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lab48
I/O; reading into arrays
and, comparing to Java

Bored over the summer? Want to keep familiar with your programming-skills, and/or learn more? Some good sites are:

www.codingbat.com/ has both Java and Python, w/ more advance problems, but only very-short tutorials.
Codecademy (python, Java, and more)
Khan Academy (javascript)
Barland's ITEC120 (Java; work through all lectures, labs, and hw problems)
Want to take a community-college class? ITP 120. (One nearby-offerer is virginiawestern.edu.)

Reading input from the keyboard:

# raw_input : string -> string
# Prompt the user to type something at the keyboard.

nm = raw_input( "name, please: " )
print( nm )

Arrays: We have seen that strings are arrays-of-characters (using square-brackets to look up individual characters); you can also make your own arrays:

# The rainfall measurements for Apr.24-30, in cm:
#
rainfall = [ 1.1, 0.3, 0.0, 0.0, 3.7, 4.8, 1.2 ]

rainfall[3]

# Compute the total rainfall:
#
totalSoFar = 0.0
for daynum in range( len(rainfall) ):
  totalSoFar = totalSoFar + rainfall[daynum]


# We can also change the contents of the array:
rainfall[2] = 93.2

Here is a program saed in a file; if you have command-line python installed, you can either run it from Windows via Start > cmd and then type python nameOfFile.py, or you can configure your computer to run it as a program when you double-click it.

# sumArray: float[] -> float
# Return the sum of all the numbers in the array `data`:
#
def sumArray( data ):
  totalSoFar = 0.0
  for i in range( len(data) ):
    totalSoFar = totalSoFar + data[i]
  return totalSoFar

# avgArray: float[] -> float
# Return the average of all the numbers in the array `data`.
# The array must be non-empty.
#
def avgArray( data ):
  return sumArray(data) / len(data)


##########

scores = [ 0,0,0,0,0 ]   # Yuck -- 0 isn't correct, but we must fill the array with *something*?!

for hwNumber in range( len(scores) ):
  newScore = raw_input( "Enter score for hw#" + str(hwNumber+1) + " " )
  scores[hwNumber] = int(newScore)


print( "The scores are: " )
for hwNumber in range( len(scores) ):
  print( "hw#" + str(hwNumber+1) + ": " + str( scores[hwNumber] ) )


print("The average score is: " + str( avgArray( scores ) ) )

Python vs. Java

You have to formally declare the type of every variable (including parameters), and a function's return-type:

python Java

LBS_PER_KG = 2.2 
IN_PER_FT = 12 
CM_PER_IN = 2.54 

# bmi : float, int, float -> float
# Calculate somebody's bmi, given their weight (in pounds)
# and their height as a number of (whole) feet and also the additional inches.
#
def bmi( weightLbs, wholeFt, extraIn ):
  weightKg = weightLbs / LBS_PER_KG 
  heightIn = float(wholeFeet)*IN_PER_FT + extraInches 
  heightCm = heightIn * CM_PER_IN 
  heightM = heightCm/100 
  return weightKg / pow(heightM,2)

double LBS_PER_KG = 2.2;
double IN_PER_FT = 12;
double CM_PER_IN = 2.54;

double bmi( double weightLbs, int wholeFt, double extraIn ) {
    double weightKg;
    double heightIn;
    double heightCm;
    double heightM;

    weightKg = weightLbs / LBS_PER_KG;
    heightIn = float(wholeFeet)*IN_PER_FT + extraInches;
    heightCm = heightIn * CM_PER_IN;
    heightM = heightCm/100;
    return weightKg / Math.pow(heightM,2);
    }

(Actually this isn't quite correct, since technically the word “static” should be in front of the three named-constants and the function bmi.)

Another big difference is that Java tends to use “methods” instead of functions — which just means (for all of ITEC120) that you stick the first argument in front of the method-name:

python

Java

 
universityName = "Penn State"
substring( universityName, 2, 7 )   # evaluates to "nn St"

substring( "Chupacabra", 2, 7 )     # evaluates to "upaca"

String universityName;
universityName = "Penn State";

universityName.substring( 2, 7 )   // evaluates to "nn St"
"Chupacabra".substring( 2, 7 )     // evaluates to "upaca"

(This should seem awkward to you. It actually serves a good purpose, in object-oriented programming: it allows you to “override methods” — but that's not a concept that is covered in ITEC120; you just have to suffer through its weirdness.)

Finally, all code in Java has to be inside a class declaration.

python

Java

# countVowels: string -> int
# Return the number of vowels in a string.
#
def countVowels( txt ):
    vowelsSoFar = 0
    for i in range( len(txt) ):
        if isVowel( txt[i] ):
            vowelsSoFar = vowelsSoFar + 1
        else:
            None

    return vowelsSoFar


# isVowel : char -> boolean
# Is `ch` a vowel?
#
def isVowel( ch ):
    chLower = string.lower(ch)
    if (chLower=='a') or (chLower=='e') or (chLower=='i') or (chLower=='o') or (chLower=='u') or (chLower=='y'):
        return True
    else:
        return False

class CharCounter {

    // countVowels: string -> int
    // Return the number of vowels in a string.
    //
    int countVowels( String txt ) {
        int vowelsSoFar;
        vowelsSoFar = 0;
        for (int i=0;  i < txt.length();  i=i+1 ) {
            if (isVowel( txt.charAt(i) )) {
                vowelsSoFar = vowelsSoFar + 1;
                }
            else {
                // None
                }
            }

        return vowelsSoFar;
        }


    // isVowel : char -> boolean
    // Is `ch` a vowel?
    //
    boolean isVowel( char ch ) {
        char chLower;
        
        chLower = Character.toLowerCase(ch);
        if ((chLower=='a') || (chLower=='e') || (chLower=='i') || (chLower=='o') || (chLower=='u') || (chLower=='y')) {
            return true;
            }
        else {
            return false;
            }
        }
        
        
    }

This example also show that Java's for-loop still forces you to initialize and increment the loop's index-variable. (So Java's for and while are quite similar; they only differ in that the the for-loop gathers everything involving the index-variable “loop control” onto a single line.)

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©2014, Ian Barland, Radford University
Last modified 2014.May.01 (Thu) Please mail any suggestions
(incl. typos, broken links)
to ibarlandradford.edu

lab48 I/O; reading into arraysand, comparing to Java

Python vs. Java

lab48
I/O; reading into arrays
and, comparing to Java