Whirlwind Tour of Ada - Part 3a

Structured Statements

Input of Integers and Characters

getcountpzn.adb

getcountpzn.adb and prettified

Reads a specified number of integers and counts the positives, zeros, and negatives in the input

Demonstrates:

Subtype Natural, constant, multiple declarations
if statements, for loops
integer get procedure

Subtype Natural

Variables of type Natural holds values in the range 0 .. Integer'last

Use type Natural for counting
Natural is a subtype of the integers: it is a subset of the values in the type Integer
Compiler allows a variable of type Natural anywhere a variable of type Integer can be used, and vice versa

Examples - these compile:

        i,j: Integer;
        n: Natural
        ...
        i := n;
        n := j;

but what about runtime:

FYI: here's how Natural is declared:

        subtype Natural is 0 .. Integer'Last;

Constants and Multiple Declarations

Constants must be initialized and can't appear on the left of an assignment

Can assign values to multiple variables in a single declaration

For Loops

Example for loop:

        for i in lo_value .. hi_value loop
            put(i);
        end loop;

Parens not needed around loop condition

For loop specifies precise range of values taken by loop variable

Loop variable is implicitly declared

Loops are marked with loop ... end loop;

True for all 3 kinds of loops: for, while, general

End loop is required, even if body has one statement

More on For Loops

Other increments: Some options

Use a while loop
Increment another variable in a for loop

Reverse loop: output 10, 9, ..., 1

        for i in reverse 1 .. 10 loop
           put(i);
        end loop;

Can have an empty range (eg for i in low .. hi, where low > hi)

If Statements

Parens not needed around condition

Explicit then keyword

Syntax: if ... elsif ... else ... end if

elsif and else are optional

Can have 0 or more elsif
Can have 0 or 1 else

End if required, even if body has one statement

Compound Parts of If Statements

Compound parts don't need explicit brackets
Count grades in Ada:

        if g >= 90 then
            put_line("found an A");
            numA := numA + 1;
        elsif g > 80 then
            put_line("found an B");
            numB := numB + 1;
        elsif g > 70 then
            put_line("found an C");
            numC := numC + 1;
        else
            put_line("found another");
            numOther := numOther + 1;
        end if;

In java compound parts require brackets:

        if (g >= 90){
            S.o.p("found an A");
            numA++;
        } 
        else if (g >= 80){
            S.o.p("found a B");
            numB++;
        } 
        else if (g >= 70){
            S.o.p("found a C");
            numC++;
        } 
        else {
            S.o.p("found another grade");
            numOther++;
        }

Java we use else if instead of elsif

Contrast elsif and else if

An if is a single statement, and so it can be used in the else part of a Java if statement without brackets
What would Java look like if brackets were required:

        if (g >= 90){
            S.o.p("found an A");
            numA++;
        } 
        else { if (g >= 80"){
            S.o.p("found a B);
            numB++;
        } 
        else { if (g >= 70"){
            S.o.p("found a C);
            numC++;
        } 
        else {
            S.o.p("found another grade");
            numOther++;
        }}}   // Careful with the braces!

Or like this:

        if (g >= 90){
            S.o.p("found an A");
            numA++;
        } 
        else { 
            if (g >= 80"){
                S.o.p("found a B);
                numB++;
            } 
            else { 
                if (g >= 70"){
                    S.o.p("found a C);
                    numC++;
                } 
                else {
                    S.o.p("found another grade");
                    numOther++;
                }
            }
        }   // Careful with the braces!

Languages that require closing brackets tend to use elsif

Examples: Ada and perl use elsif; python uses elif

Case Statement

Like switch

        case i is
            when 1 => 
                ...
            when others => 
                ...
        end case;

Must account for each value in type of i

Ada.Integer_text_io.get

Ada.Integer_Text_io.get(v) reads an integer from standard input and stores the value of that integer into variable v

Ada.Integer_Text_io.get is needed to read an integer, as an integer (as opposed to a string)

Get reads from standard input (see below for more info on standard input)

A call to Get skips white space until it finds an integer (or an error occurs)

Exceptions than can occur with a call to get:

In a numeric value that is out or range is found, a CONSTRAINT_ERROR occurs
If a non-numeric character is found, a DATA_ERROR exception occurs.
If eof is reached before a number is found, an END_ERROR exception occurs.

Standard Input

Statement get(x) reads from standard input

Standard input normally reads from the keyboard

To read from a file, there are two options:

Redirect the input (see below)
Explicitly declare and open the file and name it in the get statement

Example: get(myfile, x);
Discussed later

Redirecting Standard Input

Standard input can be redirected to read from a file

Example: running the program like this
```
 getcountpzn < mydata.txt
```
will cause a get from standard input to read from the file mydata.txt instead of from the keyboard.

Testing for End of File

Test for end of file with Ada.text_io.end_of_file

Example showing basic structure for testing for end of file:

    with ada.text_io; use ada.text_io;
    with ada.integer_text_io; use ada.integer_text_io;
    procedure eofloop is
        i: Integer;
    begin
        while not ada.text_io.end_of_file loop
            get(i);
            put(i);
            new_line;
        end loop;
    end eofloop;

Example shown in next program

geteofcountpzn.adb

geteofcountpzn.adb and prettified

Counts positive, zero, and negative integers, until end of file is reached

Demonstrates while loop and the end of file test:

While loop

Things to note:

No parens

Loop structure: loop ... end loop;

Interactive I/O and End of File

When doing interactive I/O, end_of_file will return true when you enter

control-D (linux and unix)
control-Z [on a line by itself] (windows)

In interactive I/O, there is no file, but the I/O routines are written to treat the special symbols as EOF

Interactive I/O: Using Prompts and EOF

What happens if we add a prompt?

getinteofcountpzn.adb and prettified

What's happening:

The EOF test must examine the input to test for EOF
In interactive I/O, it must read an entire line to test for EOF

How to solve???

General Loops, Sentinal Loops, Loop and a Half

Let's solve the same problem with a general loop

getgeninteofcountpzn.adb and prettified

Let's try a sentinal loop instead of an EOF loop

Write a procedure that echos positive numbers and halts on non-positive

This is called a loop and a half

Sequence: input, test, process, input, test, process, ...

Character Input

If c is declared as c: Character, then get(c) reads exactly one character.

Example program (prettified)

The example inputs and outputs characters and inputs an integer.

The output is a line containing the following: a single character, a character repeated a given number of times, and a final character.

If the final character is a 'u', then all of the characters are output in upper case.

Input specification:

Specification for each input line is as follows:

a character that is the first character to output
the number of times to repeat the middle character
a character that is the character to output in the middle
a character that is the last character to output (and the key for upper case output)

The last character also specifies whether to convert output to upper case

The program runs until end of file

Sample input:

A 11bc
1 11b2
1 11 2
a 11bu
a11bx

The input file above will produce the folowing output:

Abbbbbbbbbbbc
1bbbbbbbbbbb2
1           2
ABBBBBBBBBBBU
abbbbbbbbbbbx
done

Execution: If the data is in repeat_middle.txt, then run with

repeat_middle < repeat_middle.txt