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Lecture 4
Input and output
1 Introduction
The “deliverable” of a computer program is its output. Output may be in graphical form as in a
two-dimensional function plot, or it may be in text form as in a table of data values. In addition
we often need to provide a program with input data, either interactively from the console or from
a disk file. We will cover graphics in future lectures. Here we look at various ways to input and
output data to and from the console and disk files.
2 Basic input/output (I/O)
Scilab/Matlab automatically displays the value of a variable when you type its name at the
command line. For example
-->x = 2.3; -->x x =
In a program, on the other hand, the appearance of a variable name does not produce printed
output. Moreover, even with console I/O we may want more control over the output format. We
also need a way for a program to display values and ask for input, either from the user or from a
file. There are various ways to approach this. We start with the simplest.
2.1 The disp function
Most basic console I/O can be implemented with two functions: disp and input. The disp
function can be used to display the values of variables without the extra "ans = " prefix. For
example
-->[x,y] ans = 2.3 4. -->disp([x,y]) 2.3 4.
This works from within a program also. You can also combine it with the string() function
(num2str() in Matlab) and the string concatenate operation. This creates one long string as in
the following
-->disp(string(x)+' plus '+string(y)+' equals '+string(x+y)); 2.3 plus 4.1 equals 6.
This is generally good enough for most basic program output.
Exercise 1 :
2.2 The input function
The input() function can be used to prompt the user for input. We will illustrate this with a
few examples
-->x = input('please enter the value of x :'); please enter the value of x :3. -->disp(x)
You can input arrays
-->y = input('enter a 1-by-3 vector : ') enter a 1-by-3 vector : [1,2,3] y =
and strings
-->fname = input('output file : ') output file : 'test.txt' fname = test.txt
Here's a little snippet of code that prompts the user for an array of data and prints the average
value.
z = input('enter a 1-by-n array of numbers : '); disp('the average value is '+string(mean(z)));
this produces
enter a 1-by-n array of numbers : [1,2,3,4,5,6] the average value is 3.
Exercise 2 :
2.3 The save and load functions
The save and load functions allow you to “dump” and “recover” any or all variables you have
created.
-->A = [1,2;3,4];
-->x = 1:5; -->s = 'hello there'; -->save('test.dat');
The save command saves all variable names and values, essentially your entire Scilab session.
You can exit Scilab and in a later session use the load command to recove these saved values.
-->load('test.dat') -->A A =
these are replaced by the variable values. Here's another example
x = 1.23; y = 4; z = %pi*1e10; str = 'testing one two three'; mfprintf(fd,'%f %d %e %s\n',x,y,z,str); 1.230000 4 3.141593e+010 testing one two three
The \n symbol denotes a "new line." If you omit this then subsequent mfprintf commands
will be appended to the same line. Consider the following.
mfprintf(fd,'the value of x is %f',x); mfprintf(fd,' and y is %f\n',y);
produces
the value of x is 1.230000 and y is 3.
Finally, if you want to control the precise format of the numerical output, the syntax for a
floating point number is %m.nf where m is the total number of spaces (you need one for the
decimal point and you might need one for the sign) and n is the number of decimal places. For
example
-->mfprintf(fd,'%4.2f\n',x);
-->mfprintf(fd,'%6.2f\n',x);
-->mfprintf(fd,'%6.3f\n',x);
For %d and %s formats you can use the syntax %md or %ms where m is the total number of
spaces to be displayed. Note that if you don't allocate enough, the full value will be printed
anyway. If you allocate "too much" then blank space will be added. In the following example we
generate a formatted table of trig values.
N = 4;
x = linspace(0,%pi/2,N); y = sin(x); z = cos(x); mfprintf(fd,'\n'); //creates blank line at start mfprintf(fd, '%6s %6s %6s\n','x','sin','cos'); for i=1:N mfprintf(fd,'%6.3f %6.3f %6.3f\n',x(i),y(i),z(i)); end
Note the mfprintf(fd,'\n'); statement used to clear any previously "open" lines of
output. The output is
x sin cos 0.000 0.000 1. 0.524 0.500 0. 1.047 0.866 0. 1.571 1.000 0.
A couple more points. Consider the following.
-->mfprintf(fd,'%3d\n',k); 5 -->mfprintf(fd,'%-3d\n',k); 5 -->mfprintf(fd,'%03d\n',k); 005
The format %-3d causes the output to be left aligned as opposed to the default right alignment.
The format %03d causes the output to be right aligned but all remaining space to the left is filled
with zeros.
In keeping with the “vectorized” nature of Scilab/Matlab, the mfprintf (and fprintf)
function is also vectorized. For example
-->x = 1: x =
-->mfprintf(fd,'%d %d %d\n',x) 1 2 3
Scilab/Matlab recognizes that x is an array. It fills in the 3 %d formats with x(1), x(2) and
x(3). Now consider this
A = [1,2;3,4];
-->mfprintf(fd,'%f %f\n',A) 1.000000 2. 3.000000 4.
mfprintf repeats itself for each row of matrix A. In addition to mfprintf there is a Scilab
function mprintf that does not require the file descriptor argument and prints directly to the
console.
-->x = 2; -->mprintf('%f %f %f\n',x,x^2,x^3) 2.000000 4.000000 8.
The advantage of using mfprintf with fd = %io(2) for console output is that it is very
simple to modify your code to output to a file. You merely need to assign the fd variable using
the mopen command described below.
Exercise 4 :
3.2 msprintf (Scilab) & sprintf (Matlab)
The msprinf function (sprintf in Matlab) is similar to the mfprintf function except that
instead of writing output to the console or a file, it writes it to a string that can be assigned to a
variable. Consider the following
-->k = 3; -->name = msprintf('file%03d.txt',k) name = file003.txt
ans = T -->isfile('test2.txt') ans = F
We might use this as follows
name = 'test.txt'; if (isfile(name)) [fd,err] = mopen(name,'rt'); if (err) mfprintf(%io(2),'cannot open file %s\n',name); else mfprintf(%io(2),'file %s is now open\n',name); end else mfprintf(%io(2),'file %s does not exist\n',name); end
This produces the output
file test.txt is now open
whereas changing the first line to
name = 'test2.txt';
gives us the message
file test2.txt does not exist
Exercise 5 :
3.4 mfscanf (Scilab) and fscanf (Matlab)
The mfscanf function is a modification of the C fscanf function and allows you to perform
formatted input from a file. Suppose we use a text editor to create a text file named data.txt that
contains
We can read these numbers into a vector x as follows (note that for compactness we are not
including error checking for the mopen command).
[fd,err] = mopen('data.txt','rt'); [n,x] = mfscanf(6,fd,'%d'); mclose(fd); disp(x');
Variable n indicates the number of successful reads. It is -1 if the end of file was reached before
all desired data were read. The 6 tells mfscanf to read six times in the %d format. These are
assigned as the elements of x. Here a variation
[fd,err] = mopen('data.txt','rt'); [n,x] = mfscanf(3,fd,'%d'); [n,y] = mfscanf(3,fd,'%d'); mclose(fd); -->x' ans = 1. 2. 3. -->y' ans = 4. 5. 6.
This reads 3 numbers and assigns them to x, then another 3 numbers are read and assigned to y.
In Matlab the ordering is slightly different
[fd,err] = fopen('data.txt','rt'); [n,x] = fscanf(fd,'%d',3); [n,y] = fscanf(fd,'%d',3); fclose(fd);
Here another example in Scilab. This time the file data.txt looks like this
east 2 23. west 4 -94. south 1 8. north 3 -7.
The following commands
[fd,err] = mopen('data.txt','rt'); [n,s,d,x] = mfscanf(4,fd,'%s %d %f'); mclose(fd);
fill the arrays s, d and x with the corresponding string, decimal and floating point entries
-->s' ans = !east west south north! -->d' ans = 2. 4. 1. 3. -->x' ans = 23.75 - 94.5 8.1999998 - 7. 4 meof (Scilab) and feof (Matlab)
Suppose we have a file named 'test.txt' which has the following contents
What do we do if we know the file contains some numbers but we don't know how many? One
way to read all the available numbers in the file is to read them one at a time followed by a check
To write a csv file we use the command
csvWrite(M,'new.csv');
It is possible to specify a separator other than a comma and to read and write strings. See the
Spreadsheet section of the help menu for more information.
