Lecture 7: Variable Scope B Burlingame March 16, 2016

Announcements Midterm Today – next Tuesday Homework #3 due up front Homework #4 due after break

Learning Objectives Discuss the importance of style in code Define scope and variable duration

Programming Style The layout of program directives using white space, syntax, & punctuation to visually highlight a programmer’s intention Good Style is  Consistent – A given structure always looks the same  Illustrative – The intent of the code is shown by the grammar  Clear – Any reasonable programmer should be able to follow the intent

Programming Style Why does it matter?  Reduces bugs – the human eye is excellent at detecting patterns  Reduces development time  Allows others to understand how to approach your code  Style is a fundamental part of code documentation  9 different style documented

No style #include "stdio.h" #include "math.h" int main() { double time, max_height, velocity_at_apex, time_at_apex,height,velocity = 0; printf( "%4s %10s %14s\n", "time", "height (m)", "velocity (m/s)" ); printf( "%4s %10s %14s\n", "----", " ", " " ); for( time = 0.0; time < 49.0; time = time ) { height = * pow( time, 4.0 ) * pow( time, 3.0 ) ; velocity = (-0.48 * pow( time, 3.0 ) * pow( time, 2.0 ); if( height > max_height ) { max_height = height; velocity_at_apex = velocity; time_at_apex = time; } } printf( "Max height %.2lf, at %.3lf hours, while moving %.3lf m/s\n",max_height, time_at_apex, velocity_at_apex); return(0); }

K&R Style #include "stdio.h" #include "math.h" int main() { double time = 0; double max_height = 0; double velocity_at_apex = 0; double time_at_apex = 0; double height,velocity = 0; printf( "%4s %10s %14s\n", "time", "height (m)", "velocity (m/s)" ); printf( "%4s %10s %14s\n", "----", " ", " " ); for( time = 0.0; time < 49.0; time = time ) { height = * pow( time, 4.0 ) * pow( time, 3.0 ) ; velocity = (-0.48 * pow( time, 3.0 ) * pow( time, 2.0 ); if( height > max_height ) { max_height = height; velocity_at_apex = velocity; time_at_apex = time; } printf( "Max height %.2lf, at %.3lf hours, while moving %.3lf m/s\n",max_height, time_at_apex, velocity_at_apex); return(0); }

Allman Style/BSD Style #include "stdio.h" #include "math.h" int main() { double time = 0; double max_height = 0; double velocity_at_apex = 0; double time_at_apex = 0; double height,velocity = 0; printf( "%4s %10s %14s\n", "time", "height (m)", "velocity (m/s)" ); printf( "%4s %10s %14s\n", "----", " ", " " ); for( time = 0.0; time < 49.0; time = time ) { height = * pow( time, 4.0 ) * pow( time, 3.0 ) ; velocity = (-0.48 * pow( time, 3.0 ) * pow( time, 2.0 ); if( height > max_height ) { max_height = height; velocity_at_apex = velocity; time_at_apex = time; } printf( "Max height %.2lf, at %.3lf hours, while moving %.3lf m/s\n",max_height, time_at_apex, velocity_at_apex); return(0); }

Identifiers and Scope Identifier  The name of a variable, function, label, etc. int my_var1;/* a variable */ pow_table();/* a function */ start:/* a label */ Question:  Does it make a difference where in a program an identifier is declared? YES! --> concept of ‘scope’

Scope of Identifiers Scope of a declaration of an identifier  The region of the program that the declaration is active (i.e., can access the variable, function, label, etc.) Five types of scope:  Program (global scope)  File  Function prototype  Function  Block (“between the { } scope”)

Scope of Identifiers - Block Scope Block (local) scope  A block is a series of statements enclosed in braces { }  The identifier scope is active from the point of declaration to the end of the block ( } )  Nested blocks can both declare the same variable name and not interfere  ex. from Ch var_scope_block.c scope_nested_blocks.c #include double product(double x, double y); int main() { int a = 10; double var1 = 3.0, var2 = 5.0; double ans; ans = product(var1, var2); printf("var1 = %.2f\n" "var2 = %.2f\n",var1,var2); printf("var1*var2 = %g\n", ans); } /* function definition */ double product(double x, double y) { double result; result = x * y; return result; }

Scope of Identifiers - Program (Global) Scope Program (global) scope  if declared outside of all functions  "Visible" to all functions from point of declaration  Visible to functions in other source files  Use only when necessary and then very carefully!!  ex. from Ch var_scope.c #include int a = 10; double product(double x, double y); int main() { double var1 = 3.0, var2 = 5.0; double ans; ans = product(var1, var2); printf("var1 = %.2f\n" "var2 = %.2f\n",var1,var2); printf("var1*var2 = %g\n", ans); } /* function definition */ double product(double x, double y) { double result; result = x * y; return result; }

Function scope  Applies only to labels start: * goto start;  Active from the beginning to the end of a function  Ex. Statement labels in a switch selection structure Scope of Identifiers - Function Scope #include int main() { int user_sel; /* prompt user for entry */ /* get user entry */ switch( user_sel ) { case 1: printf("\n message..."); /* call game function1 here */ break; case 2: printf("\n message..."); /* call game function2 here */ break; default: printf("Error"); break; }

Storage Duration How long the identifier exists in memory Static storage class  Identifier exists when program execution begins For variables:  Storage allocated and variable is initialized once  Retains their values throughout the execution of the program #include void just_count(void); /* proto */ int main() { int i; for(i=0;i<10;i++) { just_count(); } return 0; } void just_count(void) { static int count_a; int count_b; count_a = count_a + 1; count_b = count_b + 1; printf("count_a== %d\n", count_a); printf("count_b== %d\n", count_b); } just_count.c

For functions:  function name exists when execution begins For variables with global scope:  i.e., declared outside of all functions and uses static keyword  "Visible" to all functions from point of declaration in this source file only  Keeps data ‘private’ to this file only Storage Duration, cont. #include static int a = 10; double product(double x, double y); int main() { double var1 = 3.0, var2 = 5.0; double ans; ans = product(var1, var2); printf("var1 = %.2f\n" "var2 = %.2f\n",var1,var2); printf("var1*var2 = %g\n", ans); } /* function definition */ double product(double x, double y) { double result; result = x * y; return result; }

References Modular Programming in C math.h /xsh/math.h.html /xsh/math.h.html e.html e.html