Play a game on a phone and watch what it keeps track of: your score, your lives, your name on the high-score list, the level you have reached. Then watch what it decides. Did you answer right? Then add a point. Did you run out of lives? Then the game is over. A game without memory or choices would be no game at all: it would do the same thing every time, whatever you did.
In Grade 6 you planned algorithms that remember things in variables and make choices, and you made a turtle draw with blocks. In this unit you put the two together. You will write programs that ask you questions, remember your answers, check them, and decide what to do next. By the end you will have built a quiz game that keeps your score and a number-guessing game, and you will be able to follow any short program by hand and say what it does.
Variables in a program
A cricket scorer keeps one number in mind all through an innings: the runs. It starts at 0. A four makes it 4; a six on the next ball makes it 10. The scorer never writes a new page for every ball. There is one place for the runs, and its number changes.
A program does the same with a variable: a named place in the computer’s memory that holds one value. The program can look at the value, and can put a new value in. As in Grade 6, think of a box with a name on its lid.
In our blocks, two blocks put values into a variable:
- set runs to 0 puts 0 into the variable runs. Whatever was in it before is gone.
- change runs by 4 adds 4 to the number already in runs. If runs held 6, it now holds 10. To take away, change by a minus number: change runs by -1.
To use the value inside a variable, drag the variable’s own round block (runs) into a slot of another block. Wherever that block runs, the computer looks in the box and uses whatever value is in it at that moment.
The say block shows something in the output box, the way a phone app shows a message. join puts two pieces together, one after the other: join “Runs: ” runs makes the words Runs: followed by whatever number is in runs, such as Runs: 11. The space after the colon is inside the quotes, so it is part of the text.
Good names
You can name a variable almost anything, but choose names that say what is inside: runs, score, price, name. A program with variables called a, b and x2 works just as well for the computer, but a person reading it, including you a week later, can’t tell what each one is for. To make a new variable, press Make a variable at the start of the blocks and type its name.
A program runs set score to 5, then change score by 3, then set score to 0, then change score by 2. What is in score at the end?
Pick an answer to see why.
Why should a variable be called cups rather than c?
Pick an answer to see why.
Three types of value
In Grade 6 you met three data types: numbers, text, and yes/no. A program uses the same three. The yes/no type is written true or false in a program, and it gets its own name, Boolean (after George Boole, an English mathematician of the 1800s). Each type is good for different jobs:
| Type | Examples | What you can do with it |
|---|---|---|
| number | 12, 0, 7.5, −30 | add, take away, multiply, divide; ask which is bigger |
| text | “Sita”, “Bhaktapur”, “Runs: ” | join it to other text; check whether it is the same as other text |
| true/false (Boolean) | true, false | decide what to do next: if it is true, do this |
Text is usually called a string by programmers (a string of letters), and you will see that word in Grade 8.
In our blocks, what you type into a slot is kept as a number if it looks like a number, and as text if it doesn’t: type 12 and you get the number 12, type Sita and you get the text “Sita”. True and false have a block of their own. The boxes over the editor show each variable’s type beside its value.
What happens when you put a number and text together depends on the block. 12 + 1 is 13, because + works on numbers. join 12 1 is 121, because join works on text: it puts the 1 after the 12, side by side, as you would write them on paper.
What does join 10 5 make?
Pick an answer to see why.
A program keeps whether a student has paid their exam fee. Which type of variable suits it best?
Pick an answer to see why.
Input: asking a question
The cricket program always does the same thing: the runs are written into its blocks. Most useful programs work with input, information that comes in from outside while the program runs: a number you type, a button you tap, a photo you choose. A mobile wallet asks how much you want to send; a bus-ticket app asks where you are going. The program can’t know the answer in advance, so it keeps it in a variable and uses it from there.
Our input block is ask … and keep the answer in …. It shows the question, waits until you type an answer and press OK, then puts what you typed into the variable you chose. Then the program carries on.
Here is the tea shop’s bill from Grade 6, now as a real program, for milk tea only. (The price, Rs 30 a cup, is the same made-up one.)
set bill to cups × 30 works out the bill from whatever cups holds, and keeps the answer in bill. A block that works something out like this, from values and variables, is called an expression.
What you type arrives as characters: as you saw in Unit 2, the digit 2 is stored as its character code, 50. Our blocks change an answer that looks like a number into a real number before using it, so 2 cups can be multiplied by 30. But if a customer types a word where the program needs a number, such as two instead of 2, the program stops with a message: the text “two” can’t be multiplied by 30. The type of a value matters even when it comes from input.
What is the input in the tea shop program?
Pick an answer to see why.
Conditions: true or false
Before a program can choose, it needs a question with only two answers. A question like that is called a condition, as in Grade 6: it is either true or false. In a program, conditions are usually comparisons:
| Condition | Read it as | When age is 12 |
|---|---|---|
| age < 13 | age is less than 13 | true |
| age > 12 | age is more than 12 | false |
| age = 12 | age is equal to 12 | true |
| name = “Sita” | name is the same as “Sita” | depends on name |
The three signs, <** (less than), **> (more than) and = (equal to), are called comparison operators. The pointed end of < and > points at the smaller number: in 5 < 9, the 5 is at the point.
<** and **> compare two numbers. If age holds the text “four”, nobody can say whether it is less than 13, so our blocks stop with a message instead of guessing.
A condition is itself a value, of the true/false type, so you can keep it in a variable. set child to age < 13 works out whether age is less than 13 and keeps true or false in child.
At the edge
Rules in real life often say “5 or more” or “up to 12”. Our blocks have only <, > and =, as some block languages do. For whole numbers that is enough:
- “5 or more” is the same as > 4: 5, 6, 7… are all more than 4.
- “12 or less” is the same as < 13.
The edge is where most mistakes hide. > 5 leaves out 5 itself. This is the bug you found in Grade 6, in the tea shop’s “5 cups or more”.
marks holds 40. Which condition is true?
Pick an answer to see why.
A bus gives a free ride to children under 5. Which condition is true exactly for the children who ride free?
Pick an answer to see why.
If and if/else
Now the program can choose. The if block has a slot for a condition and a mouth for blocks:
- if condition then …: the blocks in its mouth run only when the condition is true. When it is false they are skipped, and the program goes on below.
- if condition then … else …: two mouths. When the condition is true, the first runs; when it is false, the blocks under else run instead. Exactly one of the two runs, never both and never neither.
Choosing which blocks to run like this is called selection (in Grade 6 you called it a choice). With sequence and repetition, it is one of the three ways every program is built.
Here is a bus fare: children under 5 ride free, everyone else pays Rs 30 (made-up prices).
An if/else inside another if/else, as in the answer above, is called a nested choice, like the nested loops of Grade 6. Each question is asked only when the one before it has sent the program that way.
A program says: if score > 7 then say “Gold” else say “Keep trying”. score holds 7. What does it say?
Pick an answer to see why.
Find the bug
A school lets students pass with 40 marks or more. Here is a program for it. Does it work?
Tracing a program
A program with variables and choices can do something different every time, depending on its input. To be sure what it does, you trace it, as in Grade 6: follow it one block at a time, exactly as the computer would, and write the value of each variable after each step in a trace table.
Here is a program. A bus ticket costs Rs 30, and you have Rs 100. How many tickets can you buy?
set tickets to 0set money to 100repeat 4 times if money > 29 then change tickets by 1 change money by -30say join “Tickets: ” tickets
In a trace table, what does each column show?
Pick an answer to see why.
Build a quiz game
Now you have everything a quiz needs:
- a variable score, set to 0 at the start;
- for each question, an ask that keeps the answer in answer;
- an if/else that checks the answer: if it is right, say so and change score by 1; otherwise, say the right answer;
- at the end, say the score.
Each question is the same few blocks with different words. That is a pattern you will turn into one reusable block in Grade 8, called a function.
Your quiz has 5 questions and starts with set score to 0. A player gets 3 right. Which block ran 3 times?
Pick an answer to see why.
A scoring game: guess my number
In this game the computer thinks of a number from 1 to 20 and you have 5 guesses to find it. After each wrong guess it tells you whether to go bigger or smaller. You start with 5 points and lose one for each wrong guess.
The program uses a new block, random number from 1 to 20, which picks a whole number by chance, a different one each game. Its variables are of two types, numbers and true/false:
| Variable | Type | What it holds |
|---|---|---|
| secret | number | the number the computer picked |
| guess | number | your latest guess |
| points | number | your points so far |
| found | true/false | whether you have found it yet |
found starts as false. When you guess right, it is set to true, and from then on the if found = false inside the loop skips the rest of the rounds.
In the guessing game, why is found a true/false variable and not a number?
Pick an answer to see why.
A variable that grows
Variables work with the turtle too. In this program the length of each line is a variable, side, which grows by 10 every time round the loop. The turtle draws a square spiral.
In the spiral, side starts at 10 and the loop runs 20 times, changing side by 10 at the end of each round. How long is the last line the turtle draws?
Pick an answer to see why.
Plan, then build
The quiz and the guessing game were designed before they were built. For a program of your own, plan the same way:
- Say what it should do, in a sentence or two. What does it ask? What does it decide? What does it say at the end?
- List the variables, each with its type and what it holds, in a table like the one for the guessing game.
- Write the steps in plain words, with the choices as “if … then … otherwise …”.
- Choose tests: inputs whose right answers you know, including the edges of every condition.
- Build it one part at a time, running it after each part. Then run all your tests.
You are planning a game. Which should you write first?
Pick an answer to see why.
In short
Exercise
What does change lives by -1 do when lives holds 3?
Pick an answer to see why.
Which of these is a condition?
Pick an answer to see why.
name holds “Asha” and age holds 13. What does join name age make?
Pick an answer to see why.
A program says if temp > 30 then say “Hot!” and nothing else. temp holds 25. What happens?
Pick an answer to see why.
Which tests best check a rule “free entry for children under 10”?
Pick an answer to see why.
Explain each term with an example: variable, input, condition, selection.
Answer
What is the difference between set and change? A variable coins holds 8. What does it hold after set coins to 3, and what after change coins by 3 (each starting from 8)?
Answer
Name the data type (number, text or true/false) best suited to each variable, and give a reason for one of them: a student’s name; the number of students in a class; whether the school bus has left; the price of a notebook; a district’s name; whether a player has a ticket.
Answer
x holds 6 and y holds 9. Is each condition true or false? i. x < y ii. x > 6 iii. y = 9 iv. x + 3 = y v. y < x
Answer
Write a trace table for this program when the user types 7. What does it say?
ask “Pick a number” and keep the answer in nset total to n × 2if total > 10 then change total by -10else change total by 5say totalAnswer
A canteen gives a free banana with any order of Rs 100 or more. Rojan writes: if bill > 100 then say “Free banana!”. Find the bug, give an input that shows it, and fix it.
Answer
Design a program that asks for a student’s marks out of 100 and says “Distinction” for 80 or more, “Pass” for 40 to 79, and “Not yet” below 40. List its variables with their types, write its blocks in plain words, and give five tests with the answers they should give.
Answer
Look at this quiz program. What is wrong with it, and what will a player see if they play two games in a row and answer right both times? How would you fix it?
ask “What is 9 × 7?” and keep the answer in answerif answer = 63 then change score by 1say join “Score: ” scoreAnswer
Plan a game of your own that uses at least three variables of at least two different types. Write: what the game does, a table of its variables (name, type, what it holds), its steps, and three tests.
Answer
Why do programs need input and if/else? Explain with an example of an app you have used.
Answer
Quiz yourself: Algorithms and programming, Grades 6–12 →
Practice every question in the book, and keep track of the ones you miss →