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C++ Programming Code Examples

C++ > Algorithms Code Examples

Read Text Files

/* Read Text Files */ void main() { char pcMyLines[10000]; ReadLines("C:\\MyTestFile.txt", 10, 5, pcMyLines, 12345); } // This function reads n lines from a file // Returns the amount of bytes read int ReadLines(const char *pcFile, const int iStartLine, const int iTotLines, char *pcBuffer, const int iBufLen) { FILE *fSrc; int iLines = 0, j, iPos, iByteCount = 0; // Try to open the file if(fSrc = fopen(pcFile, "r")) { // Try to locate the starting line if(iPos = GetLinePos(fSrc, iStartLine)) { // Set the starting position if(!fseek(fSrc, iPos, SEEK_SET)) { // Read the lines for(iLines=0;iLines<iTotLines;iLines++) { // Read the line if((j = ReadLine(fSrc, iPos, pcBuffer, iBufLen - iByteCount)) == -1) break; iByteCount += j; } } } // Close the file fclose(fSrc); } // Return the amount of bytes in the buffer, or -1 for failure return iLines == iTotLines ? iByteCount : -1; } // This function reads a single line from a file int ReadLine(FILE *fSrc, const int iStartPos, char *pcBuffer, const int iBufLen) { int iCount = 0; // Set the starting position if(!fseek(fSrc, iStartPos, SEEK_SET)) { // Keep reading characters while we can while(!feof(fSrc) && (iCount < iBufLen)) { // Read the character pcBuffer[iCount] = (char)fgetc(fSrc); if(pcBuffer[iCount++] == '\n') break; } } else iCount = -1; return iCount; } // This function locates a line in a file // Returns the position of the line in the file int GetLinePos(FILE *fSrc, const int iLine, const int iStartPos /* 0 */) { char pcBuffer[4096]; int i, iPos = 0, iLines = 0, iBytes; // Set the start position if(!fseek(fSrc, iStartPos, SEEK_SET)) { // Read the (next) block of data while(iBytes = fread(pcBuffer, sizeof(char), 4096, fSrc)) { // Go through the block for(i=0;i<iBytes;i++) { // Do we have a new line? if(pcBuffer[i] == '\n') { // Update the line count iLines++; // Have we reached the desired line? if(iLines == iLine) { // Store the position iPos += i; break; } } } // Update the position iPos += iBytes; } } // Return the position of the line in the file, or -1 for failure return iLines == iLine ? iPos : -1; }

In computer programming, we use the if statement to run a block code only when a certain condition is met. An if statement can be followed by an optional else statement, which executes when the boolean expression is false. There are three forms of if...else statements in C++: • if statement, • if...else statement, • if...else if...else statement, The if statement evaluates the condition inside the parentheses ( ). If the condition evaluates to true, the code inside the body of if is executed. If the condition evaluates to false, the code inside the body of if is skipped.

Reposition stream position indicator. Sets the position indicator associated with the stream to a new position. For streams open in binary mode, the new position is defined by adding offset to a reference position specified by origin. For streams open in text mode, offset shall either be zero or a value returned by a previous call to ftell, and origin shall necessarily be SEEK_SET. If the function is called with other values for these arguments, support depends on the particular system and library implementation (non-portable). The end-of-file internal indicator of the stream is cleared after a successful call to this function, and all effects from previous calls to ungetc on this stream are dropped.

Logical Operators are used to compare and connect two or more expressions or variables, such that the value of the expression is completely dependent on the original expression or value or variable. We use logical operators to check whether an expression is true or false. If the expression is true, it returns 1 whereas if the expression is false, it returns 0. Assume variable A holds 1 and variable B holds 0:

Close file. Closes the file associated with the stream and disassociates it. Closes the given file stream. Any unwritten buffered data are flushed to the OS. Any unread buffered data are discarded. Whether or not the operation succeeds, the stream is no longer associated with a file, and the buffer allocated by std::setbuf or std::setvbuf, if any, is also disassociated and deallocated if automatic allocation was used. All internal buffers associated with the stream are disassociated from it and flushed: the content of any unwritten output buffer is written and the content of any unread input buffer is discarded.

The header file graphics.h contains line() function which is used to draw a line from a point(x1, y1) to point(x2, y2) i.e. (x1, y1) and (x2, y2) are end points of the line. The function line() draws a line on the graphics screen between two specified points. So this function requires four parameters namely x1, y1, x2, and y2 to represent two points. This function draws a line from (x1, y1) coordinates to (x2, y2) coordinates on the graphics screen.

Check whether eofbit is set. Returns true if the eofbit error state flag is set for the stream. This flag is set by all standard input operations when the End-of-File is reached in the sequence associated with the stream. Note that the value returned by this function depends on the last operation performed on the stream (and not on the next). Operations that attempt to read at the End-of-File fail, and thus both the eofbit and the failbit end up set. This function can be used to check whether the failure is due to reaching the End-of-File or to some other reason.

The sizeof() is an operator that evaluates the size of data type, constants, variable. It is a compile-time operator as it returns the size of any variable or a constant at the compilation time. The size, which is calculated by the sizeof() operator, is the amount of RAM occupied in the computer. The sizeof is a keyword, but it is a compile-time operator that determines the size, in bytes, of a variable or data type. The sizeof operator can be used to get the size of classes, structures, unions and any other user defined data type. The data_type can be the data type of the data, variables, constants, unions, structures, or any other user-defined data type.

Read block of data. Extracts n characters from the stream and stores them in the array pointed to by s. This function simply copies a block of data, without checking its contents nor appending a null character at the end. If the input sequence runs out of characters to extract (i.e., the end-of-file is reached) before n characters have been successfully read, the array pointed to by s contains all the characters read until that point, and both the eofbit and failbit flags are set for the stream. Internally, the function accesses the input sequence by first constructing a sentry object (with noskipws set to true). Then (if good), it extracts characters from its associated stream buffer object as if calling its member functions sbumpc or sgetc, and finally destroys the sentry object before returning.

Break statement in C++ is a loop control statement defined using the break keyword. It is used to stop the current execution and proceed with the next one. When a compiler calls the break statement, it immediately stops the execution of the loop and transfers the control outside the loop and executes the other statements. In the case of a nested loop, break the statement stops the execution of the inner loop and proceeds with the outer loop. The statement itself says it breaks the loop. When the break statement is called in the program, it immediately terminates the loop and transfers the flow control to the statement mentioned outside the loop.

In computer programming, loops are used to repeat a block of code. For example, when you are displaying number from 1 to 100 you may want set the value of a variable to 1 and display it 100 times, increasing its value by 1 on each loop iteration. When you know exactly how many times you want to loop through a block of code, use the for loop instead of a while loop. A for loop is a repetition control structure that allows you to efficiently write a loop that needs to execute a specific number of times.

In while loop, condition is evaluated first and if it returns true then the statements inside while loop execute, this happens repeatedly until the condition returns false. When condition returns false, the control comes out of loop and jumps to the next statement in the program after while loop. The important point to note when using while loop is that we need to use increment or decrement statement inside while loop so that the loop variable gets changed on each iteration, and at some point condition returns false. This way we can end the execution of while loop otherwise the loop would execute indefinitely. A while loop that never stops is said to be the infinite while loop, when we give the condition in such a way so that it never returns false, then the loops becomes infinite and repeats itself indefinitely.

C supports nesting of loops in C. Nesting of loops is the feature in C that allows the looping of statements inside another loop. Any number of loops can be defined inside another loop, i.e., there is no restriction for defining any number of loops. The nesting level can be defined at n times. You can define any type of loop inside another loop; for example, you can define 'while' loop inside a 'for' loop. A loop inside another loop is called a nested loop. The depth of nested loop depends on the complexity of a problem. We can have any number of nested loops as required. Consider a nested loop where the outer loop runs n times and consists of another loop inside it. The inner loop runs m times. Then, the total number of times the inner loop runs during the program execution is n*m.

The if...else statement executes two different codes depending upon whether the test expression is true or false. Sometimes, a choice has to be made from more than 2 possibilities. The if...else ladder allows you to check between multiple test expressions and execute different statements. In C/C++ if-else-if ladder helps user decide from among multiple options. The C/C++ if statements are executed from the top down. As soon as one of the conditions controlling the if is true, the statement associated with that if is executed, and the rest of the C else-if ladder is bypassed. If none of the conditions is true, then the final else statement will be executed.

A program shall contain a global function named main, which is the designated start of the program in hosted environment. main() function is the entry point of any C++ program. It is the point at which execution of program is started. When a C++ program is executed, the execution control goes directly to the main() function. Every C++ program have a main() function.

As the name already suggests, these operators help in assigning values to variables. These operators help us in allocating a particular value to the operands. The main simple assignment operator is '='. We have to be sure that both the left and right sides of the operator must have the same data type. We have different levels of operators. Assignment operators are used to assign the value, variable and function to another variable. Assignment operators in C are some of the C Programming Operator, which are useful to assign the values to the declared variables. Let's discuss the various types of the assignment operators such as =, +=, -=, /=, *= and %=. The following table lists the assignment operators supported by the C language:

In the C++ Programming, The static keyword allows a variable to maintain its value among different function calls. If the value of a static variable changes when the variable has been