C++ · Variables and types

Constants, variables and assignment

Distinguish C++ initialization from assignment, trace copied and overwritten values, choose suitable types and try self-checks before revealing answers.

Written by InfoCode · Technical references below

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Variables, types and assignment

A variable is like a labelled box: int score = 80; creates an integer variable named score and initializes it to 80.

A declaration tells the compiler the type and name. Initialization gives a new variable its first value; a later score = 90; is assignment. score = score + 5; first calculates with the old value and then stores the result. It is not a mathematical equality.

Assignment normally copies the current value. After int b = a;, changing a does not change b. To swap values, first preserve the value that would otherwise be overwritten in a temporary variable. A fixed value can be declared as const int DAYS = 7; and cannot be assigned to afterwards.

Choose a type, then trace each assignment

int commonly stores integers, double stores floating-point approximations, char represents one character, and bool represents true or false. Types affect both storage and expression evaluation. Integer ranges depend on the implementation; double cannot represent every decimal exactly.

Initialization gives a new variable its first value; assignment updates an existing variable. Predict a and b before tracing these statements:

After	a	b
int a = 4;	4	Not declared
int b = a;	4	4
a = a + 3;	7	4

b copies the value at that moment; it does not stay linked to a. a = a + 3 reads the old value 4, computes 7, then overwrites a. Initialize an ordinary local variable before reading it.

Run and check: copy the current value

#include <iostream>
int main() {
    int a = 4;
    int b = a;
    a = a + 3;
    std::cout << a << " " << b << "\n";
}

Output: 7 4. Updating a does not assign a new value to b.

Try before revealing each answer

Next execute b = a; a = 1;. What are a and b?

a is 1 and b is 7. b copied a at the time of the second assignment.

Does a = 3; b = 8; a = b; b = a; swap the values?

No: both end as 8. The first assignment overwrites 3; preserve the old value in a temporary variable.

Which types could store a book page count and whether it is finished?

Use an int with a suitable range for the page count and bool for whether it is finished. Choose types by meaning and range.

Assignment and swapping examples

Example 1: trace assignments line by line

#include <iostream>
using namespace std;
int main() {
    int a = 6;
    int b = a;
    a = a + 4;
    cout << a << " " << b << endl;
}

The output is 10 6. b stores the value 6 from its initialization; it is not an alias for a.

Example 2: preserve the old value before swapping

#include <iostream>
using namespace std;
int main() {
    int a = 3, b = 8;
    int oldA = a;
    a = b;
    b = oldA;
    cout << a << " " << b << endl;
}

The output is 8 3. Writing a=b; b=a; directly would make both values 8, losing the original 3.

Tips

Reading an uninitialized ordinary local variable can cause undefined behavior. Do not assume its value is zero.
The left side of an assignment must be modifiable. You cannot write 3 = a.
You cannot define the same name twice in one scope. Variable names are case-sensitive.

Technical references

Scope note

This lesson is written by InfoCode and is not an official syllabus; the reference level is a site mapping and does not represent official levels, certification or eligibility. Practice records do not establish mastery or eligibility.

Reference level: GESP Level 1 (site mapping)

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