to reserve, or allocate, space in memory for the array produced by zeros. As we see from the dialogue, thearraygets the nameyand will have three elements, all with a floating pointzerovalue.Eachelement iny isnextoverwrittenby“thenumbersweactually wanted”. Strictly speaking, the assignmenty[0] = 0.0was redundanthere, since thevalueofy[0]wasknowntobe0.0frombefore.It shouldbementioned, though, thatprogrammersdeliberatelywritesuchseeminglyredundantstatementsfromtime to time,meaning to tell a humancode interpreter that thevalue isnot“accidental”. In our dialogue here, one explicit statement was written for each element value we changed, i.e. as y[0] = 0.0; y[1] = 1.0; y[2] = 2.0. For arrays with just a few elements, this is acceptable. However, with many elements, this is obviously not the way to go. Longer arrays are typically first generated byzeros (or otherwise), before individual array elements get new values in some loop arrangement(explainedin Chap.3). The Array Function Another handy way of creating an array, is by using the functionarray, e.g., like this, In [1]: from numpy import array In [2]: x = array([0, 1, 2]) # get array with integers In [3]: x Out[3]: array([0, 1, 2]) In [4]: x = array([0., 1., 2.]) # get array with real numbers 11 Thelen function may also be used withother objects, e.g., lists (Sect.5.1).