Lesson 5 - Assignment Statement
5.1 Assignment statement
// ex5-1.cpp - assignment statement
#include <iostream>
using namespace std;
int main()
{
cout << "Enter two numbers : ";
double num1, num2;
cin >> num1;
cin >> num2;
double bigger;
if (num1 > num2)
bigger = num1;
else
bigger = num2;
cout << "Bigger is " << bigger << endl;
return(0);
}
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ex5-1
Enter two numbers : 35 46
Bigger is 46
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Example ex5-1 introduces a new statement which allows us to directly change
the contents of a variable. It is called the 'assignment' statement
and it has the syntax:
variable = expression;
where 'variable' means the name of an existing variable, and 'expression'
means some kind of value made up from numbers, variables or strings, combined
with arithmetic operations such as add, multiply, etc. In general
the type of the expression has to match the type of the variable: if
the variable is an integer, the expression needs to be an integer; if the
variable is a double, the expression needs to be of double type (i.e. a
fractional number); if the variable is a string, then the expression must
evaluate to a sequence of characters. In the example, the variable
'bigger' is assigned one of two values depending on the test performed
on the input numbers num1 and num2. If num1 is bigger than num2,
then the variable bigger is set to num1, otherwise it is set to num2.
This logic ensures that the variable 'bigger' contains a value equal to
the larger of the two input numbers. |
5.2 Assignment statement changes variable
// ex5-2.cpp - assignment statement changes variable
#include <iostream>
using namespace std;
int main()
{
cout << "Enter a number : ";
double num;
cin >> num;
double numcopy = num;
cout << "Initial values:" << endl;
cout << "num=" << num;
cout << " numcopy=" << numcopy << endl;
num = num+1;
cout << "After assignment:" << endl;
cout << "num=" << num;
cout << " numcopy=" << numcopy << endl;
return(0);
}
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ex5-2
Enter a number : 10
Initial values:
num=10 numcopy=10
After assignment:
num=11 numcopy=10
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Example ex5-2 demonstrates an important characteristic of the assignment
statement: when the assignment statement is used to assign a value to a
variable, the old contents of that variable are lost. In this example,
a number is input into the variable 'num', and a copy of that value is
stored in the variable 'numcopy'. The statement 'double numcopy =
num' is a kind of joint declaration and assignment statement: it simultaneously
declares a new variable 'numcopy' and initialises it to the same value
as 'num'. The program then outputs the values of num and numcopy
to show they are the same. The statement 'num = num + 1' looks mysterious
to many people. It is not a test on the value of num (this would
actually look like 'num==num+1' and would always be false), but an assignment
statement which says evaluate the value of num+1 and assign that value
to num. The consequence is that the value of num is increased by
one. This is shown by the second set of output statements. |
5.3 Exercises
a. Write a program (assign.cpp) that given a number
x, calculates x to the power 2, x to the power 3,
and x to the power 4, using no more than three multiplications,
as in:
assign
Enter value x : 3
x to power 2 = 9
x to power 3 = 27
x to power 4 = 81
b. Write a program (stars.cpp) that prints 5 rows
of asterisks, with each row containing twice as many asterisks as the previous
row, but with your program only containing a single asterisk, as
in:
stars
*
**
****
********
****************
Hint: for strings, the '+' operator means concatenate; so "Hello "+"there"
=> "Hello there".
c. Adapt exercise 4.5b (small2.cpp) to use
assignment rather than multiple print statements.
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