Programming basics: Class 10 Computer Science

Key points

A program is a set of instructions written in a programming language to solve a problem. A programmer follows these steps: understand the problem, plan the solution as an algorithm, write the code, test and debug it, and document it. An algorithm is a step by step procedure that solves a problem in a finite number of steps.

A flowchart shows an algorithm with symbols:

• Oval: start or end.
• Parallelogram: input or output.
• Rectangle: process or calculation.
• Diamond: decision with yes and no branches.
• Arrows: show the direction of flow.

Pseudocode is an algorithm written in simple English-like statements. For example: start, read two numbers, add them, print the sum, end. It is not tied to any language.

Types of programming error:

• Syntax error: breaking the rules of the language, such as a missing semicolon. The compiler reports it.
• Logical error: the program runs but gives a wrong result because the logic is wrong.
• Runtime error: the program stops while running, for example by dividing by zero.

Structure of a simple C program:

• #include <stdio.h> adds the library for input and output.
• int main(void) is where execution starts.
• Statements are inside curly braces { } and each ends with a semicolon.
• return 0; ends the program normally.
• Comments are written between /* and */ or after // and are ignored by the compiler.

Here is a complete example: #include <stdio.h> then int main(void) { int a = 7, b = 3; printf("Sum = %d\n", a + b); return 0; } This prints Sum = 10. The code %d is a format specifier for an integer and \n moves to a new line.

Data types and variables. A variable is a named memory location that stores a value, and it must be declared with a type before use:

• int: whole numbers, for example 25. Format %d.
• float: decimal numbers, for example 3.14. Format %f.
• double: decimals with greater precision. Format %lf in scanf.
• char: a single character in single quotes, for example 'A'. Format %c.
• Variable names use letters, digits and underscore, cannot start with a digit, cannot contain spaces, and cannot be a keyword such as int or while. C is case sensitive, so Age and age differ.

A constant is a value that does not change. It can be defined with the const keyword, for example const float PI = 3.14;. A statement such as x = 5; is an assignment: the value on the right is stored in the variable on the left.

Operators:

• Arithmetic: +, -, *, / and % (remainder, only for integers). Integer division drops the fraction, so 17 / 5 gives 3 and 17 % 5 gives 2.
• Relational: >, <, >=, <=, == (equal to), != (not equal to). They give true or false.
• Logical: && (and), || (or), ! (not).
• Increment and decrement: i++ adds 1 and i-- subtracts 1.

Input is read with scanf and output is written with printf. To read an integer: scanf("%d", &num); The & sign gives the address of the variable and must be used with scanf for numbers and characters. Without the right format specifier, the value will be read or shown wrongly.

Decision making lets a program choose between actions:

• if (condition) { ... } runs the block only when the condition is true.
• if ... else runs one block when true and another when false.
• else if checks further conditions in turn.
• switch (value) with case labels and break chooses among many fixed values.
• Example: if (marks >= 40) printf("Pass"); else printf("Fail");

Loops repeat a block of statements:

• for (i = 1; i <= 5; i++) { ... }: used when the number of repetitions is known. It has initialisation, condition and update.
• while (condition) { ... }: tests the condition first, then repeats.
• do { ... } while (condition);: runs the body at least once, then tests.
• break leaves a loop early and continue skips to the next repetition.
• Take care that the condition eventually becomes false, or an infinite loop results.

Study tip: Practise writing short programs by hand: sum of numbers, even or odd, largest of three numbers, table of a number, and factorial. Always include the header line, main, declarations, input prompt and return statement. Trace loops in a table of values, because dry runs are common in papers.

Definitions and flashcards

Say the meaning first, then open the card to check.

Short questions with model answers

These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.

Numericals with worked steps

Common mistakes

• Using = instead of == in a condition. The single sign assigns a value, the double sign compares.
• Missing semicolons at the end of statements or putting a semicolon right after for or if, which changes the meaning.
• Leaving out & in scanf for an int, which can crash the program.
• Expecting 17 / 5 to give 3.4. Dividing two integers gives an integer.
• Writing a loop whose condition never becomes false, creating an infinite loop.

Practise this chapter

Practice questions and timed test Printable worksheet