Understanding for Loops and Array Management in C Programming

1. CSCI 171 Presentation 4

2. Execution of a C Program • Execution starts in main( ) • Top down style • sequential flow • Unrealistic to expect sequence in more complicated programs

3. Program Control • Change order in which program statements are executed • logic may no longer be top down • Done by using Program Control Statements • for loops • while loops

4. for statement • Executes a block of code a predetermined number of times • number of executions is known prior to iterations being performed • Counted Repetition Structure

5. for Statement • Format: • for (initial; condition; increment) • 1. initial evaluated (usually assignment) • 2. condition evaluated (usually relational) • 3. condition is false - terminate for loop • condition is true - C statements within • loop execute • 4. increment executed - return to step 2

6. Example of for loop //Write the C code to display the first 3 positive integers //DEMONSTRATING A FOR LOOP #include <stdio.h> void main() { int count = 0; for (count = 1; count <= 3; count ++) { printf(“\n%d”, count); } }

7. Example of for loop //Write the C code to display the first 3 positive integers in descending order //DEMONSTRATING A FOR LOOP #include <stdio.h> void main() { int count = 0; for (count = 3; count >= 1; count --) { printf(“\n%d”, count); } }

8. Sample Program 4.1 #include <stdio.h> int main( void ) { int index = 0; for (index = 2; index <= 40; index+=2) { printf("%d\n", index); } }

9. Flexibility of for statement int count = 1; for (; count < 1000; count++) IS equivalent to: int count = 0; for (count = 1; count < 1000; count ++)

10. Scope of Variables • Index value holds a value after loop void main() { int count = 0; for (count = 1; count <= 3; count ++) { printf(“\n%d”, count); } printf(“\n%d”, count); }

11. Sample Program 4.2 #include <stdio.h> int main( void ) { int index = 0; for (index = 2; index <= 38; index+=2) printf("%d\n", index); printf("%d\n", index); }

12. Omission of increment void main() { int count = 0; for (count = 1; count <= 3;) { printf(“\n%d”, count); } }

13. Omission of Increment - cont’d void main() { int count = 0; for (count = 1; count <= 3;) { printf(“\n%d”, count); count++; } }

14. Sample Program 4.3 #include <stdio.h> int main( void ) { int index = 0; for (index = 2; index <= 38; index+2) printf("%d ", index); }

15. Quick Introduction to Arrays • Indexed group of storage locations • Same type of variables • Identified by subscript (i.e. the index) • int data[1000]; • creates a storage location for 1000 integer elements in the variable data • First location in an array is 0 (not 1)

16. Referencing elements in an array • int i; • int data[3]; • data[0] = 3; • data[1] = –4; • data[2] = 7; • for (i = 0; i < 3; i++) • { • printf(“%d “, data[i]); • }

17. Referencing elements in an array • Can use variables or integers • int count = 2; • data[count] • is equivalent to: • data[2];

18. Sample Program 4.4 #include <stdio.h> int main( void ) { int my_array[5], sum = 0; my_array[0] = 1; my_array[1] = 2; my_array[2] = 3; my_array[3] = 4; my_array[4] = 5; sum = my_array[0] + my_array[1] + my_array[2] + my_array[3] + my_array[4]; printf("The sum of the elements in the array is: %d", sum); }

19. Sample Program 4.5 #include <stdio.h> int main( void ) { int my_array[5], sum = 0, index = 0; for (index = 0; index < 5; index++) my_array[index] = index + 1; for (index = 0; index < 5; index++) sum+=my_array[index]; printf("The sum of the elements in the array is: %d", sum); }

20. Logical operators in a for loop • Assume a 10 element array with random integers has been defined • Search the array to determine if the input value (searchElement) is in the array: • searchFlag = ‘N’; • scanf(“%d”, &searchElement); • for (cnt = 0; cnt < 10; cnt++) • { • if (array[cnt] == searchElement) • searchFlag = ‘Y’; • }

21. Logical operators in a for loop • Logical operators can sometimes be used to increase performance • searchFlag = ‘N’; • scanf(“%d”, &searchElement); • for (cnt = 0; cnt < 10 && searchFlag = = ‘N’; cnt++) • { • if (array[cnt] == searchElement) • searchFlag = ‘Y’; • }

22. The break in a for loop • The break statement will unconditionally exit the structure it is inside of • scanf(“%d”, &searchElement); • for (cnt = 0; cnt < 10; cnt++) • { • if (array[cnt] == searchElement) { • searchFlag = ‘Y’; break; • } • }

23. Null statement following for • For loop does not necessarily have to have programming statements within the body of the loop • Example: • for (cnt = 0; cnt < 10; array[cnt++] = 50);

24. Commas in for loops • Expressions can be separated by commas • Subexpressions evaluated left to right • Write a for loop to copy the contents of a 100 element array called x into a 100 element array called y in the reverse order • for (i = 0, j = 99; i < 100; i++, j--) • { • y[i] = x[j]; • }

25. Nested for statements • One loop may be embedded within another loop • Nested for loops may be inefficient

26. Nested for example • void main() { • int i = 0, j = 0; • for (i = 0; i < 3; i++) • { • for (j = 0; j < 3; j++) • { • printf(“X”); • } • printf(“\n”); • } • }

27. Sample Program 4.6 #include <stdio.h> int main( void ) { int row = 1, column = 1; printf("Multiplication Table"); printf("\n--------------------"); for (; row < 6; row++) { printf("\n"); for (column = 1; column < 6; column++) printf("%d\t", row*column); } }