A part of my program (I can add more details if necessary) contains this line:
if((e->start->explored = false) || (e->end->explored = false)){
//do action...
}
This is part of a graph algorithm, where e is a directed edge with incident vertices "start" and "end." I would like the 'action' to happen if at least one of the incident vertices of e is unexplored, but this logic appears to be faulty. Although I used a small example and verified that, indeed, the start and end vertices of my edges were unexplored to start with, my overall function is going into an infinite loop.
So then I tested it like this:
if((e->start->explored = false) || (e->end->explored = false)){
//do action...
}
else cout << "FAIL";
...and, of course, it printed a screen of "FAIL." What is my logic error here?
You're assigning false to your properties instead of testing them against false. This is a mistake often made, and quite hard to debug. Change your = assignment operator to the equality operator ==:
if((e->start->explored == false) || (e->end->explored == false)) {
// Do action...
} else {
cout << "FAIL";
}
Instead of comparing the values to false, it's clearer to use the ! not operator instead. The inner brackets are done away with, too:
if(!e->start->explored || !e->end->explored) {
// Do action...
} else {
cout << "FAIL";
}
As the others have expounded you accidentally used assignment instead of comparison. However, the real solution is not to compare at all:
Comparing bool values to literals true and false is nonsensical!
Instead, write:
if(! e->start->explored || ! e->end->explored)
You have used the assignment operator = not the comparison operator ==.
You are assigning values here:
if((e->start->explored = false) || (e->end->explored = false)){
Should be:
if((e->start->explored == false) || (e->end->explored == false)){
Related
bool x = someFuncThatReturnsTrueorFalse();
if (!x && (str1.length() != str2.length()) {
// do nothing
} else {
// do something
}
How would I rearrange the code above to eliminate the else statement?
I need to do nothing if bool = false and str1 and str2 have different lengths. Otherwise, I need to do something (e.g. function call).
For the life of me, I can't think of any way to change this. Let me know if you need further details.
The simplest way is putting !() around the condition.
bool x = someFuncThatReturnsTrueorFalse();
if (!(!x && (str1.length() != str2.length())) {
// do something
}
Another way is using De Morgan's laws: !(A && B) is equivalent to !A || !B.
bool x = someFuncThatReturnsTrueorFalse();
if (x || (str1.length() == str2.length()) {
// do something
}
There is nothing wrong leaving as is. The code will still compile
So I know that generally speaking, I should prefer an else-if over and if if. But what if the two conditions aren't related? For example, these would be considered "related" conditionals:
if (line[a] == '{'){
openCurly = true;
}
else if (line[a] == '}'){
closeCurly = false;
}
Notice how the two conditionals in the if-statements are related in a way such that when one is true, the other must be false. This is because line[a] can either be { or } but not both.
Here is another example:
if (line[a] == '{')
{
openCurly = true;
}
else if ((line[a] == ';' && !openCurly) || (line[a] == '}' && openCurly))
{
DoSomething(line);
line = "";
}
The second condition will never evaluate to true if the first condition if true, so it makes sense to have an else-if. However, those two conditionals look vastly different.
So, should I prefer something like this?
if (line[a] == '{')
{
openCurly = true;
}
if ((line[a] == ';' && !openCurly) || (line[a] == '}' && openCurly))
{
DoSomething(line);
line = "";
}
You should use an else-if statement. This is because an if-else construct only checks the second statement if the first one doesn't evaluate to true.
In the example you give,
if (line[a] == '{')
{
openCurly = true;
}
else if ((line[a] == ';' && !openCurly) || (line[a] == '}' && openCurly))
{
DoSomething(line);
line = "";
}
replacing the else if with an if statement would result in the second condition being checked even if the first one is true, which is completely pointless and would also lose you some time.
In the future, make decisions to use else-if statements based on whether the conditions are mutually exclusive or not.
You could do something like this:
#include <stdint.h>
#define COMBINATION(x, y) ((uint16_t(x) << 8) | (uint16_t(y) << 0))
...
switch (COMBINATION(line[a], openCurly))
{
case COMBINATION('{', false):
...
break;
case COMBINATION(';', false):
case COMBINATION('}', true):
...;
break;
}
}
Some may say it's a bit of an overkill, but I think that it may actually help splitting up the logical operation of your program into a set of distinct cases, thus make it easier to handle each case precisely as desired.
I want to do two string compare and used two different if condition. Is there any better way to do string compare in one if condition
if (strcmp(Buff1(), Config1) == 0)
{
if (strcmp(Buff2, Config2) == 0)
{
// my code goes here
}
}
The equivalent code is:
if ((strcmp(Buff1(), Config1) == 0)) &&
(strcmp(Buff2, Config2) == 0))
{
// my code goes here
}
Note: The compiler should generate the same machine code for both code samples. The difference is cosmetic and primarily aimed at the reader of the code.
You do get a difference when you add else clauses:
if (strcmp(Buff1(), Config1) == 0)
{
if (strcmp(Buff2, Config2) == 0)
{
// my code goes here
}
else
{
// else 1
}
}
else
{
// else 2
}
Compared to:
if ((strcmp(Buff1(), Config1) == 0)) &&
(strcmp(Buff2, Config2) == 0))
{
// my code goes here
}
else
{
// Single else clause
}
In addition to Klas's answer(just in case you're not familiar with the AND operator) - the AND operator ('&&') checks the first condition and it continues to check the second condition -only if- the first condition is true.
So in your specific question, it checks if the first couple of strings are equal and only if true (are equal), it checks if the second couple are also equal.
The obvious optimization (not mentioned yet), if you know anything about those strings, is to first perform the compare that is more likely to fail.
I have a quick question regarding proper use of syntax, basically i am trying to summarise these two if statements into one if statement.
if (sc.LastCallToFunction) {
if (p_LowRectanglesList != NULL) {
free(p_LowRectanglesList);
sc.PersistVars->i1 = 0;
}
if (p_HighRectanglesList != NULL) {
free(p_HighRectanglesList);
sc.PersistVars->i2 = 0;
}
return;
Would it be syntactically correct to rewrite this as:
if (sc.LastCallToFunction) {
if (p_LowRectanglesList || p_HighrectangleList != NULL) {
free(p_LowRectanglesList && p_HighRectanglesList);
sc.PersistVars->i1 && sc.PersistVars->i2 = 0;
}
return;
Or would the compiler not accept this / Is my Logic faulty?
you can't do it the way you have given
if (p_LowRectanglesList || p_HighrectangleList != NULL)
this logically ORs the first pointer ( treats it as true or false ) with the comparison of the seond pointer to NULL
free(p_LowRectanglesList && p_HighRectanglesList);
this logically &&s the pointers together and then tries to free the result of that operation. ie, you are trying to free "true" or "false"
sc.PersistVars->i1 && sc.PersistVars->i2 = 0;
this logically ands the two things together, which will result in true or false and then trys to assign 0 to it..... doesn't make any sense at all.
Also, in your original code....after the free, you should put p_LowRectanglesList=NULL;
Here is an extremely simplified version of a section of code that I am having trouble with.
int i = 0;
int count = 0;
int time = 50;
int steps = 1000;
double Tol = 0.1;
bool crossRes = false;
bool doNext = true;
for (int i=0; i<steps; i++) {
//a lot of operations are done here, I will leave out the details, the only
//important things are that "dif" is calculated each time and doNext either
//stays true or is switched to false
if (doNext = true) {
if (dif <= Tol) count++;
if (count >= time) {
i = steps+1;
crossRes = true;
}
}
}
if (crossRes = true) {
printf("Nothing in this loop should happen if dif is always > Tol
because count should never increment in that case, right?");
}
My issue is that every time it gets done with the for loop, it executes the statements inside the "if (crossRes = true)" brackets even if count is never incremented.
You've made a common (and quite frustrating) mistake:
if (crossRes = true) {
This line assigns crossRes to true and returns true. You're looking to compare crossRes with true, which means you need another equals sign:
if (crossRes == true) {
Or more concisely:
if (crossRes) {
I stand corrected:
if (crossRes)
You wouldn't have this problem if your condition was
if (true = crossRes)
because it wouldn't compile.
`crossRes = true` always evaluates to `true` because it's an assignment, to `true`.
You want `crossRes == true`:
if (crossRes == true) {
printf("Nothing in this loop should happen if dif is always > Tol
because count should never increment in that case, right?");
}
= is assignment, == is equality comparison. You want:
if (crossRes == true) {
You make the same mistake here:
if (doNext = true) { // Bad code
The other answers here have told you the problem. Often your compiler will warn you but a way to ensure that you do not do this is to put the constant term on the left
true == crossRes
that way you get a compiler error instead of a warning and so it can't escape unnoticed since
true = crossRes
wont compile.
First, although a number of people have pointed to the problem with if (crossRes = true), for some reason they haven't (yet, anyway) pointed to the same problem with if (doNext = true).
I'll stick to pointing out that you really want if (crossRes) rather than if (crossRes == true) (or even if (true == crossRes)).
The first reason is that it avoids running into the same problem from a simple typo.
The second is that the result of the comparison is a bool -- so if if (crossRes==true) is necessary, you probably need if (((((crossRes == true) == true) == true) == true) just to be sure (maybe a few more -- you never know). This would, of course, be utterly silly -- you're starting with a bool, so you don't need a comparison to get a bool.
I'd also note for the record, that if you insist on doing a comparison at all, you should almost always use if (x != false) rather than if (x == true). Though it doesn't really apply in C++, in old C that doesn't have an actual Boolean type, any integer type can be used -- but in this case, a comparison to true can give incorrect results. At least normally, false will be 0 and true will be 1 -- but when tested, any non-zero value will count as equivalent to true. For example:
int x = 10;
if (x) // taken
if (x == true) // not taken, but should be.
If you're not starting with a Boolean value as you are here, then the if (<constant> <comparison> <variable>) makes sense and is (IMO) preferred. But when you're starting with a Boolean value anyway, just use it; don't do a comparison to produce another of the same.