I'm attempting to compare two string arrays. Whenever I get to the while loop inside of the if statement, I get stuck in an infinite loop because even though I have an iterator inside of the loop, it doesn't increment. I have the cout<< finder; in the loop just to see what finder is at, and it never increments above zero. If anyone could help I'd really appreciate it.
if (memory[p] == "J")
{
if (is_number(memory[p+1]))
{
worker = atoi(memory[p+1].c_str());
p = worker;
continue;
}
else
{
int finder = 0;
while (memory[p+1] != Symtablelab[finder])
{
cout << finder;
finder = finder + 1;
}
if (memory[p+1] == Symtablelab[finder])
{
int k = Symtablepos[finder];
worker = atoi(memory[k].c_str());
p = worker;
continue;
}
}
}
You said finder never increments above zero. Does it print finder = 0 at all? If it does, it means
memory[p+1] = Symtablelab[1]
just after 1st iteration, so the while loop gets terminated and finder sticks at 1.
EDIT
If you say, it prints finder = 0 continuously inside the while statement, then probably you have if (memory[p] == "J") inside an outer for or while (looping) statement.
If it is continuously printing finder and it is 0, then I must ask if this whole code snippet you posted is enclosed in a while statement that you did not post. It makes absolutely no sense that the while loop included in the statement you posted would not be incrementing finder if it is the loop that gets stuck in an infinite loop.
Or the other possibility is that Symtablelab has overriden the '[' ']' operators. If neither of these things are true, that something incredibly wonky is going on.
Related
Is break the most efficient way to exit a loop? In the code snippet below, would line A or line B be the more efficient way to exit the loop? Any links to material on how the break instruction works under the hood would be appreciated.
for (int i = 0; i < 10; i++) {
cout << i << endl;
if (i == 3) {
break; // A
i = 15; // B
}
}
I assume the difference is trivial in most situations, and that A is faster because B requires an assignment, an increment, and then a comparison, but I don't actually know how break works and it's better to know than assume. Thanks!
Let's compile the following code and look at the assembly:
#include <stdio.h>
int loop_with_break() {
for (int i = 0; i < 10; i ++) {
puts("Hello, world!");
if (i == 3) {
break;
}
}
}
int loop_with_assignment() {
for (int i = 0; i < 10; i ++) {
puts("Hello, world!");
if (i == 3) {
i = 10;
}
}
}
int main() {
loop_with_break();
loop_with_assignment();
}
As you can see, when you use break, if i == 3, it'll jump straight out of the loop to the end of the function, whereas if you use i = 10, it'll set i to 10, increment it by 1, then do the comparison, which is slower. However, this was compiled with no optimizations. If you use optimizations, they both end up becoming the same thing. However, since break is more readable and these optimizations are not guaranteed, you should use it for breaking out of loops.
The keyword break will just quit the loop without any comparison made in the loop condition. If you use i = 15 syntax, the loop will iterate next time to verify whether the variable i is greater than 10, if so, then quit.
In short words, break will break the loop without thinking anything, whereas i = 15 will lead one more iteration to look if the condition satisfies it.
You're right! Actually break keyword is very faster for breaking loop!
In your example, if you use line A, then as soon as control reaches this statement, it will immediately break the loop.
On the other hand, if you use line B, then first the assignment will be performed and again the control will go to the condition checking and when the condition will get false then it will exit from the loop!
if you choose while loop you can prefer to make condition false in the while loop. But in this case using break make sense.
Had a new problem with the while function. As easy as it sounds, I still can't wrap my head around it.
Like my last program, this one closes unexpectedly after the correct and wrong messages.
I want this to loop after entering a number, so that the program won't stop.
Thanks for the help, if any.
#include <iostream>
using namespace std;
int main()
{
int X = 0; //setting the first variable
int num; //setting the second
while (X == 0) //this should happen whenever X is equal to 0
{
cout << "Type a number bigger than 3. "; //output
X++; //This should increase X, so that the next while function can happen
}
while (X == 1) //again, since I increased x by one, (0+1=1 obviously) this should happen
{
cin >> num; //standard input
if (num > 3) //if function: if num is bigger than three, then this should happen
{
cout << "Correct! Try again!" <<endl; //output
X--; //Here I'm decreasing x by one, since it was 1 before, now it becomes 0. This should make the "while (X == 0)" part happen again, so that another number bigger than three can be entered
}
if (num <= 3) //if function: if num is lesser than or equal to 3, this should happen
{
cout << "Wrong! Try again!" <<endl; //output
X--; //This is supposed to work like the "X--;" before, repeating the code from "while (X==0)"
}
}
}
now it becomes 0. This should make the "while (X == 0)" part happen again
Nope. While loops don't magically take effect at any point during execution of the program. You only enter a while loop when you've reached it from code above. Programs are executed top-to-bottom, generally.
You would need a loop around the entire program if you want to keep going round and round. Those whiles you have now should probably be ifs.
Merge the two while loops into one, while(true).
Put each previous while body into an if state with the clause from the old while in it.
while(true) {
if (X==0) {
// the X==0- case
} else if (X==1) {
// the X==1 case
}
}
in order to end your loop, do a break;.
You have to think of C++ programs as a sequence of instructions, like a recipe. while just means a loop: you check the condition. If true, you run the body. After running the body, you check only that condition again, and run the body if true. Whenever the condition is false at the start or end of the body of the while (the {} enclosed code after it), you end the loop and proceed to the next one.
The first loop runs, finishes, then the second loop runs in your code. Once the first loop exits, you do not go back into it just because the condition becomes true.
Understanding flow control is one of the "hard" steps of learning to program, so it is ok if you find this tricky.
There are many improvements you can do your code beyond getting it working -- there is, actually, little need for X at all. But baby steps! Once you get it working, you can ponder "how could I remove the variable X?".
Before making such fundamental changes to your program, you should get it working, and save a copy of it so you can "go back" to the last working version.
You want to wrap all that code in it's own while loop:
while (true /* or something */)
{
while (X == 0) //this should happen whenever X is equal to 0
{
// ...
}
At least put your second while loop inside the first one to get it working as intended. Otherwise your program has no reason to go back again.
Nevertheless it's not a good design.
This is the snippet of code that I'm puzzled about. I'm checking for how long an incoming string is. I've appended * in order to have a sentinel value to stop the while loop. Yet, I'm consistently getting a length value that is inclusive of the * and I don't understand why, since the while loop with the nested if ought to stop prior to the *. Can someone point out what I'm doing wrong and why I'm having this issue?
void conversion(string romanIn)
{
length=0;
romanIn.append("*");
while(item!="*")
{
if(item != "*")
{
item = romanIn[length];
length++;
}
cout<<item;
}
you are naturally going to get a +1 the first time through the loop because you aren't initializing the variable "item". Also make it a do while instead of a while loop.
Try this:
do
{
// This line moves out of the if statement
item = romanIn[length];
if(item != "*")
{
length++;
}
cout<<item;
}while(item!="*")
What is the initial value of item?
Let's assume it's 0. You enter the loop
item == 0 != marker, so you enter the if as well, and you say
item = romanIn[0], length++
If romanIn[0] == "*" you will exit the loop, but your length now says 1 which includes the marker
Sorry for this very simple looking problem, but I have no idea what causes it:
In a C++ project I have a loop in a loop in a loop and have to leave the inner two so I have a variable for a query. In the first iteration it works fine, but in the second the assign from dtime to abbruch does not work. In the Debugger dtime has correctly the value "1" and abbruch "0" but this stays after the assignment. Both are of type long.
for (sect = 0; sect <= sections; sect++)
{
abbruch = 0;
for(line = 0; line < maxlines ; line ++)
{
abbruch = dtime[sect][0];
if(abbruch != 0)
{
break;
}
for (index = 0; index < 30; index ++)
{
if (complicated query)
{
dtime[sect][0] = DiffTime[maxlines * sect + line];
break;
}
}
}
}
I use VS2012 Ultimate.
Has anyone an idear how this can happen ot how to solve it?
Did you maybe mean to put this?
abbruch = dtime[sect][line];
(line instead of 0)
But also what Bathseba said is true. A break will only break one for-loop.
break will only take you out of the current for loop. In your case, the loop over index will not be called following a break when abbruch != 0 as that break will take you out of the loop over line. The other break statement will take you out of the loop over index.
That's the rationale, but, by far the best thing to do is to step through with a debugger. I wouldn't use break statements in this way as it's too confusing. Consider breaking the triple loop structure into function calls using return statements in place of breaks.
Also, it's a good idea to localise the interating variables in the for loops, e.g.
for (int sect = 0; sect <= sections; sect++)
I am very curious to learn why the below code does not run in a continuous loop. And I'm also looking for some ways to achieve what I want to achieve--which is resetting a loop inside of the loop. I need to do this because I need to account for each element in a container. The reason why this is because I might start off in the middle, and need to loop back around to check the others / and need to recheck other information too. So on with my little test example:
for ( int i = 0; i != 10; i++ ) {
std::cout << std::endl << "TEST: " << i << std::endl;
if ( i++ == 10 ) {
i = 0;
} else {
i--;
}
}
Is there any particular reason why the above does not work? I am very interested in knowing why, so I can learn how everything works. This also leads into a much bigger problem I am facing. Which is the below code. I am using MSVC++ 2010 Express. Also, this is one thread, so other data is not accessing it. It is an unordered_map using STL. its size if 2 (i checked).
for (game_player_client_map::const_iterator it = gpc_map_ptr->begin(); it != gpc_map_ptr->end(); ++it) {
if ( it++ == gpc_map_ptr->end() ) {
cout << endl << "IT == gpc_map_ptr->end()" << endl;
it = gpc_map_ptr->begin();
} else {
it--;
}
}
I appreciate any feedback SO has to offer, and any new things to learn :-) If further information is needed I will provide. Thank you for your time.
Because the condition is checked before the body of the loop is entered. When i == 10, the loop is broken, before your code can execute at the time that i++ would evaluate to 10.
Remember that postincrement increments the variable and returns the old value. So if i is 9, i++ evaluates to 9 also, but the next time you use i, it will be 10.
If you want the variable to be incremented and use the new value in an expression, use preincrement:
if (++i == 10) // changes i to i + 1 and checks if the new value of i is 10
You could completely ditch the increment however, and just use i + 1. That way you don't have to de-increment i in the else block.
Your misunderstanding of postincrement is probably also the source of the bug in the second block of code you posted. You can change it to preincrement, or if it is a random-access iterator, you can do the same thing as mentioned above and check if it + 1 == gpc_map_ptr->end() and not have to de-increment it in the else block.