- IS12 Introduction to Programming Lecture 10: Simple Programs
Peter Brusilovsky
http://www2.sis.pitt.edu/~peterb/0012-041/
Exchange Kiosk: Assignment
void main() { float dollars_for_mark; /* exchange rate */ int commission; /* commission in dollars */ float marks; /* marks given */ float dollars; /* dollars returned */ /* get data */ dollars_for_mark = 0.666; commission = 3; marks = 100; /* calculate USD */ dollars = marks * dollars_for_mark commission; /* print result */ printf("For %6.2f marks you will get %6.2f dollars!\n" , marks, dollars); }
Exchange Kiosk: Initialization
void main() { float dollars_for_mark = 0.666; /* exchange rate */ int commission = 3; /* commission in dollars */ float marks = 100; /* marks given */ float dollars; /* dollars returned */ /* calculate USD */ dollars = marks * dollars_for_mark commission; /* print result */ printf("For %6.2f marks you will get %6.2f dollars!\n" , marks, dollars); }
Exchange Kiosk: #define
#define DOLLARS_FOR_MARK 0.666 #define COMMISSION 3 #define MARKS 100 void main() { float dollars; /* dollars returned */ /* calculate USD */ dollars = MARKS * DOLLARS_FOR_MARK COMMISSION ; /* print result */ printf("For %6.2f marks you will get %6.2f dollars!\n" , MARKS, dollars); }
Defined constants
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#define is a preprocessor directive for defining constants Example: #define COMMISSION 3 COMMISSION is a defined constant
Now every appearance of commission is literally replaced by 3 It is not a variable, it can't be assigned a value
Why to use defined constants?
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Remember example 2.1?
/* Temperature Converter */ #include <stdio.h> void main() { printf("100 Fahrenheit = %6.2f Celsius\n", (5.0/9.0)*(10032)); }
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Need to replace 100 to 150 in two places (in real programs in dozens places!)
Example: Temperature Converter
/* Example: Temperature Converter Author: Peter Brusilovsky Objective: use of #define and printf */ #include <stdio.h> #define FAHR 100.0 /* the temperature to be converted */ void main() { printf("%6.2f Fahrenheit = %6.2f Celsius\n", FAHR, (5.0 / 9.0) * (FAHR 32)); }
More About Preprocessor
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Preprocessor is a separate part of C compiler, which differs from the rest of it Makes file insertions #include <stdio.h> system files #include "myfile.h" user files Resolves definitions #define NUM 100.0 Removes comments
Example after preprocessing
...content of the file stdio.h...
void main() { printf("%6.2f Fahrenheit = %6.2f Celsius\n", 100.0, (5.0 / 9.0) * (100.0 32)); }
Constants are not variables!
#include <stdio.h> #define FAHR 100.0 /* the temperature to be converted */ void main() { printf("%6.2f Fahrenheit = %6.2f Celsius\n", FAHR, (5.0 / 9.0) * (FAHR 32)); FAHR = 80; printf("%6.2f Fahrenheit = %6.2f Celsius\n", FAHR, (5.0 / 9.0) * (FAHR 32)); }
... after preprocessing
...content of the file stdio.h...
void main() { printf("%6.2f Fahrenheit = %6.2f Celsius\n", 100.0, (5.0 / 9.0) * (100.0 32)); 100.0 = 80; printf("%6.2f Fahrenheit = %6.2f Celsius\n", 100.0, (5.0 / 9.0) * (100.0 }
Usage 32)); Rules
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Defined constant: #define COMMISSION 3
This value stays the same in the program.
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Initialized variable: int commission = 3;
The value can be changed in the program, but the initial value is known before the start of the program
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Assignment: commission = 3;
Initial value is not known before the start of the program
Example: Temperature Converter
/* Example: Temperature Converter with input Author: Peter Brusilovsky */ #include <stdio.h> void main() { float fahr; /* the temperature to be converted */ /* Data input */ printf("Enter temperature in Fahrenheit: "); /* prompt */ scanf("%f",&fahr); /* input */ /* Calculation and printing */ printf("%6.2f Fahrenheit = %6.2f Celsius\n", FAHR, (5.0 / 9.0) * (FAHR 32)); }
Getting Changeable Data Inside
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Defined constant Initialization of variables Assignment Input data
scanf("%d",&marks); The computer will read the symbols from standard input, consider it as an integer (%d tells about it) and assign to the given variable marks. Treat & as a magic sign for now
Example: Better Exchange Kiosk
#include <stdio.h> #define DOLLARS_FOR_MARK 0.666 /* exchange rate */ #define COMMISSION 3 /* commission in dollars */ void main() { int marks; float dollars; /* get data */ printf("Marks to exchange?: "); /* prompt */ scanf("%d",&marks); /* input */ /* calculate USD */ dollars = marks * DOLLARS_FOR_MARK COMMISSION; /* print results */ printf("For %d marks you will get %6.2f dollars!\n" , marks, dollars); }
Example: Capital growth
#include <stdio.h> void main() { float interest_rate; /* interest rate in percents */ int capital; /* starting capital in dollars */ float annual_interest; /* annual interest in dollars */ printf("Capital ($$) and interest rate (%%xx.x): "); scanf("%d...