C Programming Midterm Exam Review Guide

To manage large projects, declarations are often placed in header files (.h) and implemented in source files (.c). To prevent a header from being processed multiple times, which would cause errors, you must use inclusion guards:

#ifndef UTILS_H
#define UTILS_H
int add(int a, int b);
#endif

Control Flow: Conditions and Loops

Logic in C is driven by Boolean expressions using relational operators ($<, >, \le, \ge, ==, \neq$) and logical operators ($&&, ||, !$).

• Conditions: Use if/else for branching and switch for multiple discrete values. Be careful: = is an assignment, while == is a comparison.
• Loops: Use for loops for count-controlled repetition (like iterating through an array), while loops for condition-based repetition, and do-while loops when the body must execute at least once.

Arrays and Memory Management

Arrays are fixed-size, homogeneous, and contiguous blocks of memory. Because indexing starts at 0, an array of size $N$ has valid indices from $0$ to $N-1$; accessing arr[N] is an "out of bounds" error.

You can use the sizeof operator to calculate the number of elements in a local array by dividing the total bytes of the array by the bytes of one element: $\text{count} = \frac{\text{sizeof(array)}}{\text{sizeof(array[0])}}$ However, be warned: when an array is passed to a function, it "decays" into a pointer, and this sizeof trick will no longer return the original array length.

Common Mistake: When traversing an array with a loop, never use i <= size. Always use i < size to avoid accessing memory outside the array bounds.

Pointers and Parameter Passing

Pointers are one of the most critical concepts in C. A pointer stores a memory address. You use the & (address-of) operator to get an address and the * (dereference) operator to access the value stored at that address.

A helpful mental shortcut is:

• & = WHERE is the variable?
• * = WHAT IS THERE at that address?

Pass by Value vs. Pass by Pointer

By default, C uses pass by value, meaning the function receives a copy of the data. To allow a function to modify the original variable in the caller's scope, you must use output parameters by passing the address of the variable using & and receiving it as a pointer.

void setValue(int *out) { *out = 100; } // Function definition
int x = 5;
setValue(&x); // Function call: x is now 100

Strings and File I/O

In C, a string is simply a char array that ends with the null terminator '\0'. You should use the <string.h> library for manipulations:

• strlen: Get length.
• strcpy: Copy one string to another.
• strcat: Append (join) strings.
• strcmp: Compare strings (returns $0$ if they are equal).

For persistent data, use File Streams via the FILE * type. The standard workflow is: Open (fopen), Check for NULL, Operate (fscanf/fprintf or fgets/fputs), and Close (fclose).

Compilation, Linking, and Debugging

The process of turning code into a running program involves several steps:

1. Source Code: Your human-readable .c files.
2. Object Code: Machine code produced by the compiler (e.g., using gcc -c file.c to create file.o).
3. Executable: The final program created by the linker, which combines object files and libraries.

A typical gcc command to compile and link multiple files is: gcc main.c utils.c -o program.

Debugging Checklist

When your code doesn't work, check for these common issues:

• Compiler Errors: Syntax mistakes that stop the build.
• Warnings: Suspicious code (use gcc -Wall to see them).
• Logic Errors: The program runs but gives the wrong answer (e.g., using = instead of ==).
• Memory Errors: Dereferencing a NULL pointer or accessing an array out of bounds.

Final Study Priority: Focus your remaining time on arrays, loops, pointer memory diagrams, and the difference between & and *.