Strings Additions File I/O. Strings and I/O. K. Cooper 1. 1 Department of Mathematics. Washington State University. Strings

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1 and I/O K. 1 1 Department of Mathematics 2018

2 Character We have already used character strings a few times. We have seen them as simple collections of characters, surrounded by quotation marks. e.g. cat, dog, asdf jkl;. Creating strings is easy: just enclose characters in quotes. can be assigned to variables as usual. str1 = Four score and seven years ago

3 Quotation Marks There are many types of quotation marks in Python. We can use single- or double-quotes, or in a pinch, we can use triplets of those symbols. The following strings all work. Sinead said: "She is serious!" "Sinead said: She is serious! " We can even go to extremes: Sinead said: "She s serious!" """Sinead said: "She s serious!" """ We can also escape quotations, if we prefer. Sinead said: "She\ s serious!"

4 Lists are managed as lists by Python. That means that list arithmetic works. a = always A = Alyssa print(a+a) yields Alyssa always. Continuing... aaaaa = A+2*a+ abhors alliteration. print(aaaaa) gives Alyssa always always abhors alliteration. Note that we needed to include the appropriate spaces as part of the strings.

5 Special Characters We need the usual ways of producing non-printing characters, such as tab spaces. \n newline \r carriage return; return to beginning of line, without moving down. \t tab space \b backspace

6 Numbers We have discussed how numbers are stored: integer or floating point. Note that e.g. floating point numbers require one word to store in memory, but to store a character representation of that might require 20 bytes three to 20 words, depending on the scheme used. The point is that we need the str() command to convert numbers into character representations. If we read numbers as strings, we must convert them using int() or float().

7 Number Example num = print(4*num) gives By contrast f_num = float(num) print(4*f_num) gives

8 Replacement We typically are not interested in plain text. We are printing results of computations. There are a few ways to do this. integ = simpson(lambda x:x*x,0.,1.,2) s = The integral of xˆ2 is result out = s.replace( result,str(integ)) print(out) yields The integral of xˆ2 is The replace() method is more generally useful, and certainly not the best way to do this.

9 Percent One way you will see this done often uses a percent sign. s = The integral of xˆ3 is %f print(s % integ) gives The integral of xˆ2 is We can control the format of the output by inserting a numeric specification: s = The integral of xˆ3 is %4.2f print(s % integ) gives The integral of xˆ2 is In %A.Bf, A is the total width of the number field; B is the number of digits to appear after the decimal point. In %.Bf the number field is as wide as it needs to be.

10 Number Fields In these specifications, f denotes a floating point representation; d represents an integer; and e represents a scientific notation. print("x = %4.1f" % pi) x = 3.1 print("x = %4.0f" % pi) x = 3 print("x = %4d" % pi) x = 3 print("x = %d" % pi) x = 3 print("x = %4.1e" % pi) x = 3.1e+00 print("x = %4.1e and i = %d" % (pi,17)) x = 3.1e+00 and i = 17

11 Format() Using the percent signs is at best an old way of doing things. A more modern and more powerful way to do this is to use the format method of string objects. There are several ways to go about this. print( x = {}.format(pi)) x = print( x = {:.3f}.format(pi)) x = print( x = {:8.3f}.format(pi)) x = 3.142

12 Position Association of values with formatting is by position. x = {} and i = {}.format(pi,17) x = and i = 17 x = {} and i = {:4d}.format(pi,17) x = and i = 17 We can make the dependence explicit by using position numbers (Python 2.5+). x = {0} and i = {1}.format(pi,17) x = and i = 17 x = {1} and i = {0:.3f}.format(pi,17) x = 17 and i = 3.142

13 Keys We can also specify variables by names we assign. x = {p} and i = {i}.format(p=pi,i=17) x = and i = 17 x = {p} so x {p:.3f}.format(p=pi,i=17) x = so x Note that once you pick a scheme: automatic position, manual position, or by name, then you must stick with it.

14 Files So far we have talked only about printing to standard output. Of course, if we need to write to a file, we can do so using a standard output redirect: #!/usr/bin/python2.7 print( Hello, world! ) is a simple script called hi.py. If we run it as hi.py > hi.out then the file hi.out will contain the output.

15 File Objects For more sophisticated output we need to write directly to a file. We can then write to that file using methods from the file object. #!/usr/bin/python2.7 f = open( hello.out, w ) f.write( Hello, world! ) makes a file called hello.out containing that text. The open function takes two arguments: the first is the (string) name of the file; the second tells whether we want to write to the file ( w ), read from the file ( r ), or both ( r+ ). We can close files, (e.g. f.close), but garbage collection seems to be a problem in my experience it is a bit unreliable.

16 Common Output Often we need to write a file containing e.g. a matrix of values. One way to do that goes like this. [m,n] = shape(a) for i in arange(m): for j in arange(n): f.write( {0},{1},{2}\n.format(i,j,A[i,j])) We could make a for loop that reads that kind of format in as well... but it has already been done.

17 CSV Files It is probably best to load the csv module for this input. Reads each row of a CSV file as a list of strings. Takes care of counting fields Numbers must be converted from strings

18 CSV Function Here is a function that reads a CSV file as a floating point Numpy array. import csv def read_csv(filename): A = [] infile=csv.reader(open(filename, r )) for row in infile: A += [row] return array(a).astype(float) Call it as e.g. read_csv( myfile.csv ) Note that we used the properties of lists to make our array.

19 CSV Writer Here is a function that writes a CSV file from an array. import csv def write_csv(a,filename): data = csv.writer(open(filename, w )) for row in A: data.writerow(row) return Note that this assumes that A has a list of lists structure. It works for Numpy arrays. Non-strings are converted automatically.