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ICDL Coding Principles study materials
ICDL Coding Principles study resources

ICDL Coding Principles (ICDL): Data, Variables and Control Flow

ICDL Coding Principles · Work through your exam, one topic at a time.

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Data, Variables and Control Flow

Data and variables

A variable is a named store whose value can change; a constant is fixed. Common data types are integer, real (float), character, string, Boolean, and sometimes date and array. Declaring a variable reserves it and fixes its type; assigning gives it a value. Casting converts between types, and conversion can lose information, for example a real truncated to an integer.

Operators

Arithmetic operators include + - * / and often MOD (remainder) and DIV (whole-number division). Comparison operators (=, <> or !=, <, >, <=, >=) produce Boolean results. Boolean operators are AND, OR and NOT, evaluated in the order NOT, then AND, then OR, unless brackets say otherwise. Watch the classic confusion between = as assignment and == as comparison.

Control flow

Selection: if / else-if / else, and case statements; conditions can be nested. Iteration: count-controlled loops (repeat a known number of times, often FOR i = 1 TO n), condition-controlled loops (WHILE tests before the body and may run zero times; REPEAT ... UNTIL tests after and always runs at least once). Arrays and lists store many values under one name, and indexing commonly starts at 0, so the last index of an array of n items is n-1.

Traps

  • "5" + 3 concatenates to give "53" in many languages, not 8.
  • A loop that counts from 0 to n inclusive runs n+1 times.
  • WHILE may never execute; REPEAT ... UNTIL always executes at least once.
  • Accessing index n in an array of n items is out of range.

Rule of thumb

Write the loop condition and the counter update together, and check the first and last iteration on paper before trusting the loop.